Thomas Meixner

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Scholarly Contributions

Chapters

• Fenn, M. E., Driscoll, C. T., Zhou, Q., Rao, L. E., Meixner, T., Allen, E. B., Yuan, F., & Sullivan, T. J. (2015). Use of Combined Biogeochemical Model Approaches and Empirical Data to Assess Critical Loads of Nitrogen. In Critical Loads and Dynamic Risk Assessments(pp 269--295). Springer.
• Fenn, M. E., Nagel, H., Koseva, I., Aherne, J., Jovan, S. E., Geiser, L. H., Schlutow, A., Scheuschner, T., Bytnerowicz, A., Gimeno, B. S., & others, . (2014). A Comparison of Empirical and Modelled Nitrogen Critical Loads for Mediterranean Forests and Shrublands in California. In Nitrogen Deposition, Critical Loads and Biodiversity(pp 357--368). Springer Netherlands.

Journals/Publications

• Dwivedi, R., Knowles, J. F., Eastoe, C., Minor, R., Abramson, N., Mitra, B., Wright, W. E., McIntosh, J., Meixner, T., Ferre, P. A., & others, . (2020). Ubiquitous Fractal Scaling and Filtering Behavior of Hydrologic Fluxes and Storages from A Mountain Headwater Catchment. Water, 12(2), 613.
• Chorover, J., Hu, J., Stanley, M., Papuga, S., Abramson, N., Meixner, T., Barron-Gafford, G. A., Nui, G., Castro, C., Ferre, P., Mitra, B., Minor, R., Hammon, L., Wright, W., Knowles, J., Eastoe, C., & Dwivedi, R. (2019). Vegetation source water identification using isotopic and hydrometric observations from a sub-humid mountain Catchment. Ecohydrology, e2167, 17. doi:10.1002/eco.2167
• Dwivedi, R., Meixner, T., Mcintosh, J. C., Ferre, P. A., Eastoe, C. J., Niu, G., Minor, R. L., Barron-Gafford, G. A., & Chorover, J. D. (2019). Hydrologic functioning of the deep Critical Zone and contributions to streamflow in a high elevation catchment: testing of multiple conceptual models. Hydrological Processes. doi:10.1002/hyp.13363
• White, A., Moravec, B., McIntosh, J., Olshansky, Y., Paras, B., Sanchez, R. A., Ferr'e, T., Meixner, T., & Chorover, J. (2019). Distinct stores and the routing of water in the deep critical zone of a snow-dominated volcanic catchment.
• Williams, Z. C., Pelletier, J. D., & Meixner, T. (2019). Self-affine fractal spatial and temporal variability of the San Pedro River, southern Arizona. Journal of Geophysical Research: Earth Surface, 124(6), 1540--1558.
• Chang, L., Dwivedi, R., Knowles, J. F., Fang, Y., Niu, G., Pelletier, J. D., Rasmussen, C., Durcik, M., Barron-Gafford, G. A., & Meixner, T. (2018). Why Do Large-Scale Land Surface Models Produce a Low Ratio of Transpiration to Evapotranspiration?. Journal of Geophysical Research: Atmospheres, 123(17), 9109--9130.
• Driscoll, J., Meixner, T., Molotch, N., Ferre, P. A., Williams, M. W., & Sickman, J. O. (2018). Event-response ellipses: a method to quantify and compare the role of dynamic storage at the catchment scale in snowmelt-dominated systems. Water, 10(12), 1824. doi:10.3390/w10121824
• Meixner, T., Barron-Gafford, G. A., Durcik, M., Rasmussen, C., Pelletier, J. D., Niu, G., Fang, Y., Knowles, J. F., Dwivedi, R., & Chang, L. (2018). Why Do Large‐Scale Land Surface Models Produce a Low Ratio of Transpiration to Evapotranspiration?. Journal of Geophysical Research: Atmospheres, 123(17), 9109-9130. doi:10.1029/2018JD029159
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  Most land surface models (LSMs) used in Earth System Models produce a lower ratio of transpiration (T) to evapotranspiration (ET) than field observations, degrading the credibility of Earth System Model‐projected ecosystem responses and feedbacks to climate change. To interpret this model deficiency, we conducted a pair of model experiments using a three‐dimensional, process‐based ecohydrological model in a subhumid, mountainous catchment. One experiment (CTRL) describes lateral water flow, topographic shading, leaf dynamics, and water vapor diffusion in the soil, while the other (LSM like) does not explicitly describe these processes to mimic a conventional LSM using artificially flattened terrain. Averaged over the catchment, CTRL produced a higher T/ET ratio (72%) than LSM like (55%) and agreed better with an independent estimate (79.79 ± 27%) based on rainfall and stream water isotopes. To discern the exact causes, we conducted additional model experiments, each reverting only one process described in CTRL to that of LSM like. These experiments revealed that the enhanced T/ET ratio was mostly caused by lateral water flow and water vapor diffusion within the soil. In particular, terrain‐driven lateral water flows spread out soil moisture to a wider range along hillslopes with an optimum subrange from the middle to upper slopes, where evaporation (E) was more suppressed by the drier surface than T due to plant uptake of deep soil water, thereby enhancing T/ET. A more elaborate representation of water vapor diffusion from a dynamically changing evaporating surface to the height of the surface roughness length reduced E and increased the T/ET ratio.
• Perdrial, J., Brooks, P. D., Swetnam, T., Lohse, K. A., Rasmussen, C., Litvak, M., Harpold, A. A., Zapata-Rios, X., Broxton, P., Mitra, B., & others, . (2018). A net ecosystem carbon budget for snow dominated forested headwater catchments: linking water and carbon fluxes to critical zone carbon storage. Biogeochemistry, 138(3), 225--243.
• Soule, D., Darner, R., O'Reilly, C., Bader, N. E., Meixner, T., Gibson, C. A., & McDuff, R. E. (2018). EDDIE modules are effective learning tools for developing quantitative literacy and seismological understanding. Journal of Geoscience Education, 66(2), 97--108.
• Balocchi, F., Pizarro, R., Meixner, T., & Urbina, F. (2017). Annual and monthly runoff analysis in the Elqui River, Chile, a semi-arid snow-glacier fed basin. Tecnolog'ia y Ciencias del Agua, 8(6), 23--35.
• Chorover, J. D., Derry, L., Pelletier, J. D., Meixner, T., Brooks, P., Zapata-Rios, X., Vinson, D., Rasmussen, C., Vazquez-Ortega, A., Harpold, A., Perdrial, J., Schaumberg, C., & Mcintosh, J. C. (2017). Geochemical evolution of the Critical Zone across variable time scales informs concentration-discharge relationships: Jemez River Basin Critical Zone Observatory. Water Resources Research, 53, 1-28. doi:10.1002/2016WR019712
• Duan, J. G., Bai, Y., Dominguez, F., Rivera, E., & Meixner, T. (2017). Framework for incorporating climate change on flood magnitude and frequency analysis in the upper Santa Cruz River. Journal of hydrology, 549, 194--207.
• Gougis, R. D., Stomberg, J. F., O’Hare, A. T., O’Reilly, C. M., Bader, N. E., Meixner, T., & Carey, C. C. (2017). Post-secondary science students’ explanations of randomness and variation and implications for science learning. International Journal of Science and Mathematics Education, 15(6), 1039--1056.
• McIntosh, J. C., Schaumberg, C., Perdrial, J., Harpold, A., V'azquez-Ortega, A., Rasmussen, C., Vinson, D., Zapata-Rios, X., Brooks, P. D., Meixner, T., & others, . (2017). Geochemical evolution of the Critical Zone across variable time scales informs concentration-discharge relationships: Jemez River Basin Critical Zone Observatory. Water Resources Research, 53(5), 4169--4196.
• Niraula, R., Meixner, T., Ajami, H., Rodell, M., Gochis, D., & Castro, C. L. (2017). Comparing potential recharge estimates from three Land Surface Models across the western US. Journal of hydrology, 545, 410--423.
• Niraula, R., Meixner, T., Dominguez, F., Bhattarai, N., Rodell, M., Ajami, H., Gochis, D., & Castro, C. (2017). How might recharge change under projected climate change in the western US?. Geophysical Research Letters, 44(20).
• O'Reilly, C. M., Gougis, R. D., Klug, J. L., Carey, C. C., Richardson, D. C., Bader, N. E., Soule, D. C., Castendyk, D., Meixner, T., Stomberg, J., & others, . (2017). Using Large Data Sets for Open-Ended Inquiry in Undergraduate Science Classrooms. BioScience, 67(12), 1052--1061.
• Stromberg, J. C., Setaro, D. L., Gallo, E. L., Lohse, K. A., & Meixner, T. (2017). Riparian vegetation of ephemeral streams. Journal of Arid Environments, 138, 27--37.
• Biederman, J. A., Meixner, T., Harpold, A. A., Reed, D. E., Gutmann, E. D., Gaun, J. A., & Brooks, P. D. (2016). Riparian zones attenuate nitrogen loss following bark beetle-induced lodgepole pine mortality. Journal of Geophysical Research: Biogeosciences, 121(3), 933--948.
• Gomaah, M., Meixner, T., Korany, E. A., Garamoon, H., & Gomaa, M. A. (2016). Identifying the sources and geochemical evolution of groundwater using stable isotopes and hydrogeochemistry in the Quaternary aquifer in the area between Ismailia and El Kassara canals, Northeastern Egypt. Arabian Journal of Geosciences, 9, 1--12.
• Guido, Z., Guido, Z., Mcintosh, J. C., Mcintosh, J. C., Papuga, S. A., Papuga, S. A., Meixner, T., & Meixner, T. (2016). Seasonal glacial meltwater contributions to surface water in the Bolivian Andes: A case study using environmental tracers. Journal of Hydrology: Regional Studies, 8, 260-273.
• Meixner, T., Manning, A. H., Stonestrom, D. A., Allen, D. M., Ajami, H., Blasch, K. W., Brookfield, A. E., Castro, C. L., Clark, J. F., Gochis, D. J., & others, . (2016). Implications of projected climate change for groundwater recharge in the western United States. Journal of Hydrology, 534, 124--138.
• Weber, M., Meixner, T., & Stromberg, J. (2016). Valuing Instream-Related Services of Wastewater. Ecosystem Services, 59-71. doi:10.1016/j.ecoser.2016.07.016
• Field, J. P., Breshears, D. D., Law, D. J., Villegas, j. C., Lopez Hoffman, L. -., Brooks, P. D., Chorover, J., Barron-Gafford, G. A., Gallery, R. E., Litvak, M. E., Lybrand, R., Mcintosh, J. C., Meixner, T. -., Niu, Y. -., Papuga, S. A., Pelletier, J. D., Rasmussen, C. -., & Troch, P. A. (2015). Critical zone services: Expanding context, constraints, and curency beyond ecosystem services.. Vadose Zone Journal, 1-7.
• Gallo, E. L., Meixner, T., Aoubid, H., Lohse, K. A., & Brooks, P. D. (2015). Combined impact of catchment size, land cover, and precipitation on streamflow and total dissolved nitrogen: A global comparative analysis. Global Biogeochemical Cycles, 29, 1109--1121.
• Guido, Z., Mcintosh, J. C., Papuga, S. A., & Meixner, T. (2016). Seasonal glacial meltwater contributions to surface water in the Bolivian Andes: A case study using environmental tracers. Journal of Hydrology: Regional Studies, 8, 260-273.
• Nakolan, L. E., Meixner, T., & Thompson, D. (2015). Response of Infiltration Rate to Effluent Water Quality Improvement at the Sweetwater Recharge Facility and Santa Cruz River.
• Niraula, R., Meixner, T., & Norman, L. M. (2015). Determining the Importance of Model Calibration for Forecasting Absolute/Relative Changes in Stream flow from LULC and Climate Changes. Journal of Hydrology.
• Gallo, E. L., Lohse, K. A., Ferlin, C. M., Meixner, T., & Brooks, P. D. (2014). Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways. Biogeochemistry, 121(1), 189--207.
• Homyak, P. M., Sickman, J. O., Miller, A. E., Melack, J. M., Meixner, T., & Schimel, J. P. (2014). Assessing Nitrogen-Saturation in a Seasonally Dry Chaparral Watershed: Limitations of Traditional Indicators of N-Saturation. Ecosystems, 17(7), 1286--1305.
• Hopkins, C. B., McIntosh, J. C., Eastoe, C., Dickinson, J. E., & Meixner, T. (2014). Evaluation of the importance of clay confining units on groundwater flow in alluvial basins using solute and isotope tracers: the case of Middle San Pedro Basin in southeastern Arizona (USA). Hydrogeology Journal, 1-21.
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  Abstract: As groundwater becomes an increasingly important water resource worldwide, it is essential to understand how local geology affects groundwater quality, flowpaths and residence times. This study utilized multiple tracers to improve conceptual and numerical models of groundwater flow in the Middle San Pedro Basin in southeastern Arizona (USA) by determining recharge areas, compartmentalization of water sources, flowpaths and residence times. Ninety-five groundwater and surface-water samples were analyzed for major ion chemistry (water type and Ca/Sr ratios) and stable (18O, 2H, 13C) and radiogenic (3H, 14C) isotopes, and resulting data were used in conjunction with hydrogeologic information (e.g. hydraulic head and hydrostratigraphy). Results show that recent recharge (
• Perdrial, J. N., McIntosh, J., Harpold, A., Brooks, P. D., Zapata-Rios, X., Ray, J., Meixner, T., Kanduc, T., Litvak, M., Troch, P. A., & Chorover, J. (2014). Stream water carbon controls in seasonally snow-covered mountain catchments: Impact of inter-annual variability of water fluxes, catchment aspect and seasonal processes. Biogeochemistry, 118(1-3), 273-290.
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  Abstract: Stream water carbon (C) export is one important pathway for C loss from seasonally snow-covered mountain ecosystems and an assessment of overarching controls is necessary. However, such assessment is challenging because changes in water fluxes or flow paths, seasonal processes, as well as catchment specific characteristics play a role. For this study we elucidate the impact of: (i) changes in water flux (by comparing years of variable wetness), (ii) catchment aspect [north-facing (NF) vs. south-facing (SF)] and (iii) season (snowmelt vs. summer) on all forms of dissolved stream water C [dissolved organic C (DOC), chromophoric dissolved organic matter (CDOM) and dissolved inorganic C (DIC)] in forested catchments within the Valles Caldera National Preserve, New Mexico. The significant correlation between annual water and C fluxes (e.g. DOC r2 = 0.83, p < 0.02) confirms annual stream water discharge as the overarching control on C efflux, likely from a well-mixed ground water reservoir as indicated by previous research. However, CDOM exhibited a dominantly terrestrial fluorescence signature (59-71 %) year round, signaling a strong riparian and near stream soil control on CDOM composition. During snowmelt, the role of water as C transporter was superimposed on its control as C reservoir, when the NF stream transported significantly more soil C (40 % DOC, 56 % DIC) than the SF stream as a result of hillslope flushing. Inter-annual variations in winter precipitation were paramount in regulating annual stream C effluxes, e.g., reducing C effluxes three-fold after a dry (relative to wet) winter season. During the warmer summer months % dissolved oxygen saturation decreased, δ13CDIC increased and CDOM assumed a more microbial signature, consistent with heterotrophic respiration in the stream and riparian soils. As a result of stream C incubation and soil respiration, PCO2 increased up to 12 times atmospheric values leading to substantial degassing. © 2013 Springer Science+Business Media Dordrecht.
• Perdrial, J. N., McIntosh, J., Harpold, A., Brooks, P. D., Zapata-Rios, X., Ray, J., Meixner, T., Kanduc, T., Litvak, M., Troch, P. A., & others, . (2014). Stream water carbon controls in seasonally snow-covered mountain catchments: impact of inter-annual variability of water fluxes, catchment aspect and seasonal processes. Biogeochemistry, 118(1-3), 273--290.
• Riha, K. M., Michalski, G., Gallo, E. L., Lohse, K. A., Brooks, P. D., & Meixner, T. (2014). High Atmospheric Nitrate Inputs and Nitrogen Turnover in Semi-arid Urban Catchments. Ecosystems, 17(8), 1309--1325.
• Xue, L., Bao, R., Meixner, T., Yang, G., & Zhang, J. (2014). Influences of topographic index distribution on hydrologically sensitive areas in agricultural watershed. Stochastic Environmental Research and Risk Assessment, 28(8), 2235--2242.
• Ali, G., Oswald, C. J., Spence, C., Cammeraat, E. L., McGuire, K. J., Meixner, T., & Reaney, S. M. (2013). Towards a unified threshold-based hydrological theory: necessary components and recurring challenges. Hydrological Processes, 27(2), 313--318.
• Ali, G., Oswald, C. J., Spence, C., L., E., Mcguire, K. J., Meixner, T., & Reaney, S. M. (2013). Towards a unified threshold-based hydrological theory: Necessary components and recurring challenges. Hydrological Processes, 27(2), 313-318.
• Bytnerowicz, A., Fenn, M., McNulty, S., Yuan, F., Pourmokhtarian, A., Driscoll, C., & Meixner, T. (2013). Interactive effects of air pollution and climate change on forest ecosystems in the united states. current understanding and future scenarios. Developments in Environmental Science, 13, 333-369.
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  Abstract: A review of the current status of air pollution and climate change (CC) in the United States from a perspective of their impacts on forest ecosystems is provided. Ambient ozone (O3) and nitrogen (N) deposition have important and widespread ecological impacts in U.S. forests. Effects of sulphurous (S) air pollutants and other trace pollutants have significant ecological importance only at much smaller geographic scales. Complex interactive effects of air pollution and CC for selected future CC scenarios are reviewed. In addition, simulations of past, present, and future hydrologic, nutrient, and growth changes caused by interactive effects of air pollution and CC are described for two U.S. forest ecosystems. Impacts of O3, N deposition, and CC on growth and hydrology of mixed conifer forests in the San Bernardino Mountains in southern California were projected with the DayCent model. Effects of N deposition, CO2 fertilization, N deposition, and CC on northern hardwood forests at the Hubbard Brook Experimental Forest in New Hampshire were simulated with the PnET-BGC model. Projected changes in these forests can influence the provision of ecosystem services such as C sequestration and water supply. The extent of these effects will vary depending on the future intensity and extent of CC, air pollutant emission levels, the distribution of air pollution, and other factors such as drought, pest outbreaks, fire, etc. Our chapter ends with research and management recommendations intended to increase our ability to cope with uncertainties related to the future interactive effects of multiple air pollutants, atmospheric deposition, CC, and other biotic and abiotic stressors. © 2013 Elsevier Ltd.
• Gallo, E. L., Lohse, K. A., Ferlin, C. M., Meixner, T., & Brooks, P. D. (2013). Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways. Biogeochemistry, 1-19.
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  Abstract: Rapid increases in human population and land transformation in arid and semi-arid regions are altering water, carbon (C) and nitrogen (N) cycles, yet little is known about how urban ephemeral stream channels in these regions affect biogeochemistry and trace gas fluxes. To address these knowledge gaps, we measured carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4) before and after soil wetting in 16 ephemeral stream channels that vary in soil texture and organic matter in Tucson, AZ. Fluxes of CO2 and N2O immediately following wetting were among the highest ever published (up to 1,588 mg C m-2 h-1 and 3,121 μg N m-2 h-1). Mean post-wetting CO2 and N2O fluxes were significantly higher in the loam and sandy loam channels (286 and 194 mg C m-2 h-1; 168 and 187 μg N m-2 h-1) than in the sand channels (45 mg C m-2 h-1 and 7 μg N m-2 h-1). Factor analyses show that the effect of soil moisture, soil C and soil N on trace gas fluxes varied with soil texture. In the coarser sandy sites, trace gas fluxes were primarily controlled by soil moisture via physical displacement of soil gases and by organic soil C and N limitations on biotic processes. In the finer sandy loam sites trace gas fluxes and N-processing were primarily limited by soil moisture, soil organic C and soil N resources. In the loam sites, finer soil texture and higher soil organic C and N enhance soil moisture retention allowing for more biologically favorable antecedent conditions. Variable redox states appeared to develop in the finer textured soils resulting in wide ranging trace gas flux rates following wetting. These findings indicate that urban ephemeral channels are biogeochemical hotspots that can have a profound impact on urban C and N biogeochemical cycling pathways and subsequently alter the quality of localized water resources. © 2013 Springer Science+Business Media Dordrecht.
• Norman, L. M., Villarreal, M. L., Niraula, R., Meixner, T., Frisvold, G., & Labiosa, W. (2013). Framing scenarios of binational water policy with a tool to visualize, quantify and valuate changes in ecosystem services. Water (Switzerland), 5(3), 852-874.
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  Abstract: In the Santa Cruz Watershed, located on the Arizona-Sonora portion of the U.S.-Mexico border, an international wastewater treatment plant treats wastewater from cities on both sides of the border, before discharging it into the river in Arizona. These artificial flows often subsidize important perennial surface water ecosystems in the region. An explicit understanding of the benefits of maintaining instream flow for present and future generations requires the ability to assess and understand the important trade-offs implicit in water-resource management decisions. In this paper, we outline an approach for modeling and visualizing impacts of management decisions in terms of rare terrestrial and aquatic wildlife, vegetation, surface water, groundwater recharge, real-estate values and socio-environmental vulnerable communities. We identify and quantify ecosystem services and model the potential reduction in effluent discharge to the U.S. that is under scrutiny by binational water policy makers and of concern to stakeholders. Results of service provisioning are presented, and implications for policy makers and resource managers are discussed. This paper presents a robust ecosystem services assessment of multiple scenarios of watershed management as a means to discern eco-hydrological responses and consider their potential values for future generations living in the borderlands. © 2013 by the authors.
• Pelletier, J. D., Barron-Gafford, G. A., Breshears, D. D., Brooks, P. D., Chorover, J., Durcik, M., Harman, C. J., Huxman, T. E., Lohse, K. A., Lybrand, R., Meixner, T., McIntosh, J. C., Papuga, S. A., Rasmussen, C., Schaap, M., Swetnam, T. L., & Troch, P. A. (2013). Coevolution of nonlinear trends in vegetation, soils, and topography with elevation and slope aspect: A case study in the sky islands of southern Arizona. Journal of Geophysical Research F: Earth Surface, 118(2), 741-758.
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  Abstract: Feedbacks among vegetation dynamics, pedogenesis, and topographic development affect the "critical zone" - the living filter for Earth's hydrologic, biogeochemical, and rock/sediment cycles. Assessing the importance of such feedbacks, which may be particularly pronounced in water-limited systems, remains a fundamental interdisciplinary challenge. The sky islands of southern Arizona offer an unusually well-defined natural experiment involving such feedbacks because mean annual precipitation varies by a factor of five over distances of approximately 10 km in areas of similar rock type (granite) and tectonic history. Here we compile high-resolution, spatially distributed data for Effective Energy and Mass Transfer (EEMT: the energy available to drive bedrock weathering), above-ground biomass, soil thickness, hillslope-scale topographic relief, and drainage density in two such mountain ranges (Santa Catalina: SCM; Pinaleño: PM). Strong correlations exist among vegetation-soil-topography variables, which vary nonlinearly with elevation, such that warm, dry, low-elevation portions of these ranges are characterized by relatively low above-ground biomass, thin soils, minimal soil organic matter, steep slopes, and high drainage densities; conversely, cooler, wetter, higher elevations have systematically higher biomass, thicker organic-rich soils, gentler slopes, and lower drainage densities. To test if eco-pedo-geomorphic feedbacks drive this pattern, we developed a landscape evolution model that couples pedogenesis and topographic development over geologic time scales, with rates explicitly dependent on vegetation density. The model self-organizes into states similar to those observed in SCM and PM. Our results highlight the potential importance of eco-pedo-geomorphic feedbacks, mediated by soil thickness, in water-limited systems. ©2013. American Geophysical Union. All Rights Reserved.
• Pelletier, J. D., Barron-Gafford, G. A., Breshears, D. D., Brooks, P. D., Chorover, J., Durcik, M., Harman, C. J., Huxman, T. E., Lohse, K. A., Lybrand, R., Meixner, T., Mcintosh, J. C., Papuga, S. A., Rasmussen, C., Schaap, M. G., Swetnam, T. W., & Troch, P. A. (2013). Coevolution of nonlinear trends in vegetation, soils, and topography with elevation and slope aspect: A case study in the sky islands of southern Arizona. J. Geophys. Res. - Earth Surf., 118, 741-758.
• Simpson, S. C., & Meixner, T. (2013). The influence of local hydrogeologic forcings on near-stream event water recharge and retention (Upper San Pedro River, Arizona). Hydrological Processes, 27(4), 617--627.
• Simpson, S. C., & Meixner, T. (2013). The influence of local hydrogeologic forcings on near-stream event water recharge and retention (Upper San Pedro River, Arizona). Hydrological Processes, 27(4), 617-627.
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  Abstract: The rise in stream stage during high flow events (floods) can induce losing stream conditions, even along stream reaches that are gaining during baseflow conditions. The aquifer response to flood events can affect the geochemical composition of both near-stream groundwater and post-event streamflow, but the amount and persistence of recharged floodwater may differ as a function of local hydrogeologic forcings. As a result, this study focuses on how vertical flood recharge varies under different hydrogeologic forcings and the significance that recharge processes can have on groundwater and streamflow composition after floods. River and shallow groundwater samples were collected along three reaches of the Upper San Pedro River (Arizona, USA) before, during and after the 2009 and 2010 summer monsoon seasons. Tracer data from these samples indicate that subsurface floodwater propagation and residence times are strongly controlled by the direction and magnitude of the dominant stream-aquifer gradient. A reach that is typically strongly gaining shows minimal floodwater retention shortly after large events, whereas the moderately gaining and losing reaches can retain recharged floodwater from smaller events for longer periods. The moderately gaining reach likely returned flood recharge to the river as flow declined. These results indicate that reach-scale differences in hydrogeologic forcing can control (i) the amount of local flood recharge during events and (ii) the duration of its subsurface retention and possible return to the stream during low-flow periods. Our observations also suggest that the presence of floodwater in year-round baseflow is not due to long-term storage beneath the streambed along predominantly gaining reaches, so three alternative mechanisms are suggested: (i) repeated flooding that drives lateral redistribution of previously recharged floodwater, (ii) vertical recharge on the floodplain during overbank flow events and (iii) temporal variability in the stream-aquifer gradient due to seasonally varying water demands of riparian vegetation. © 2011 John Wiley & Sons, Ltd.
• Simpson, S. C., Meixner, T., & Hogan, J. F. (2013). The role of flood size and duration on streamflow and riparian groundwater composition in a semi-arid basin. Journal of Hydrology, 488, 126--135.
• Simpson, S. C., Meixner, T., & Hogan, J. F. (2013). The role of flood size and duration on streamflow and riparian groundwater composition in a semi-arid basin. Journal of Hydrology, 488, 126-135.
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  Abstract: Floods with differing sizes and durations are likely to impact riparian systems in different hydrologic and geochemical ways. Here the impact of flood size and duration was investigated. Flood-driven recharge along predominantly losing reaches of the Bill Williams River (western Arizona, USA) later reemerges as baseflow in downstream gaining reaches, and the river's longest losing reach (Planet Valley) retains and releases the most flood recharge. River discharge volume and flow intermittency downstream of Planet Valley is highly dependent upon the length of time since the last major flood. After large floods (e.g. 2004-2005), baseflow was dominated by the flood's chemical (SO4, Cl) and isotopic (d18OH2O,δ2HH2O,δ34-SSO4,d18OSO4) composition for long periods (>4 years), suggesting that the largest events result in much more flood recharge and a longer persistence of floodwater in the subsurface than after smaller, more recent events. The continued dominance of baseflow by 2004-2005 floodwater nearly 5 years later- despite three smaller floods in 2007, 2008 and 2009-highlights the long-term impacts that the largest floods have on riparian water composition. Of these three recent events, only the largest and longest (in 2009) caused observable changes in both baseflow volume and the composition of baseflow and riparian groundwater, thereby suggesting that a threshold of flood size and duration exists for floods to alter the system's state and behavior. The river's dependence on large winter floods and the tendency of a particular set of atmospheric conditions (associated with El Niño-Southern Oscillation, or ENSO) to cause the region's largest winter floods indicates the importance of ENSO to the system, and that future changes to ENSO caused by climate change could drastically alter the flood properties and overall hydrology of southwestern rivers. © 2013 Elsevier B.V.
• Stromberg, J. C., McCluney, K. E., Dixon, M. D., & Meixner, T. (2013). Dryland Riparian Ecosystems in the American Southwest: Sensitivity and Resilience to Climatic Extremes. Ecosystems, 16(3), 411-415.
• Stromberg, J., McCluney, K., Dixon, M., & Meixner, T. (2013). Dryland riparian ecosystems in the American Southwest: sensitivity and resilience to climatic extremes. Ecosystems, 16(3), 411--415.
• Ajami, H., Maddock, T., Meixner, T., Hogan, J. F., & Guertin, D. P. (2012). RIPGIS-NET: A GIS Tool for Riparian Groundwater Evapotranspiration in MODFLOW. Groundwater, 50(1), 154--158.
• Ajami, H., Meixner, T., Dominguez, F., Hogan, J., & Maddock, T. (2012). Seasonalizing Mountain System Recharge in Semi-Arid Basins-Climate Change Impacts. Groundwater, 50(4), 585--597.
• Dejwakh, N. R., Meixner, T., Michalski, G., & McIntosh, J. (2012). Using 17O to investigate nitrate sources and sinks in a semi-arid groundwater system. Environmental science \& technology, 46(2), 745--751.
• Dejwakh, N. R., Meixner, T., Michalski, G., & McIntosh, J. (2012). Using ¹⁷O to investigate nitrate sources and sinks in a semi-arid groundwater system. Environmental Science and Technology, 46(2), 745-751.
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  PMID: 22148251;Abstract: We apply a triple isotope approach for nitrate that utilizes Δ 17O as a conservative tracer, in combination with δ 18O and δ 15N, to assess source/sink dynamics of groundwater nitrate beneath alluvial washes in a semiarid urban setting. Other studies have used δ 18O and δ 15N to determine nitrate sources and cycling, but the atmospheric δ 18O signature can be overprinted by biogeochemical processes. In this study, δ 18O and δ 15N values of nitrate were coupled with δ 17O values of nitrate to quantify atmospheric nitrate inputs and denitrification amounts. Results show generally low groundwater nitrate concentrations (
• Gallo, E. L., Lohse, K. A., Brooks, P. D., McIntosh, J. C., Meixner, T., & McLain, J. E. (2012). Quantifying the effects of stream channels on storm water quality in a semi-arid urban environment. Journal of Hydrology, 470, 98--110.
• Gallo, E. L., Lohse, K. A., Brooks, P. D., McIntosh, J. C., Meixner, T., & McLain, J. E. (2012). Quantifying the effects of stream channels on storm water quality in a semi-arid urban environment. Journal of Hydrology, 470-471, 98-110.
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  Abstract: Stormwater drainage systems can have a large effect on urban runoff quality, but it is unclear how ephemeral urban streams alter runoff hydrochemistry. This problem is particularly relevant in semi-arid regions, where urban storm runoff is considered a renewable water resource. Here we address the question: how do stream channels alter urban runoff hydrochemistry? We collected synoptic stormwater samples during three rainfall-runoff events from nine ephemeral streams reaches (three concrete or metal, three grass, three gravel) in Tucson, Arizona. We identified patterns of temporal and spatial (longitudinal) variability in concentrations of conservative (chloride and isotopes of water) and reactive solutes (inorganic-N, soluble reactive phosphorous, sulfate-S, dissolved organic carbon (DOC) and nitrogen, and fecal indicator bacteria). Water isotopes and chloride (Cl) concentrations indicate that solute flushing and evapoconcentration alter temporal patterns in runoff hydrochemistry, but not spatial hydrochemical responses. Solute concentrations and stream channel solute sourcing and retention during runoff were significantly more variable at the grass reaches (CV=2.3-144%) than at the concrete or metal (CV=1.6-107%) or gravel reaches (CV=1.9-60%), which functioned like flow-through systems. Stream channel soil Cl and DOC decreased following a runoff event (Cl: 12.1-7.3μgg -1 soil; DOC: 87.7-30.1μgg -1 soil), while soil fecal indicator bacteria counts increased (55-215CFUg -1 soil). Finding from this study suggest that the characteristics of the ephemeral stream channel substrate control biogeochemical reactions between runoff events, which alter stream channel soil solute stores and the hydrochemistry of subsequent runoff events. © 2012 Elsevier B.V.
• Meixner, T., Brooks, P., Hogan, J., Soto, C., & Simpson, S. (2012). Carbon and Nitrogen Export from Semiarid Uplands to Perennial Rivers: Connections and Missing Links, San Pedro River, Arizona, USA. Geography Compass, 6(9), 546-559.
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  Abstract: Investigations of biogeochemical processes in semiarid environments have largely focused on either plot studies in the uplands or on in-stream and near stream reaction or transport studies. Recent research permits us to synthesize a conceptual model of how uplands and riparian systems are linked in semiarid climates specific to the San Pedro River basin in Arizona. These studies have demonstrated significant export of both dissolved and sediment associated carbon and nitrogen from the uplands into the channel network of semiarid river systems. Likewise research has demonstrated that riparian areas are biogeochemically active, with the potential to rapidly respire inputs of organic matter, releasing carbon back to the atmosphere and inorganic nitrogen into the water column or back to the atmosphere through denitrification. For the San Pedro, a total of more than 80% of both carbon and nitrogen export from the uplands that is observed in small catchments is not observed at the larger river system scale, indicating that this carbon and nitrogen must be either stored, taken up by vegetation or returned to the atmosphere at scales between the small catchment (1-1000ha) and large river systems scale (∼320,000ha). In summary, existing research shows that the uplands contribute significant amounts of material into the stream and near stream zone, and that this imported material influences nutrient conditions in aquatic systems. These conclusions point out significant gaps in developing a complete understanding of the reactions of carbon and nitrogen as material is transported from the uplands through ephemeral and perennial channel networks. © 2012 The Authors. Geography Compass © 2012 Blackwell Publishing Ltd.
• Meixner, T., Brooks, P., Hogan, J., Soto, C., & Simpson, S. (2012). Carbon and nitrogen export from semiarid uplands to Perennial rivers: connections and missing links, San Pedro River, Arizona, USA. Geography Compass, 6(9), 546--559.
• Niraula, R., Norman, L. M., Meixner, T., & Callegary, J. B. (2012). Multi-gauge Calibration for modeling the Semi-Arid Santa Cruz watershed in Arizona-Mexico border area using SWAT. Air, Soil and Water Research, 5, 41-57.
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  Abstract: In most watershed-modeling studies, flow is calibrated at one monitoring site, usually at the watershed outlet. Like many arid and semi-arid watersheds, the main reach of the Santa Cruz watershed, located on the Arizona-Mexico border, is discontinuous for most of the year except during large flood events, and therefore the flow characteristics at the outlet do not represent the entire watershed. Calibration is required at multiple locations along the Santa Cruz River to improve model reliability. The objective of this study was to best portray surface water flow in this semi-arid watershed and evaluate the effect of multi-gauge calibration on flow predictions. In this study, the Soil and Water Assessment Tool (SWAT) was calibrated at seven monitoring stations, which improved model performance and increased the reliability of flow predictions, in the Santa Cruz watershed. The most sensitive parameters to affect flow were found to be curve number (CN2), soil evaporation and compensation coefficient (ESCO), threshold water depth in shallow aquifer for return flow to occur (GWQMN), base flow alpha factor (Alpha_Bf), and effective hydraulic conductivity of the soil layer (Ch_K2). In comparison, when the model was established with a single calibration at the watershed outlet, flow predictions at other monitoring gauges were inaccurate. This study emphasizes the importance of multi-gauge calibration to develop a reliable watershed model in arid and semi-arid environments. The developed model, with further calibration of water quality parameters will be an integral part of the Santa Cruz Watershed Ecosystem Portfolio Model (SCWEPM), an online decision support tool, to assess the impacts of climate change and urban growth in the Santa Cruz watershed. © the author(s), publisher and licensee Libertas Academica Ltd.
• Niraula, R., Norman, L. M., Meixner, T., & Callegary, J. B. (2012). Multi-gauge Calibration for modeling the semi-arid Santa Cruz Watershed in Arizona-Mexico border area using SWAT.
• Simpson, S. C., & Meixner, T. (2012). Modeling effects of floods on streambed hydraulic conductivity and groundwater-surface water interactions. Water Resources Research, 48(2).
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  Abstract: Flood events can induce temporal changes in streambed elevation and particle-size composition, which may influence the bed's hydraulic properties and stream-aquifer fluxes during and after an event. This study combines a set of previously developed modeling approaches to create a synthetic flood event during which bed sediment is entrained and deposited as a function of hydraulic conditions and particle size. One simulated river reach in a state of approximate dynamic equilibrium is chosen to investigate the impacts of size-selective sediment transport on stream-aquifer interaction. Along this reach, the preferential entrainment of fine sediment during the flood's rising limb leads to overall bed coarsening, and increases in vertical hydraulic conductivity (K bv) and downward fluxes of floodwater into the streambed. Progressively finer sediment layers are deposited during the event's falling limb, causing the redevelopment of a colmation (clogging) layer on the bed surface and a decline in overall K bv by the event's conclusion. This reduction in K bv leads to prolonged retention of event water in the streambed (after the reach reverts from losing to gaining river conditions) when compared with what is expected if pre-event K bv values are used to estimate river-aquifer exchanges. This process of sequential bed coarsening and fining during a flood event provides a mechanistic explanation for the event size-and-duration threshold, inferred in some systems, that must be exceeded for significant amounts of flood recharge to occur. The major consequences of these processesenhanced infiltration and prolonged floodwater retentionhave potentially major implications for groundwater-surface water interactions, water quality, contaminant transport, and riparian biogeochemistry. Copyright 2012 by the American Geophysical Union.
• Simpson, S. C., & Meixner, T. (2012). Modeling effects of floods on streambed hydraulic conductivity and groundwater-surface water interactions. Water resources research, 48(2).
• Wagener, T., Kelleher, C., Weiler, M., McGlynn, B., Gooseff, M., Marshall, L., Meixner, T., McGuire, K., Gregg, S., Sharma, P., & Zappe, S. (2012). It takes a community to raise a hydrologist: The Modular Curriculum for Hydrologic Advancement (MOCHA). Hydrology and Earth System Sciences, 16(9), 3405-3418.
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  Abstract: Protection from hydrological extremes and the sustainable supply of hydrological services in the presence of changing climate and lifestyles as well as rocketing population pressure in many parts of the world are the defining societal challenges for hydrology in the 21st century. A review of the existing literature shows that these challenges and their educational consequences for hydrology were foreseeable and were even predicted by some. However, surveys of the current educational basis for hydrology also clearly demonstrate that hydrology education is not yet ready to prepare students to deal with these challenges. We present our own vision of the necessary evolution of hydrology education, which we implemented in the Modular Curriculum for Hydrologic Advancement (MOCHA). The MOCHA project is directly aimed at developing a community-driven basis for hydrology education. In this paper we combine literature review, community survey, discussion and assessment to provide a holistic baseline for the future of hydrology education. The ultimate objective of our educational initiative is to enable educators to train a new generation of "renaissance hydrologists," who can master the holistic nature of our field and of the problems we encounter. © Author(s) 2012.
• Wagener, T., Kelleher, C., Weiler, M., McGlynn, B., Gooseff, M., Marshall, L., Meixner, T., McGuire, K., Gregg, S., Sharma, P., & others, . (2012). It takes a community to raise a hydrologist: the Modular Curriculum for Hydrologic Advancement (MOCHA).
• Chorover, J., Troch, P. A., Rasmussen, C., Brooks, P. D., Pelletier, J. D., Breshars, D. D., Huxman, T. E., Kurc, S. A., Lohse, K. A., McIntosh, J. C., Meixner, T., Schaap, M. G., Litvak, M. E., Perdrial, J., Harpold, A., & Durcik, M. (2011). How water, carbon, and energy drive critical zone evolution: The Jemez-Santa Catalina critical zone observatory. Vadose Zone Journal, 10(3), 884-899.
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  Abstract: The structure of the critical zone (CZ) is a result of tectonic, lithogenic, and climatic forcings that shape the landscape across geologic time scales. The CZ structure can be probed to measure contemporary rates of regolith production and hillslope evolution, and its fluids and solids can be sampled to determine how structure affects CZ function as a living filter for hydrologic and biogeochemical cycles. Substantial uncertainty remains regarding how variability in climate and lithology influence CZ structure and function across both short (e.g., hydrologic event) and long (e.g., landscape evolution) time scales. We are addressing this issue using a theoretical framework that quantifies system inputs in terms of environmental energy and mass transfer (EEMT, MJ m -2 yr -1) in the recently established Jemez River Basin (JRB)-Santa Catalina Mountains (SCM) Critical Zone Observatory (CZO). We postulate that C and water fluxes, as embodied in EEMT, drive CZ evolution and that quanti- fying system inputs in this way leads to predictions of nonlinear and threshold effects in CZ structure formation. We are testing this hypothesis in the JRB-SCM CZO, which comprises a pair of observatories-in northern New Mexico within the Rio Grande basin (JRB) and in southern Arizona within the Colorado River basin (SCM). The JRB-SCM CZO spans gradients in climate, lithology, and biota representative of much variation found in the larger U.S Southwest. Our approach includes in situ monitoring of zero-order basins nested within larger CZO watersheds and measurement-modeling iterations. The initial data collected at the ecosystem, pedon, and catchment scales indicates a strong role of coupled C and water flux in regulating chemical denudation of catchments in the JRB site. © Soil Science Society of America.
• Chorover, J., Troch, P. A., Rasmussen, C., Brooks, P. D., Pelletier, J. D., Breshears, D. D., Huxman, T. E., Kurc, S. A., Lohse, K., Mcintosh, J. C., Meixner, T., Schaap, M. G., Litvak, M., Perdrial, J., Harpold, A., & Durcik, M. (2011). How water, carbon, and energy drive critical zone evolution: the Jemez-Santa Catalina Critical Zone Observatory. Vadose Zone Journal, 10, 884-899.
• Yuan, F., Meixner, T., Fenn, M. E., & Šimůnek, J. (2011). Impact of transient soil water simulation to estimated nitrogen leaching and emission at high- and low-deposition forest sites in Southern California. Journal of Geophysical Research G: Biogeosciences, 116(3).
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  Abstract: Soil water dynamics and drainage are key abiotic factors controlling losses of atmospherically deposited N in Southern California. In this paper soil N leaching and trace gaseous emissions simulated by the DAYCENT biogeochemical model using its original semi-dynamic water flow module were compared to that coupled with a finite element transient water flow module (HYDRUS), for two mixed conifer forests with annual deposition rates of about 70 and 9 kg N ha -1, in the San Bernardino National Forest. Numerical solution of the Richards equation implemented in HYDRUS water module could improve response of surface soil water dynamics to precipitation pattern, compared to the original, and consequently it resulted in annual N gaseous emission loss about 1.5 ∼ 2 times higher. While the two flow modules predicted similar amounts of annual water drainage, the HYDRUS water module simulated more frequent, but smaller drainage fluxes, which favors soil mineralization and downward transport. In normal precipitation years, annual leaching losses predicted by the HYDRUS coupled DAYCENT model was about 5-18 kg N ha -1 higher due to different temporal patterns of daily water drainage. In dry and wet years, leaching losses were similar. Our analysis suggests that it is necessary to fully capture dynamics of transient water flow (e.g., by numerically solving the transient Richards equation) in order to adequately estimate soil N gaseous emissions and N transport and thus leaching, although it requires more computational resources while the uncertainty in model improvement is still large due to lack of measurements. © 2011 by the American Geophysical Union.
• Fenn, M. E., Allen, E. B., Weiss, S. B., Jovan, S., Geiser, L. H., Tonnesen, G. S., Johnson, R. F., Rao, L. E., Gimeno, B. S., Yuan, F., Meixner, T., & Bytnerowicz, A. (2010). Nitrogen critical loads and management alternatives for N-impacted ecosystems in California. Journal of Environmental Management, 91(12), 2404-2423.
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  PMID: 20705383;Abstract: Empirical critical loads for N deposition effects and maps showing areas projected to be in exceedance of the critical load (CL) are given for seven major vegetation types in California. Thirty-five percent of the land area for these vegetation types (99,639km2) is estimated to be in excess of the N CL. Low CL values (3-8kgNha-1yr-1) were determined for mixed conifer forests, chaparral and oak woodlands due to highly N-sensitive biota (lichens) and N-poor or low biomass vegetation in the case of coastal sage scrub (CSS), annual grassland, and desert scrub vegetation. At these N deposition critical loads the latter three ecosystem types are at risk of major vegetation type change because N enrichment favors invasion by exotic annual grasses. Fifty-four and forty-four percent of the area for CSS and grasslands are in exceedance of the CL for invasive grasses, while 53 and 41% of the chaparral and oak woodland areas are in exceedance of the CL for impacts on epiphytic lichen communities. Approximately 30% of the desert (based on invasive grasses and increased fire risk) and mixed conifer forest (based on lichen community changes) areas are in exceedance of the CL. These ecosystems are generally located further from emissions sources than many grasslands or CSS areas. By comparison, only 3-15% of the forested and chaparral land areas are estimated to be in exceedance of the NO3- leaching CL. The CL for incipient N saturation in mixed conifer forest catchments was 17kgNha-1yr-1. In 10% of the CL exceedance areas for all seven vegetation types combined, the CL is exceeded by at least 10kgNha-1yr-1, and in 27% of the exceedance areas the CL is exceeded by at least 5kgNha-1yr-1. Management strategies for mitigating the effects of excess N are based on reducing N emissions and reducing site N capital through approaches such as biomass removal and prescribed fire or control of invasive grasses by mowing, selective herbicides, weeding or domestic animal grazing. Ultimately, decreases in N deposition are needed for long-term ecosystem protection and sustainability, and this is the only strategy that will protect epiphytic lichen communities. © 2010.
• Rao, L. E., Allen, E. B., & Thomas, M. (2010). Risk-based determination of critical nitrogen deposition loads for fire spread in southern California deserts. Ecological Applications, 20(5), 1320-1335.
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  PMID: 20666252;Abstract: Fire risk in deserts is increased by high production of annual forbs and invasive grasses that create a continuous fine fuel bed in the interspaces between shrubs. Interspace production is influenced by water, nitrogen (N) availability, and soil texture, and in some areas N availability is increasing due to anthropogenic N deposition. The DayCent model was used to investigate how production of herbaceous annuals changes along gradients of precipitation, N availability, and soil texture, and to develop risk-based critical N loads. DayCent was parameterized for two vegetation types within Joshua Tree National Park, California, USA: creosote bush (CB) and piflon-juniper (PJ). The model was successfully calibrated in both vegetation types, but validation showed that the model is sensitive to soil clay content. Despite this fact, DayCent (the daily version of the biogeochemical model CENTURY) performed well in predicting the relative response of production to N fertilization and was used to determine estimates of fire risk for these ecosystems. Fire risk, the probability that annual biomass exceeds the fire threshold of 1000 kg/ha, was determined for each vegetation type and began to increase when N deposition increased 0.05 g/m 2 above background levels (0.1 g/m2). Critical loads were calculated as the amount of N deposition at the point when fire risk began to increase exponentially. Mean critical loads for all soil types and precipitation
• Valeron, B., & Meixner, T. (2010). Overland flow generation in chaparral ecosystems: Temporal and spatial variability. Hydrological Processes, 24(1), 65-75.
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  Abstract: Fire is an important and natural process in the lifecycle of chaparral systems, removing old growth and recycling nutrients. Recent catastrophic wildfires in southern California chaparral have heightened concerns about increased runoff and nutrient export. The goal of this study was to improve understanding of how overland flow is generated in unburned and post-fire chaparral watersheds. Samples of overland flow were collected from burned and unburned watersheds after rainfall events and multiple regression analysis was used to examine the influence of individual storm characteristics and system moisture on overland flow volume. The results indicate that variation in overland flow generation in the unburned watershed is best explained by storm size, while overland flow in the burned watershed was positively related to storm size and time between storms. These findings suggest that the burned system had decreased infiltration rates and increased soil water repellency. In contrast, there is a statistically significant negative relationship between overland flow 1 year after a fire against different system and precipitation factors revealed a negative correlation with drying period and a positive relationship with rainfall intensity, a combination that suggests reduced repellency. © 2009 John Wiley & Sons, Ltd.
• Xuyong, L. i., Miller, A. E., Meixner, T., Schimel, J. P., Melack, J. M., & Sickman, J. O. (2010). Adding an empirical factor to better represent the rewetting pulse mechanism in a soil biogeochemical model. Geoderma, 159(3-4), 440-451.
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  Abstract: The rewetting of a dry soil causes a pulse in decomposition of soil organic matter (SOM). This mechanism may dominate carbon (C) and nitrogen (N) cycles in arid, semi-arid and Mediterranean ecosystems. Existing biogeochemical models perform poorly for systems characterized by pulsed events. In this study, we added a rewetting factor into the DAYCENT soil biogeochemical model to better represent the drying-rewetting pulses. Based on a 4-month laboratory incubation from a parallel study, we developed a simple rewetting factor for representing the enhanced mineralization pulse by rewetting stimulation. The rewetting factor was then incorporated into DAYCENT by modifying the soil moisture factor. The DAYCENT modification significantly improved model performance in predicting soil C respiration rates in drying-rewetting treatments through the capture of rewetting pulses. The modification also improved prediction performance for net N mineralization in treatments with shorter rewetting intervals, but did not improve predictions in treatments with longer rewetting intervals. The model modifications were validated by using a laboratory incubation data set from a different field site. The modified DAYCENT predictions showed that active and slow SOM pools were major contributors to mineralization pulses while the contribution from the passive pool was minimal. The modifications we made improved model performance and should be considered in future field representations of biogeochemical processes. © 2010 Elsevier B.V.
• Jung, H. Y., Hogue, T. S., Rademacher, L. K., & Meixner, T. (2009). Impact of wildfire on source water contributions in Devil Creek, CA: Evidence from end-member mixing analysis. Hydrological Processes, 23(2), 183-200.
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  Abstract: A geochemical and end-member mixing analysis (EMMA) is undertaken in Devil Canyon catchment, located in southern California, to further understanding of watershed behaviour and source water contributions after an acute and extensive wildfire. Physical and chemical transformations in post-fire watersheds are known to increase overland flow and decrease infiltration, mainly due to formation of a hydrophobic layer at, or near, the soil surface. However, less is known about subsurface flow response in burned watersheds. The current study incorporates EMMA to evaluate and quantify source water contributions before, and after, a catchment affected by wildfires in southern California during the fall of 2003. Pre- and post-fire stream water data were available at several sampling sites within the catchment, allowing the identification of contributing water sources at varying spatial scales. Proposed end-member observations (groundwater, overland flow, shallow subsurface flow) were also collected to constrain and develop the catchment mixing model. Post-fire source water changes are more evident in the smaller and faster responding sub-basin (interior sampling point). Early post-fire storm events are dominated by overland flow with no significant soil water or groundwater flow contribution. Inter-storm streamwater in this smaller basin shows an increase in groundwater and a decrease in soil water. In the larger, baseflow-dominated system, source water components appear less affected by fire. A slight increase in lateral flow is observed with only a slight decrease in baseflow. Changes in the post-fire flow regimes affect nutrient loading and chemical response of the basin. Relatively rapid recovery of the chaparral ecosystem is evidenced, with active re-growth and evapotranspiration evidenced by the fourth post-fire rainy season. Copyright © 2008 John Wiley & Sons, Ltd.
• Lohse, K. A., Brooks, P. D., McIntosh, J. C., Meixner, T., & Huxman, T. E. (2009). Interactions between biogeochemistry and hydrologic systems. Annual Review of Environment and Resources, 34, 65-96.
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  Abstract: Here we review the fundamental interactions between hydrology and the cycling of carbon (C) and nitrogen (N) in terrestrial and stream ecosystems. We organize this review around five commonly studied environments: land-atmosphere interface, soil, groundwater, streams, and headwater catchments. Common among all environments is that hydrological transitions, either episodic changes in water availability or hydrologic transport of reactants, result in disproportionately high rates of C and N cycling. Two major research challenges in coupling hydrological and biogeochemical research are (a) effectively scaling reactions at these spatiotemporal transitions and (b) combining the progress made within each of the five environments listed above into an integrated understanding of hydrobiogeochemical cycles. Changes in local-to-regional hydrological cycling are likely to result in unexpected surprises at the landscape scale until progress in these research areas is made. Copyright © 2009 by Annual Reviews.
• Miller, A. E., Schimel, J. P., Sickman, J. O., Skeen, K., Meixner, T., & Melack, J. M. (2009). Seasonal variation in nitrogen uptake and turnover in two high-elevation soils: Mineralization responses are site-dependent. Biogeochemistry, 93(3), 253-270.
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  Abstract: In arctic and alpine ecosystems, soil nitrogen (N) dynamics can differ markedly between winter and summer months, and nitrogen losses can be measurable during the spring and fall transitions. To explore the effect of seasonality on biogeochemical processes in a temperate alpine environment, we used a combination of field incubations (year-round) and 15N tracer additions (late fall, early spring, summer) to characterize soil N dynamics in a wet and dry meadow in the Sierra Nevada, California. The snowmelt to early summer season marked a period of high 15N uptake and turnover in the two soils, coincident with the increase in microbial N pools at the start of snowmelt (wet and dry meadow); an increase in net N mineralization and net nitrification as snowmelt progressed (wet meadow only); and measureable net production of 15N-NH4+ in mid-summer (wet and dry meadow). Whereas fluctuations in microbial biomass were generally synchronous between the wet and dry meadow soils, only wet meadow soils appeared to mineralize N in response to declines in the microbial N pool. Net N mineralization and net nitrification rates in the dry meadow soil were negligible on all but one sampling date, in spite of periodic decreases in biomass of up to 60%. Across both sites, high 15N recoveries in microbial biomass N, rapid 15N-NH4+ turnover, and low or negative net 15N-NH4+ fluxes suggested tight cycling of N, particularly in the late fall and early spring. © 2009 Springer Science+Business Media B.V.
• Treese, S., Meixner, T., & Hogan, J. F. (2009). Clogging of an effluent dominated semiarid river: A conceptual model of stream-aquifer interactions. Journal of the American Water Resources Association, 45(4), 1047-1062.
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  Abstract: Water managers in arid and semiarid regions increasingly view treated wastewater (effluent) as an important water resource. Artificial recharge basins allow effluent to seep into the ground relieving stressed aquifers, however these basins frequently clog due to physical, chemical, and biological processes. Likewise effluent is increasingly used to maintain perennial base flow for dry streambeds, however, little is known about the impact of effluent on streambed hydraulic conductivity and stream-aquifer interactions. We address this issue by investigating: if a clogging layer forms, how the formation of a clogging layer alters stream-aquifer connections, and what hydrologic factors control the formation and removal of clogging layers. We focused on the Upper Santa Cruz River, Arizona where effluent from the Nogales International Waste Water Treatment Plant sustains perennial flow. Monthly sampling, along a 30 km river reach, was done with two foci: physical streambed transformations and water source identification using chemical composition. Historical dataset were included to provide a larger context for the work. Results show that localized clogging occurs in the Upper Santa Cruz River. The clogging layers perch the stream and shallow streambed causing desaturation below the streambed. With these results, a conceptual model of clogging is established in the context of a semiarid hydrologic cycle: formation during the hot premonsoon months when flow is nearly constant and removal by large flood flows (>10 m 3-s) during the monsoon season. However, if the intensity of flooding during the semiarid hydrologic cycle is lessened, the dependent riparian area can experience a die off. This conceptual model leads us to the conclusion that effluent dominated riparian systems are inherently unstable due to the clogging process. Further understanding of this process could lead to improved ecosystem restoration and management. © 2009 American Water Resources Association.
• Fenn, M. E., Jovan, S., Yuan, F., Geiser, L., Meixner, T., & Gimeno, B. S. (2008). Empirical and simulated critical loads for nitrogen deposition in California mixed conifer forests. Environmental Pollution, 155(3), 492-511.
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  PMID: 18499320;Abstract: Empirical critical loads (CL) for N deposition were determined from changes in epiphytic lichen communities, elevated NO3- leaching in streamwater, and reduced fine root biomass in ponderosa pine (Pinus ponderosa Dougl. ex Laws.) at sites with varying N deposition. The CL for lichen community impacts of 3.1 kg ha-1 year-1 is expected to protect all components of the forest ecosystem from the adverse effects of N deposition. Much of the western Sierra Nevada is above the lichen-based CL, showing significant changes in lichen indicator groups. The empirical N deposition threshold and that simulated by the DayCent model for enhanced NO3-leaching were 17 kg N ha-1 year-1. DayCent estimated that elevated NO3- leaching in the San Bernardino Mountains began in the late 1950s. Critical values for litter C:N (34.1), ponderosa pine foliar N (1.1%), and N concentrations (1.0%) in the lichen Letharia vulpina ((L.) Hue) are indicative of CL exceedance.
• Gimeno, B. S., Yuan, F., Fenn, M. E., & Meixner, T. (2008). Chapter 19 Management Options for Mitigating Nitrogen (N) Losses from N-Saturated Mixed-Conifer Forests in California. Developments in Environmental Science, 8, 425-455.
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  Abstract: Mixed-conifer forests of southern California are exposed to nitrogen (N) deposition levels that impair carbon (C) and N cycling, enhance forest flammability, increase the risk of fire occurrence and air pollution emissions in fire, and increase nitrate runoff and soil N emissions both pre- and postfire. N-deposition abatement policies and prescribed fire treatments have been proposed to mitigate the interactive effects of fire suppression, N deposition, and wildfire occurrence. To test the most effective management options for N-enriched forests, a simulation study was done using a parameterization of the DAYCENT model for a mixed-conifer forest site currently experiencing 70 kg N ha-1 yr-1. Five N deposition scenarios were defined, ranging from 5 to 70 kg N ha-1 yr-1. Five abatement strategies ranging from 0% to 100% reductions in N deposition were considered for each N-deposition scenario. The influence of prescribed fire was tested for the selected N deposition and abatement scenarios, considering 15-, 30-, and 60-year intervals (PF15, PF30, and PF60, respectively), or no prescribed fires. When the most extreme N-deposition scenario was compared to the lowest, fuel loads were increased by 121%, resulting in 70% increases in wildfire emissions of particulate matter (PM10 and PM2.5), methane (CH4), carbon monoxide (CO), carbon dioxide (CO2), and sulfur dioxide (SO2). The estimated increase in wildfire nitrogen oxide (NOx) emissions ranged from 56% to 210%. The larger values were derived when variations in fuel N content were taken into account. The combination of reduced N deposition and prescribed fire was most effective in reducing long-term N losses to the atmosphere and in runoff. The PF15 treatment combined with 50-75% reduced N deposition were the best options for reducing N losses before and after fire. However, even prescribed fire at longer intervals and in combination with 25-50% reduced N deposition still resulted in large reductions in ecosystem losses of N. Implementation of such treatments would be considered a major achievement towards mitigating the symptoms of N saturation, even though in sites chronically exposed to 70 kg N ha-1 yr-1 a 100% reduction in N deposition may require many years to return N losses to baseline levels. © 2009 Elsevier B.V. All rights reserved.
• Griensven, A. V., Meixner, T., Srinivasan, R., & Grunwald, S. (2008). Fit-for-purpose analysis of uncertainty using split-sampling evaluations. Hydrological Sciences Journal, 53(5), 1090-1103.
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  Abstract: An uncertainty assessment method for evaluating models, the Sources of UNcertainty GLobal Assessment using Split SamplES (SUNGLASSES), is presented, which assesses predictive uncertainty that is not captured by parameter or other input uncertainties. The method uses the split sample approach to generate a quantitative estimate of the fit-for-purpose of the model, thus focusing on the purpose for which the model is used. It operates by comparing the output to be used for decision making to its observed counterpart and the associated uncertainty. The described method is applied on a Soil Water Assessment Tool (SWAT) model of Honey Creek, a tributary of the Sandusky catchment in Ohio. Water flow and sediment loads are analysed. In this case study the uncertainty estimated by the proposed method is much larger than the typically estimated parameter uncertainty. Copyright © 2008 IAHS Press.
• Molotch, N. P., Meixner, T., & Williams, M. W. (2008). Estimating stream chemistry during the snowmelt pulse using a spatially distributed, coupled snowmelt and hydrochemical modeling approach. Water Resources Research, 44(11).
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  Abstract: We used remotely sensed snow cover data and a physically based snowmelt model to estimate the spatial distribution of energy fluxes, snowmelt, snow water equivalent, and snow cover extent over the different land cover types within the Green Lakes Valley, Front Range, Colorado. The spatially explicit snowpack model was coupled to the Alpine Hydrochemical Model (AHM), and estimates of hydrochemistry at the basin outlflow were compared with the baseline AHM approach, which implicitly prescribes snowmelt. The proportions of total meltwater production from soil, talus, and rock subunits were 46, 25, and 29%, respectively, for the baseline simulation without our advanced snowmelt representation. Conversely, simulations in which the AHM was coupled to our distributed snowmelt model ascribed the largest meltwater production to talus (47%) subunits, with 37% ascribed to soil and 16% ascribed to rock. Accounting for these differences in AHM reduced model overestimates of cation concentration during snowmelt; modeled Ca2+ estimates explained 82 and 70% (P values < 0.01) of observations with and without the coupled model, respectively. Similarly, the coupled model explained more variability in nitrate concentrations, with 83 versus 70% (P values < 0.01) explained by the coupled and baseline models, respectively. Early snowmelt over talus subunits was not detected at the basin outflow, confirming earlier reports that deeper flow paths are needed in biogeochemical models of alpine systems. Realistic treatment of snowmelt within these models will allow efforts to improve understanding of flow paths and predict catchment response to increases in atmospheric deposition and climate change. Copyright 2008 by the American Geophysical Union.
• Brooks, P. D., Hogan, J. F., & Meixner, T. (2007). Water in the desert: Introduction to special section on River and Riparian Biogeochemistry. Journal of Geophysical Research G: Biogeosciences, 112(3).
• Griensven, A. v., & Meixner, T. (2007). A global and efficient multi-objective auto-calibration and uncertainty estimation method for water quality catchmnt models. Journal of Hydroinformatics, 9(4), 277-291.
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  Abstract: Catchment water quality models have many parameters, several output variables and a complex structure leading to multiple minima in the objective function. General uncertainty/optimization methods based on random sampling (e.g. GLUE) or local methods (e.g. PEST) are often not applicable for theoretical or practical reasons. This paper presents "ParaSol", a method that performs optimization and uncertainty analysis for complex models such as distributed water quality models. Optimization is done by adapting the Shuffled Complex Evolution algorithm (SCE-UA) to account for multi-objective problems and for large numbers of parameters. The simulations performed by the SCE-UA are used further for uncertainty analysis and thereby focus the uncertainty analysis on solutions near the optimum/optima. Two methods have been developed that select "good" results out of these simulations based on an objective threshold. The first method is based on χ 2 statistics to delineate the confidence regions around the optimum/optima and the second method uses Bayesian statistics to define high probability regions. The ParaSol method was applied to a simple bucket model and to a Soil and Water Assessment Tool (SWAT) model of Honey Creek, OH, USA. The bucket model case showed the success of the method in finding the minimum and the applicability of the statistics under importance sampling. Both cases showed that the confidence regions are very small when the χ2 statistics are used and even smaller when using the Bayesian statistics. By comparing the ParaSol uncertainty results to those derived from 500,000 Monte Carlo simulations it was shown that the SCE-UA sampling used for ParaSol was more effective and efficient, as none of the Monte Carlo samples were close to the minimum or even within the confidence region defined by ParaSol. © IWA Publishing 2007.
• Meixner, T., Huth, A. K., Brooks, P. D., Conklin, M. H., Grimm, N. B., Bales, R. C., Haas, P. A., & Petti, J. R. (2007). Influence of shifting flow paths on nitrogen concentrations during monsoon floods, San Pedro River, Arizona. Journal of Geophysical Research G: Biogeosciences, 112(3).
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  Abstract: Hydrologic flow paths control transport, and therefore are a major constraint on the cycling and availability of nutrients within stream ecosystems. This control is particularly evident in semiarid streams, where hydrologic connectivity between stream, riparian, and upland systems increases greatly during storms in the rainy season, We measured chloride concentrations in base flow, precipitation, soil water, and stream water to quantify the hydrologic connectivity and solute flux between soil water, groundwater, and the stream channel during six summer floods in 2001 (a wet year; 25 cm winter rain) and 2002 (a dry year; 5 cm winter rain) in the San Pedro River, southeastern Arizona. This hydrologic information was used to evaluate observed patterns in nitrate, dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) concentrations in floods. The first floods of each year showed increased stream nitrate concentration that was approximately two orders of magnitude higher than base flow concentration. DOC consistently doubled to tripled during storm events, while DON in 2001 showed no response and showed a marked increase in 2002. A chloride mixing model indicated that soil and groundwater contributions to storm water discharge were related to antecedent conditions and to flood magnitude. Soil and groundwater contributions were the highest early in the 2001 monsoon season following the wet winter, much lower early in 2002 following a dry winter, and lowest during the largest floods of the 2002 monsoon season when flows were derived primarily from precipitation and overland flow. Stream water nitrate-N concentrations during floods were consistently 0.2-0.5 mg/L higher in 2002 than during 2001, suggesting greater over-winter accumulation of soil nitrate during the drier year. This result is consistent with higher mean nitrate-N concentrations in soil water of the riparian zone in 2002 (3.1 mg/L) than in 2001 (0.56 mg/ L). These data highlight the importance of seasonal and interannual variability of hydrology in semiarid regions, and the role of water availability in driving patterns of soil nutrient accumulation and their transport to the stream. Copyright 2007 by the American Geophysical Union.
• Miller, A. E., Schimel, J. P., Sickman, J. O., Meixner, T., Doyle, A. P., & Melack, J. M. (2007). Mineralization responses at near-zero temperatures in three alpine soils. Biogeochemistry, 84(3), 233-245.
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  Abstract: Cold-season processes are known to contribute substantially to annual carbon (C) and nitrogen (N) budgets in continental high elevation and high-latitude soils, but their role in more temperate alpine ecosystems has seldom been characterized. We used a 4-month lab incubation to describe temperature (-2, 0, 5°C) and moisture [50, 90% water-holding capacity (WHC)] effects on soil C and N dynamics in two wet and one dry meadow soil from the Sierra Nevada, California. The soils varied in their capacity to process N at and below 0°C. Only the dry meadow soil mineralized N at -2°C, but the wet meadow soils switched from net N consumption at -2°C to net N mineralization at temperatures ≥0°C. When the latter soils were incubated at -2°C at either moisture level (50 or 90% WHC), net NO3- production decreased even as NH4+ continued to accumulate. The same pattern occurred in saturated (90% WHC) soils at warmer temperatures (≥0°C), suggesting that dissimilatory processes could control N cycling in these soils when they are frozen. © 2007 Springer Science+Business Media B.V.
• Sirulnik, A. G., Allen, E. B., Meixner, T., & Allen, M. F. (2007). Impacts of anthropogenic N additions on nitrogen mineralization from plant litter in exotic annual grasslands. Soil Biology and Biochemistry, 39(1), 24-32.
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  Abstract: Urban regions of southern California receive up to 45 kg N ha-1 y-1 from nitrogen (N) deposition. A field decomposition study was done using 15N-labelled litter of the widespread exotic annual grass Bromus diandrus to determine whether elevated soil N is strictly from N deposition or whether N mineralization rates from litter are also increased under N deposition. Tissue N and lignin concentrations, which are inversely related in field sites with high and low N deposition, determine the rate at which N moves from plant litter to soil and becomes available to plants. The effect of soil N on N movement from litter to soil was tested by placing litter on high and low N soil in a factorial experiment with two levels of litter N and two levels of soil N. The litter quality changes associated with N deposition resulted in faster rates of N cycling from litter to soil. Concentrations of litter-derived N in total N, NH4+, NO3-, microbial N and organic N were all higher from high N/low lignin litter than from low N/high lignin litter. Litter contributed more N to soil NH4+ and microbial N in high N than low N soil. At the end of the study, N mineralized from high N litter on high N soil accounted for 46% of soil NH4+ and 11% of soil NO3-, compared to 35% of soil NH4+ and 6% of soil NO3- from low N litter on low N soil. The study showed that in high N deposition areas, elevated inorganic soil N concentrations at the end of the summer N deposition season are a result of N mineralized from plant litter as well as from N deposition. © 2006.
• Sirulnik, A. G., Allen, E. B., Meixner, T., Fenn, M. E., & Allen, M. F. (2007). Changes in N cycling and microbial N with elevated N in exotic annual grasslands of southern California. Applied Soil Ecology, 36(1), 1-9.
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  Abstract: The impacts of nitrogen (N) fertilization and N deposition on N mineralization and microbial biomass were studied in exotic annual grasslands in southern California. The goal of the study was to understand how N deposition impacts N availability to the grasslands by studying mineralization in plots in an urban area that has received chronic N deposition for 50 years compared with N fertilized and control plots in a rural area. Fertilized plots had higher net and gross rates of N cycling than did soils from the control. The effect of soil mineral N concentrations on microbial N varied between and within growing seasons. Lower microbial N corresponded to more net N release and higher microbial N corresponded to less net N release. Urban soils often had higher NO3- concentrations than did soils from the rural site but there was no difference in NH4+ concentrations. Urban soils also had lower mineral N concentrations than the fertilized soils and mineralization patterns in the high N deposition soils did not resemble those in the fertilized soils, indicating that the levels of N deposition at this site were well below the experimental fertilization rate. The levels of soil mineral N in the rural site were considerably higher than from other studies in the same plots in recent years. This corresponds with rapidly increasing suburbanization of the rural site and increasing N deposition, as suggested from a recent air pollution model. Although the urban and rural soils were not as different in mineral N concentrations as expected, soils in exotic grasslands near urban areas across the region can have mineral N concentrations as high as the fertilized soils, indicating that increased N cycling and altered microbial N may occur under N deposition. © 2006 Elsevier B.V. All rights reserved.
• Wagener, T., Weiler, M., McGlynn, B., Gooseff, M., Meixner, T., Marshall, L., McGuire, K., & McHale, M. (2007). Taking the pulse of hydrology education. Hydrological Processes, 21(13), 1789-1792.
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  Abstract: As a group of young hydrologists, we conducted a short, online survey to understand some of the main characteristics of current hydrology education and its educators. The survey provided a very interesting view on the great diversity found in hydrology education and suggests that while an education with a common basis is desirable, it is clearly not available at the moment. Hydrology educators are challenged to identify common principles, core knowledge, and approaches that should be included, in addition to areas where clear consensus is lacking. This lack of consistency may be contributing to slow progress in hydrologic science since each hydrologist's definition of what a hydrologist should know depends on their education and background. Kirchner (2006) and Bloeschl (2006) discuss in separate papers that advancements in hydrological science will likely come from synthesis of different approaches, from 'collision' of theory and data, and from better communication. Hydrology education is clearly one way to facilitate this communication. Additional information about the Research and Education Advancement through Cooperative Hydrology (REACH) group, which initiated this survey can be found as an online supplement to this article on the survey website. All the data collected are freely available and interested parties are invited to approach any of the authors to discuss the issue of hydrology education further. The survey can be found at http:// www.ideal.forestry.ubc.ca/markus/survey.asp, as well as the data underlying the analysis presented here. The survey will remain open to new respondents. Copyright © 2007 John Wiley & Sons, Ltd.
• Wood, Y. A., Fenn, M., Meixner, T., Shouse, P. J., Breiner, J., Allen, E., & Laosheng, W. u. (2007). Smog nitrogen and the rapid acidification of forest soil, San Bernardino Mountains, southern California. TheScientificWorldJournal, 7(SUPPL. 1), 175-180.
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  PMID: 17450295;Abstract: We report the rapid acidification of forest soils in the San Bernardino Mountains of southern California. After 30 years, soil to a depth of 25 cm has decreased from a pH (measured in 0.01 M CaCl2) of 4.8 to 3.1. At the 50-cm depth, it has changed from a pH of 4.8 to 4.2. We attribute this rapid change in soil reactivity to very high rates of anthropogenic atmospheric nitrogen (N) added to the soil surface (72 kg ha-1 year-1) from wet, dry, and fog deposition under a Mediterranean climate. Our research suggests that a soil textural discontinuity, related to a buried ancient landsurface, contributes to this rapid acidification by controlling the spatial and temporal movement of precipitation into the landsurface. As a result, the depth to which dissolved anthropogenic N as nitrate (NO3) is leached early in the winter wet season is limited to within the top ∼130 cm of soil where it accumulates and increases soil acidity. ©2007 with author. Published by TheScientificWorld.
• Chinkuyu, A., Meixner, T., Gish, T., & Nejadhashemi, A. P. (2006). Prediction of NO ₃-N losses in surface runoff from a field with seepage zones using GLEAMS and RZWQM. Transactions of the ASABE, 49(6), 1779-1790.
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  Abstract: Seepage zones have been shown to be of critical importance in controlling contaminant export from agricultural watersheds. However, their impacts on water quality have not been effectively modeled. The Groundwater Loading Effects of Agricultural Management Systems (GLEAMS) model and the Root Zone Water Quality Model (RZWQM) were used to predict daily and monthly nitrate-nitrogen (NO 3-N) concentration and loss in surface runoff from an agricultural field with seepage zones. The results of the study show that calibrated GLEAMS and RZWQM predicted daily NO 3-N concentration [index of agreement (D) > 0.57] in surface runoff from the field with seepage zones. Based on the different model evaluation techniques used in this study, both GLEAMS and RZWQM performed fairly well in assessing the effects of seepage zones on daily NO 3-N losses in surface runoff. However, GLEAMS (D = 0.93) performed relatively better than RZWQM (D = 0.45) in predicting NO 3-N loss in surface runoff on a monthly basis, while both GLEAMS and RZWQM performed equally well in predicting NO 3-N loss in surface runoff on a daily basis. Both models performed poorly in predicting NO 3-N concentration in surface runoff on a monthly basis (D < 0.44). Additionally, since neither model adequately simulated monthly NO 3-N concentration in surface runoff from the field with seepage zones, their ability in water quality modeling for such fields will be compromised, and further model evaluation and development is justified.
• French, C., Laosheng, W. u., Meixner, T., Haver, D., Kabashima, J., & Jury, W. A. (2006). Modeling nitrogen transport in the Newport Bay/San Diego Creek watershed of Southern California. Agricultural Water Management, 81(1-2), 199-215.
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  Abstract: The Newport Bay/San Diego Creek watershed has had a total maximum daily load (TMDL) established for the allowable loading of nitrogen into the bay. Baseline allocations for the TMDL indicate that 30% of the total nitrogen load is from agriculture; however, observations from a monitoring study and estimates from a conceptual model show that nitrogen from agriculture contributions is more likely between 2 and 8% of the total. An analysis using a nitrogen transport model indicates that agriculture contributes 2% of total N loading directly to surface waters, and an additional 6% from leaching to shallow groundwater and subsequent mixing with surface water. The time lag involved in groundwater mixing suggests that if all agricultural production in the watershed were ceased today, nitrate loading from fertilization would continue for 10-30 years. The greatest input of nitrogen to the surface-water system was found to be from urban development with 63% rather than the 21% estimated in the TMDL baseline allocations. © 2005 Elsevier B.V. All rights reserved.
• Griensven, A. v., & Meixner, T. (2006). Methods to quantify and identify the sources of uncertainty for river basin water quality models. Water Science and Technology, 53(1), 51-59.
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  PMID: 16532735;Abstract: Worldwide, the application of river basin water quality models is increasing, often imposed by law. It is, thus, important to know the degree of uncertainty associated with these models and their application to a specific watershed. These uncertainties lead to errors that are revealed when model outputs are compared to observations. Such uncertainty is typically described by calculating the residuals. However, residuals should not be seen as an estimate of total uncertainty, since through the calibration process, the residuals may be reduced by over-adjustment to the data, which is typically the case for over-parameterised models. Over-adjustment during a calibration period can also lead to highly biased results when the model is applied to other periods or environmental conditions. The total model uncertainties are, therefore, assessed by four components: the sum of the squares of the residuals (SSQ), parameter uncertainties (that can be ignored when their error is much smaller than SSQ), input data uncertainties, and an additional predictive uncertainty that is expressed when the model appears to be biased when it is applied for data other than the data used for calibration. The sources are ranked according to a quantification criterion (magnitude) as well as an identification criterion that depends on the number of observations that are covered by the confidence region. This approach is illustrated with SWAT2003 simulations for flow and sediment of Honey Creek, a tributary of the Sandusky River basin (Ohio). The results show the dominance of the model uncertainty. The input data uncertainty is less important. © IWA Publishing 2006.
• Griensven, A. v., Breuer, L., Luzio, M. D., Vandenberghe, V., Goethals, P., Meixner, T., Arnold, J., & Srinivasan, R. (2006). Environmental and ecological hydroinformatics to support the implementation of the European Water Framework Directive for river basin management. Journal of Hydroinformatics, 8(4), 239-252.
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  Abstract: Research and development in hydroinfomatics can play an important role in environmental impact assessment by integrating physically-based models, data-driven models and other information and Communication Tools (ICT). An illustration is given in this paper describing the developments around the Soil and Water Assessment Tool (SWAT) to support the implementation of the EU Water Framework Directive. SWAT operates on the river basin scale and includes processes for the assessment of complex diffuse pollution; it is open-source software, which allows for site-specific modifications to the source and easy linkage to other hydroinformatics tools. A crucial step in the world-wide applicability of SWAT was the integration of the model into a GIS environment, allowing for a quick model set-up using digital information on terrain elevation, land use and management, soil properties and weather conditions. Model analysis tools can be integrated with SWAT to assist in the tedious tasks of model calibration, parameter optimisation, sensitivity and uncertainty analysis and allows better understanding of the model before addressing scientific and societal questions. Finally, further linkage of SWAT to ecological assessment tools, Land Use prediction tools and tools for Optimal Experimental Design shows that SWAT can play an important role in multi-disciplinary eco-environmental impact assessment studies. © IWA Publishing 2006.
• Griensven, A. v., Meixner, T., Grunwald, S., Bishop, T., Diluzio, M., & Srinivasan, R. (2006). A global sensitivity analysis tool for the parameters of multi-variable catchment models. Journal of Hydrology, 324(1-4), 10-23.
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  Abstract: Over-parameterisation is a well-known and often described problem in hydrological models, especially for distributed models. Therefore, methods to reduce the number of parameters via sensitivity analysis are important for the efficient use of these models. This paper describes a novel sampling strategy that is a combination of latin-hypercube and one-factor-at-a-time sampling that allows a global sensitivity analysis for a long list of parameters with only a limited number of model runs. The method is illustrated with an application of the water flow and water quality parameters of the distributed water quality program SWAT, considering flow, suspended sediment, total nitrogen, total phosphorus, nitrate and ammonia outputs at several locations in the Upper North Bosque River catchment in Texas and the Sandusky River catchment in Ohio. The application indicates that the methodology works successfully. The results also show that hydrologic parameters are dominant in controlling water quality predictions. Finally, the sensitivity results are not transferable between basins and thus the analysis needs to be conducted separately for each study catchment. © 2005 Elsevier B.V. All rights reserved.
• Meixner, T., Fenn, M. E., Wohlgemuth, P., Oxford, M., & Riggan, P. (2006). N saturation symptoms in chaparral catchments are not reversed by prescribed fire. Environmental Science and Technology, 40(9), 2887-2894.
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  PMID: 16719087;Abstract: Fire is a critical ecosystem process in many landscapes and is particularly dominant in the chaparral shrublands of southern California which are also exposed to high levels of atmospheric N deposition. Few studies have addressed the combined effects of these two disturbance factors. In this study we evaluate the hydrologie and biogeochemical response of a control and a prescribed burn catchment over a 15-year period. Streamwater nitrate concentrations and export in the burned catchment were higher than those in the unburned catchment for 7-10 years after the burn and concentrations remained high in both catchments during the entire study. Therefore, fire is not an effective mitigation tool for N deposition in these semi-arid systems. Additionally, the extended N export in this system indicates that chaparral ecosystems do not recover their N retention capabilities as rapidly as humid systems do when subjected to disturbance. © 2006 American Chemical Society.
• Wood, Y. A., Meixner, T., Shouse, P. J., & Allen, E. B. (2006). Altered ecohydrologic response drives native shrub loss under conditions of elevated nitrogen deposition. Journal of Environmental Quality, 35(1), 76-92.
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  PMID: 16391279;Abstract: Many regions of southern California's coastal sage scrub (CSS) are rapidly declining as exotic annual plants replace native shrubs. During this conversion, the subsurface hydrology of the seniiarid hillslopes that support CSS may be altered. This could chronically suppress the ability of native shrubland to revegetate the landscape since ecosystem processes of nutrient availability and of seedling establishment rely on spatial patterns of available soil water. In this work, soil water and nutrient N regimes were compared over a 2-yr period between a southern California site where CSS has declined (approximately 5% shrub cover) with high additions of anthropogenic N, and one where CSS remains dominant (over 50% shrub cover) with predominantly background atmospheric additions of N. These two sites have similar climate, bedrock lithology, soils, and topography, and had the same vegetation type (Riversidean CSS) 30 years ago. We found that the depth and rate of rainwater percolation into wildland hillslope soils in response to early-season storm events has been greatly reduced after loss of CSS shrubs and vegetation type conversion to invasive grassland. With decreased rainwater redistribution to soil depths of 100 to 150 cm, the predominant zone of soil water has become the upper 25 cm. This shift exacerbates vegetation type conversion by (i) concentrating smog-produced nitrogenous (N) chemicals in the uppermost soil, where they become readily available, along with high soil water, to shallow-rooted exotic grasses early in the growing season and (ii) depriving adult and juvenile shrubs of deeper regolith water. © ASA, CSSA, SSSA.
• Xuyong, L. i., Meixner, T., Sickman, J. O., Miller, A. E., Schimel, J. P., & Melack, J. M. (2006). Decadal-scale dynamics of water, carbon and nitrogen in a California chaparral ecosystem: DAYCENT modeling results. Biogeochemistry, 77(2), 217-245.
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  Abstract: The Mediterranean climate, with its characteristic of dry summers and wet winters, influences the hydrologic and microbial processes that control carbon (C) and nitrogen (N) biogeochemical processes in chaparral ecosystems. These biogeochemical processes in turn determine N cycling under chronic N deposition. In order to examine connections between climate and N dynamics, we quantified decadal-scale water, C and N states and fluxes at annual, monthly and daily time steps for a California chaparral ecosystem in the Sierra Nevada using the DAYCENT model. The daily output simulations of net mineralization, stream flow and stream nitrate (NO3-) export were developed for DAYCENT in order to simulate the N dynamics most appropriate for the abrupt rewetting events characteristic of Mediterranean chaparral ecosystems. Overall, the magnitude of annual modeled net N mineralization, soil and plant biomass C and N, nitrate export and gaseous N emission agreed with those of observations. Gaseous N emission was a major N loss pathway in chaparral ecosystems, in which nitric oxide (NO) is the dominant species. The modeled C and N fluxes of net primary production (NPP), N uptake and N mineralization, NO3- export and gaseous N emission showed both high inter-annual and intra-annual variability. Our simulations also showed dramatic fire effects on NPP, N uptake, N mineralization and gaseous N emission for three years of postfire. The decease in simulated soil organic C and N storages was not dramatic, but lasted a longer time. For the seasonal pattern, the predicted C and N fluxes were greatest during December to March, and lowest in the summer. The model predictions suggested that an increase in the N deposition rate would increase N losses through gaseous N emission and stream N export in the chaparral ecosystems of the Sierra Nevada due to changes in N saturation status. The model predictions could not capture stream NO3- export during most rewetting events suggesting that a dry-rewetting mechanism representing the increase in N mineralization following soil wetting needs to be incorporated into biogeochemical models of semi-arid ecosystems. © Springer 2006.
• Chinkuyu, A., Meixner, T., Gish, T., & Daughtry, C. (2005). Prediction of pesticide losses in surface runoff from agricultural fields using GLEAMS and RZWQM. Transactions of the American Society of Agricultural Engineers, 48(2), 585-599.
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  Abstract: Seepage zones have been shown to be of critical importance in controlling contaminant export from agricultural watersheds. To date, no multipurpose agricultural water quality model has seepage zones incorporated into its process-level representations. We chose to test two widely used models of agricultural water quality, the Groundwater Loading Effects of Agricultural Management Systems (GLEAMS) and the Root Zone Water Quality Model (RZWQM), by seeing how well each predicted solution pesticide concentration and loss in surface runoff from two agricultural fields: one with and one without seepage zones. Daily simulated atrazine and metolachlor concentration and loss in surface runoff from both calibrated and default (or non-calibrated) GLEAMS and RZWQM were compared with three years of measured data from the two fields. The results of the study show that GLEAMS and RZWQM using default input parameters were not capable of predicting atrazine and metolachlor concentration and loss in surface runoff from the fields with and without seepage zones (modeling efficiency 0.52, index of agreement >0.83, and modeling efficiency >0.53) and can be used for assessing the effects of seepage zones on pesticide loss in surface runoff from agricultural fields.
• Miller, A. E., Schimel, J. P., Meixner, T., Sickman, J. O., & Melack, J. M. (2005). Episodic rewetting enhances carbon and nitrogen release from chaparral soils. Soil Biology and Biochemistry, 37(12), 2195-2204.
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  Abstract: The short-term pulse of carbon (C) and nitrogen (N) mineralization that accompanies the wetting of dry soils may dominate annual C and N production in many arid and semi-arid environments characterized by seasonal transitions. We used a laboratory incubation to evaluate the impact of short-term fluctuations in soil moisture on long-term carbon and nitrogen dynamics, and the degree to which rewetting enhances C and N release. Following repeated drying and rewetting of chaparral soils, cumulative CO2 release in rewet soils was 2.2-3.7 times greater than from soils maintained at equivalent mean soil moisture and represented 12-18% of the total soil C pool. Rewetting frequency did not affect cumulative CO2 release but did enhance N turnover, and net N mineralization and nitrification increased with rewetting in spite of significant reductions in nitrification potential. Litter addition decreased inorganic N release but enhanced dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) from dry soils, indicating the potential importance of a litter-derived pulse to short-term nutrient dynamics. © 2005 Elsevier Ltd. All rights reserved.
• Chinkuyu, A., Meixner, T., Gish, T., & Daughtry, C. (2004). The importance of seepage zones in predicting soil moisture content and surface runoff using GLEAMS and RZWQM. Transactions of the American Society of Agricultural Engineers, 47(2), 427-438.
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  Abstract: Seepage zones have been shown to be of critical importance in controlling contaminant export from agricultural catchments. To date, no multi-purpose agricultural water quality model has incorporated seepage zones into its process-level representations. We chose to test two commonly used models of agricultural water quality, Groundwater Loading Effects of Agricultural Management Systems (GLEAMS) and the Root Zone Water Quality Model (RZWQM), by seeing how well each predicted surface runoff and soil moisture content in two agricultural fields: one with and one without seepage zones. Daily simulated surface runoff and soil moisture content from both calibrated and default (or non-calibrated) GLEAMS and RZWQM were compared with three years of measured surface runoff and soil moisture content in the two fields. The results of the study show that GLEAMS and RZWQM, using default model parameters, were not capable of predicting surface runoff and soil moisture content in either field. Site-calibrated GLEAMS and RZWQM performed well in simulating surface runoff trends from the field with and without seepage zones, but they predicted soil moisture content poorly. Several statistical tests were used that showed that although both site-calibrated GLEAMS and RZWQM performed well, RZWQM performed better than GLEAMS and is better suited in assessing the effects of seepage zones on soil moisture content and surface runoff from agricultural fields.
• Meixner, T., & Fenn, M. (2004). Biogeochemical budgets in a mediterranean catchment with high rates of atmospheric N deposition - Importance of scale and temporal asynchrony. Biogeochemistry, 70(3), 331-356.
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  Abstract: In this study biogeochemical export in a set of catchments that vary from6 ha to almost 1500 ha is investigated. Studying catchments across this large range of scales enables us to investigate the scale dependence and fundamental processes controlling catchment biogeochemical export that would not have been possible with a more limited data set. The Devil Canyon catchment, in the San Bernardino Mountains, California, has some of the highest atmospheric N deposition rates in the world (40-90 kg ha -1 year -1 at the crest of the catchment). These high rates of deposition have translated into consistently high levels of NO 3- in some streams of the San Bernardino Mountains. However, the streams of the Devil Canyon catchment have widely varying dissolved inorganic nitrogen (DIN) concentrations and export. These differences are also, to a more limited extent, present for dissolved organic carbon (DOC) but not in other dissolved species (Cl -, SO 24-, Ca 2+ and other weathering products). As catchment size increases DIN and DOC concentrations first increase until catchment area is~150 ha but then decrease as catchment scale increases beyond that size. The scale dependence ofDIN export implies that catchments at different spatial scales are at different degrees ofN saturation. The reason for this scale effect appears to be the dominance of flushing of DIN out of soil at small scales due to the temporal asynchrony between nutrient availability and biological N demand, the groundwater exfiltration of this flushed DIN at intermediate scales and the removal of this DIN from streamflow through in-stream processes and groundwater-surface water interaction at larger scales.While the particular scale effect observed here may not occur over the same range in catchment area in other ecosystems, it is likely that other ecosystems have similar scale dependant export for DIN and DOC. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.
• Meixner, T., Gupta, H. V., Montanari, A., & Jackson, B. (2004). Understanding Hydrologic Model Uncertainty: A Report on the IAHS-PUB Workshop. EOS Transactions.
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  Meixner T, H Gupta, A Montanari and B Jackson (2004), Understanding Hydrologic Model Uncertainty: A Report on the IAHS-PUB Workshop, EOS Transactions, v85, p556
• Meixner, T., Gutmann, C., Bales, R., Leydecker, A., Sickman, J., Melack, J., & McConnell, J. (2004). Multidecadal hydrochemical response of a Sierra Nevada watershed: Sensitivity to weathering rate and changes in deposition. Journal of Hydrology, 285(1-4), 272-285.
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  Abstract: To address the responses of the very dilute waters in the Sierra Nevada, California, to acidic atmospheric deposition, the Alpine hydrochemical model (AHM) was used to simulate 47 years of runoff and solute concentrations in the Emerald Lake catchment. The AHM is a semi-distributed model of alpine watersheds that incorporates representations of the major hydrologic and biogeochemical processes that control stream chemical composition. Proxy data of discharge and snowfall were used to develop the necessary inputs for the 47-year runs. The long-term simulations were stable, but conflicts in the simulation of base cation and silica concentrations indicate that the model has a missing process or misrepresents mineral weathering. Sensitivity analysis of the weathering parameters indicates that a weathering rate of approximately 80% of the value fitted based on a one-year calibration would match the observed base saturation and the initial one year estimate had incorrect stoichiometry. Additionally, comparison of annual modeled mass flux to observed mass flux indicates that the model overestimates cation and silica export in dry years and underestimates export in wet years. Our results indicate that the Emerald Lake watershed, as represented by AHM, is not sensitive to chronic acidification with atmospheric deposition at current levels and that there would be little episodic acidification with a doubling in atmospheric deposition. However, in the simulations climate variability had an impact on stream water pH and this sensitivity should be taken into account in assessing alpine catchment sensitivity to changes in atmospheric deposition. © 2003 Elsevier B.V. All rights reserved.
• Meixner, T., Shaw, J. R., & Bales, R. C. (2004). Temporal and spatial variability of cation and silica export in an alpine watershed, Emerald Lake, California. Hydrological Processes, 18(10), 1759-1776.
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  Abstract: A reaction set of possible mineral weathering reactions is proposed to explain observed cation and silica export for the Emerald Lake watershed, a small Sierra Nevada, California catchment. The reaction set was calculated through a stoichiometric mole-balance method, using a multiyear record of stream flow and snowpack chemical analyses and site-specific mineral compositions. Reaction-set calculations were intended to explore how the processes controlling stream cation and silica export depend on differing bedrock mineralogy across the catchment as snowmelt and runoff patterns change over the year. Different regions within the watershed can be differentiated by lake inflow subdrainages, each exhibiting different stream-flow chemistry and calculated weathering stoichiometry, indicating that different silica and cation generation processes are dominant in wet steep portions of the catchment. Short-term differences in stream concentrations were assumed to reflect ion exchange equilibria and rapid biological processes, whereas long-term persistent stream concentration differences in different areas of the catchment were assumed to reflect spatial variability in mineral weathering stoichiometry. Mineralogical analyses of rock samples from the watershed provided site-specific chemical compositions of major mineral species for reaction calculations. Reaction sets were evaluated by linear regression of calculated versus observed differences between snowmelt and stream-flow chemistry and by a combined measure. Initially, single weathering reactions were balanced and evaluated to determine the reactions that best explained observed stream chemical export. Next, reactions were combined, using mineral compositions from different rock types to estimate the dependence of ion fluxes on lithology. The seasonal variability of major solute calculated fluxes is low, approximately one order of magnitude, relative to the observed three orders of magnitude variability in basin discharge. Reaction sets using basin-averaged lithology and Aplite lithologies gave superior explanations of stream chemical composition. © 2004 John Wiley and Sons, Ltd.
• Michalski, G., Meixner, T., Fenn, M., Hernandez, L., Sirulnik, A., Allen, E., & Thiemens, M. (2004). Tracing Atmospheric Nitrate Deposition in a Complex Semiarid Ecosystem Using Δ¹⁷O. Environmental Science and Technology, 38(7), 2175-2181.
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  PMID: 15112822;Abstract: The isotopic composition of nitrate collected from aerosols, fog, and precipitation was measured and found to have a large 17O anomaly with Δ17O values ranging from 20‰p to 30‰ (Δ17O = δ17O - 0.52(δ18O)). This 17O anomaly was used to trace atmospheric deposition of nitrate to a semiarid ecosystem in southern California. We demonstrate that the Δ17O signal is a conserved tracer of atmospheric nitrate deposition and is a more robust indicator of N deposition relative to standard δ18O techniques. The data indicate that a substantial portion of nitrate found in the local soil, stream, and groundwater is of atmospheric origin and does not undergo biologic processing before being exported from the system.
• Stephens, S. L., Meixner, T., Poth, M., McGurk, B., & Payne, D. (2004). Prescribed fire, soils, and stream water chemistry in a watershed in the Lake Tahoe Basin, California. International Journal of Wildland Fire, 13(1), 27-35.
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  Abstract: Before Euro-American settlement fire was a common process in the forests of the Lake Tahoe Basin. The combination of drought, fire suppression, and past harvesting has produced ecosystems that are susceptible to high-severity wildfires. Consequently, a program of prescribed fire has been recommended but there is incomplete understanding of the ecological effects of fuels treatments, especially with regard to how treatments will affect the flow of nutrients to Lake Tahoe. Nitrogen and phosphorus are the most important nutrients affecting algal growth, and thus lake clarity. Existing data demonstrate a long-term shift from a co-limitation by both nitrogen and phosphorus to phosphorus limitation. Two high-consumption, moderate-intensity prescribed fires were conducted to determine their effects on soil and stream water chemistry. Stream water calcium concentrations increased in burned watersheds whereas soluble reactive phosphorus concentrations were not significantly different. Prescribed fires released calcium and raised soil pH and this may have resulted in the incorporation of phosphorus into insoluble forms. Stream monitoring data indicates water quality effects last for ∼3 months. Prescribed fires did not significantly increase the amount of soluble reactive phosphorus in stream waters. However, additional research is needed to determine if prescribed fire increases erosion or movement of particulate P, particularly in areas with steep slopes.
• Fenn, M. E., Baron, J. S., Allen, E. B., Rueth, H. M., Nydick, K. R., Geiser, L., Bowman, W. D., Sickman, J. O., Meixner, T., Johnson, D. W., & Neitlich, P. (2003). Ecological effects of nitrogen deposition in the western United States. BioScience, 53(4), 404-420.
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  Abstract: In the western United States vast acreages of land are exposed to low levels of atmospheric nitrogen (N) deposition, with interspersed hotspots of elevated N deposition downwind of large, expanding metropolitan centers or large agricultural operations. Biological response studies in western North America demonstrate that some aquatic and terrestrial plant and microbial communities are significantly altered by N deposition. Greater plant productivity is counterbalanced by biotic community changes and deleterious effects on sensitive organisms (lichens and phytoplankton) that respond to low inputs of N (3 to 8 kilograms N per hectare per year). Streamwater nitrate concentrations are elevated in high-elevation catchments in Colorado and are unusually high in southern California and in some chaparral catchments in the southwestern Sierra Nevada. Chronic N deposition in the West is implicated in increased fire frequency in some areas and habitat alteration for threatened species. Between hotspots, N deposition is too low to cause noticeable effects or has not been studied.
• Meixner, T., & Bales, R. C. (2003). Hydrochemical modeling of coupled C and N cycling in high-elevation catchments: Importance of snow cover. Biogeochemistry, 62(3), 289-308.
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  Abstract: Several ecosystems in the western US are already undergoing nitrogen (N) saturation, a condition where previously N limited ecosystems are no longer N limited. This state of N saturation leads to adverse impacts on terrestrial ecology and water quality. Due to the complexities of terrestrial carbon-nitrogen cycling, integrated hydrologic-biogeochemical modeling provides a tool to improve our understanding and discern between the impacts of changes in N deposition from changes in other ecosystem processes. A model of biogeochemical processing in alpine watersheds was developed and applied to the Emerald Lake watershed. Simulations of major terrestrial carbon and nitrogen pools and fluxes were adequate. The use of snow cover information to estimate soil temperatures improved model simulations indicating that snow cover processes need to be incorporated into biogeochemical models of seasonally snow covered areas. The model simulated mineral nitrogen processes well but significant changes in denitrification and dissolved organic nitrogen export processes appear to be necessary. Our results also showed that variations in snow cover duration have more of an impact on mineral N export, plant uptake and mineralization than appears possible due to changes in atmospheric deosition.
• Meixner, T., Allen, E. B., Tonnessen, K., Fenn, M., & Poth, M. (2002). Atmospheric nitrogen deposition: Implications for managers of western U.S. parks. Park Science, 21(2), 30-33.
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  Abstract: A comparison of three ecosystems highlights differences in the susceptibility of natural resources to the effects of atmospheric nitrogen deposition.
• Meixner, T., Fenn, M., & Poth, M. (2002). Landscape level controls on nitrate-nitrogen in forested and chaparral catchments of Southern California. Report - University of California Water Resources Center, 73-75.
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  Abstract: The water samples from Devil Canyon watershed in the semiarid Southern California with varying Nitrate concentrations were examined to investigate the landscape scale dynamics of biochemical and hydrological processes that controls nitrate export from the semiarid forested catchments with elevated N deposition. Nitrate in the catchments enters the stream through the flushing of winter rains and its concentrations are induced by the mass dilution. The nitrate in the water is dominated by groundwater seeping to the surface and its concentrations remain high throughout the season. The nitrate and dissolved organic carbon (DOC) concentrations of water increases as the stream flow increases. The concept of a denitrification control on stream nitrate and DOC concentration as a result from longitudinal surveys and mass balance analysis indicated plant uptake and riparian zone, rather than a mass dilution process.
• Meixner, T., Fenn, M., & Poth, M. (2001). Nitrate in polluted mountainous catchments with Mediterranean climates.. TheScientificWorldJournal [electronic resource], 1 Suppl 2, 564-571.
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  PMID: 12805811;Abstract: The mountains of southern California receive some of the highest rates of nitrogen (N) deposition in the world (approximately 40 kg ha(-1) year(-1)). These high rates of deposition have translated into consistently high levels of nitrate (NO3-) in some streams of the San Bernardino Mountains. However, not all streams are exhibiting these high levels of NO3-. Perennial streams have high NO3- concentrations (approximately 200 micromoles l(-1)) while ephemeral streams do not (approximately 20 micromoles l(-1)). This difference points to groundwater as the source of the NO3- observed in streams. Furthermore, the evidence indicates a differential impact of N deposition on terrestrial and aquatic systems in Mediterranean climates, with aquatic systems being impacted more quickly. The primary reason for this difference involves the asynchrony between the time that atmospheric deposition occurs (summer), the time period of maximum soil NO3- availability and leaching (winter), and the time of maximum plant N demand (spring). Our results indicate that semiarid Mediterranean climate systems behave differently from more humid systems in that, because of this asynchrony, aquatic systems may not be indicative of changes in terrestrial ecosystem response. These differences lead us to the conclusion that the extrapolation of impacts from humid to Mediterranean climates is problematic and the concept of N saturation may need to be revisited for semiarid and seasonally dry systems.
• Meixner, T., Bales, R. C., Williams, M. W., Campbell, D. H., & Baron, J. S. (2000). Stream chemistry modeling of two watersheds in the Front Range, Colorado. Water Resources Research, 36(1), 77-87.
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  Abstract: We investigated the hydrologic, geochemical, and biogeochemical controls on stream chemical composition on the Green Lakes Valley and Andrews Creek watersheds using the alpine hydrochemical model (AHM). Both sites had comparable data sets from 1994 and 1996, including high-resolution spatial data and high-frequency time series of hydrology, geochemistry, and meteorology. The model of each watershed consisted of three terrestrial subunits (soil, talus, and rock), with the routing between the subunits determined by spatial land cover data. Using 1994 data for model calibration and 1996 data for evaluation, AHM captured the dominant processes and successfully simulated daily stream chemical composition on both watersheds. These results confirm our procedure of using spatial and site-specific field and laboratory data to generate an initial catchment model and then calibrating the model to calculate effective parameters for unmeasured processes. A net source of nitrogen was identified in the Andrews Creek watershed during the spring snowmelt period, whereas nitrogen was immobilized in the Green Lakes Valley. This difference was most likely due to the larger and more dominant area of talus in the Andrews Creek watershed. Our results also indicate that routing of snowmelt through either soil or talus material is sufficient for retention of H+ and release of base cations but that N retention is more important on areas mapped as soil. Owing to the larger ionic pulse and larger fraction of surface runoff the Green Lakes Valley was more sensitive to a doubling of wet deposition chemistry than the Andrews Creek watershed.
• Meixner, T., Brown, A., & Bales, R. C. (1998). Importance of biogeochemical processes in modeling stream chemistry in two watersheds in the Sierra Nevada, California. Water Resources Research, 34(11), 3121-3133.
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  Abstract: Two small (0.22 and 0.48 ha) alpine watersheds in the Sierra Nevada of California were studied during the 1992 and 1993 snowmelt seasons to evaluate the importance of soil properties and processes on chemical concentrations in- the discharges from each watershed. Watershed 1 was surveyed as having 26% soil cover, whereas watershed 2 was 10% soil covered. Watershed 2 had greater H+ and nitrogen consumption than watershed 1 but similar cation and sulfate concentrations despite having one fourth the surveyed soil volume per unit area as watershed 1. Daily stream concentrations simulated with the Alpine Hydrochemical Model (AHM) matched the data well, after a systematic model calibration with a subset of the data. We found that the structure of the AHM and the hydrologic parameters developed for the nearby 1.2 km2 Emerald Lake watershed could be applied to these watersheds with only small adjustments; chemical parameters required considerably more adjustment, reflecting a greater degree of chemical versus physical heterogeneity at this scale. Calibration for watershed 2 gave a higher percent base saturation (19 versus 4%) and lower stream P(CO2) (10-3.1 versus 10-2.6 atm) than for watershed 1 and three times the soil reactivity (expected) of a field survey. Areas mapped as exposed bedrock in the catchments apparently contributed cations and alkalinity to stream water to a greater extent than did neighboring areas of soil. Areas of exposed bedrock were a larger nitrogen sink than the adjoining areas of soil. The pH and acid-neutralizing capacity of surface runoff in both catchments were less sensitive to changes in atmospheric deposition than at the nearby Emerald Lake watershed. This decreased sensitivity was due to (1) a less pronounced ionic pulse, (2) less retention of sulfate in the soil, and (3) greater nitrate retention.

Proceedings Publications

• Dwivedi, R., Eastoe, C. J., Knowles, J. F., Mcintosh, J. C., Meixner, T., Ferre, T. P., Minor, R. L., Gafford, G. B., Abramson, N., Stanley, M., & others, . (2019). A comparison of transit time distribution vs. fraction of young water to characterize storage in a mountain headwater catchment: does the tail matter?. In AGU Fall Meeting 2019.
• Dziubanski, D., Gallo, E. L., & Meixner, T. (2019). Impact of Green Infrastructure on Hydrologic Responses of a Highly Engineered Commercial Catchment. In AGU Fall Meeting 2019.
• Gupta, N., Meixner, T., Gallo, E. L., Dziubanski, D., & Korgaonkar, Y. (2019). Hydrological Evaluation of Influence of Green Infrastructure on Neighborhood-Scale Semi-Arid Nested Catchments. In AGU Fall Meeting 2019.
• Lowry, C. S., Avellaneda, P., Ficklin, D., Hall, D., Knouft, J., Pastel, R., Ruddell, B. L., Doerry, E., Chester, M., Garcia, M., & others, . (2019). USING CITIZEN SCIENCE AS A CORE TOOL FOR WATER RESOURCE MANAGEMENT AND FORECASTING: CLOSING THE PROFESSIONAL AND CITIZEN SCIENCE GAP. In GSA Annual Meeting in Phoenix, Arizona, USA-2019.
• Meixner, T., Gupta, N., Anderson, J., Swartz, S., & Rockhill, T. (2019). GREEN INFRASTRUCTURE PRESENCE AND MAINTENANCE AFFECTS SOIL HYDROLOGIC AND BIOGEOCHEMICAL PROPERTIES. In GSA Annual Meeting in Phoenix, Arizona, USA-2019.
• Meixner, T., Swartz, S. K., Anderson, J., Gupta, N., & Solis-Arroyo, S. S. (2019). Bioswales modify soil properties, increasing soil organic matter, increase infiltration rates, and maintenance has the potential to decrease these benefits. In AGU Fall Meeting 2019.
• Sanchez, R. A., Meixner, T., Mcintosh, J. C., & Chorover, J. (2019). Influence of wildfire disturbances on solute mobilization through the critical zone. In AGU Fall Meeting 2019.
• White, A., Moravec, B. G., Olshansky, Y., Paras, B., Sanchez, A., Ma, L., McIntosh, J., Ferre, P. A., Meixner, T., & Chorover, J. (2019). WATER ROUTING THROUGH THE CRITICAL ZONE: A HYDROMETRIC, HYDROCHEMICAL, AND ISOTOPIC INVESTIGATION IN NORTHERN NM. In GSA Annual Meeting in Phoenix, Arizona, USA-2019.
• Gupta, N., Meixner, T., Gallo, E. L., Canfield, E., & Spinti, R. (2018). Runoff increases due to urbanization in a semi-arid city. In AGU Fall Meeting Abstracts.
• Gupta, N., Solis-Arroyo, S. S., Meixner, T., Anderson, J., & Gallo, E. L. (2018). Comparative Assessment of Water Quality at the Rainwater-Harvesting Basin and Neighborhood Wash Scales. In AGU Fall Meeting Abstracts.
• Ibsen, P., Jenerette, D., Santelmann, M. V., Greydanus, H., Hondula, D., Wright, M., Swan, C., Borowoy, D., Sukop, M. C., Dell, T., & others, . (2018). Regional Aridity Drives Urban Nighttime Vegetation Derived Air Cooling. In AGU Fall Meeting Abstracts.
• Meixner, T., Rockhill, T., Anderson, J., Gupta, N., Salgado, L., & Swartz, S. K. (2018). Green Stormwater Infrasturcture Function Interactions with Maintenance. In AGU Fall Meeting Abstracts.
• Orr, C. H., O'Reilly, C., Carey, C., Gougis, R., Soule, D. C., Meixner, T., Farrell, K., Klug, J., Richardson, D., Bader, N., & others, . (2018). Environmental Data-Driven Inquiry and Exploration (Project EDDIE): Using Large Datasets to Build Quantitative Literacy. In AGU Fall Meeting Abstracts.
• Smith, B. K., Pillich, J., Cherrier, J., Meixner, T., & Berkowitz, A. R. (2018). Assessing Green Infrastructure Function Across North America with Citizen Scientists and Affordable Sensors. In AGU Fall Meeting Abstracts.
• Chorover, J. D., Barron-Gafford, G. A., Minor, R., Eastoe, C. J., Ferre, P. A., Mcintosh, J. C., Meixner, T., & Dwivedi, R. (2017, December). Hydrologic functioning of the deep Critical Zone and contributions to streamflow in a high elevation catchment: testing of multiple conceptual models. In AGU Fall Meeting.
• Chorover, J. D., Mcintosh, J. C., Meixner, T., & Sanchez, A. (2017, December). Impact of Wildfire on Solute Release in Forested Catchments, Jemez River, New Mexico, USA. In AGU Fall Meeting.
• Meixner, T., Niraula, R., Norman, L., Pivo, G., Gerlak, A., Pavao-Zuckerman, M., & Henry, A. (2015). Semi-Arid Water Resource Challenges-Can Water Harvesting Close the Gap?. In AGU Fall Meeting Abstracts.
• Niraula, R., Meixner, T., Rodell, M., Ajami, H., Gochis, D., & Castro, C. (2015). How Might Recharge Change Under Projected Climate Change in Western US?. In AGU Fall Meeting Abstracts.
• Sanchez, R., & Meixner, T. (2015). Spatial and Temporal influence of Redondo Peak headwaters in the East Fork Jemez River using Principal Component Analysis approach, Valles Caldera, New Mexico. In EGU General Assembly Conference Abstracts, 17.
• Soule, D., Bader, N., Carey, C., Castendyk, D., Fuller, R., Gibson, C., Gougis, R., Klug, J., Meixner, T., Nave, L., & others, . (2015). Project EDDIE: Improving Big Data skills in the classroom. In AGU Fall Meeting Abstracts.

Presentations

• Beal, S., Meixner, T., Brusseau, M. L., Taylor, S., & Dontsova, K. M. (2018, September). Integrative Approach to Quantifying Fate of Munitions Constituents on Training Ranges (ER19-1074). Technical Review Board Meeting. Alexandria, VA: Strategic Environmental Research and Development Program.
• Chorover, J. D., Mcintosh, J. C., Meixner, T., Ferre, P. A., Sanchez, A., Olshansky, Y., Moravec, B., & White, A. (2018, Fall). THE INFLUENCE OF CRITICAL ZONE STRUCTURE ON ITS HYDROLOGIC FUNCTION: INSIGHTS INTO THE STORAGE AND ROUTING OF WATER THROUGH THE CRITICAL ZONE. Geological Society of America Conference. Indianapolis, IN.
• Dwivedi, R., Meixner, T., Mcintosh, J. C., Ferre, P. A., Eastoe, C., Castro, C. L., Wright, W., Niu, G., Minor, R., Knowles, J., Barron-Gafford, G. A., Abramson, N., Mitra, B., Stanley, M., & Chorover, J. D. (2018, Fall). An improved and practical approach for estimating catchment-scale response functions through power spectral analysis. American Geophysical Union Fall Meeting. Washington DC.
• Mclain, J. E., Fitzsimmons, K., Meixner, T., Abrell, L. M., & Lynch, R. (2018, March). Might recycled water solve the problem of toxin-producing algae?. Science of the Environment Earth Day Student Presentation. Tucson, Arizona: University of Arizona Department of Soil, Water and Environmental Science.
• Sanchez, R. A., Meixner, T., Mcintosh, J. C., & Chorover, J. D. (2018, Fall). Impact of wildfire on solute fluxes in forested catchments, Jemez River Basin, New Mexico, USA. American Geophysical Union Fall Meeting. Washington DC.
• Dwivedi, R., Meixner, T., Mcintosh, J. C., Ferre, P. A., & Chorover, J. D. (2016, Fall). A multi-tracer approach coupled to numerical models to improve understanding of mountain block processes in a high elevation, semi-humid catchment. Geological Society of America (GSA) Annual Meeting. Denver, CO.
• Chorover, J. D., Chorover, J. D., Pelletier, J. D., Pelletier, J. D., Breshears, D. D., Breshears, D. D., Mcintosh, J. C., Mcintosh, J. C., Rasmussen, C., Rasmussen, C., Brooks, P. D., Brooks, P. D., Barron-Gafford, G. A., Barron-Gafford, G. A., Gallery, R. E., Gallery, R. E., Ferre, P. A., Ferre, P. A., Meixner, T., , Meixner, T., et al. (2014, September). The Catalina-Jemez CZO: Transformative Behavior of Energy, Water and Carbon in the Critical Zone II. Interactions between Long and Short Term Processes that Control Delivery of Critical Zone Services.. National Critical Zone Observatory All-Hands Meeting.
• Chorover, J. D., Pelletier, J. D., Breshears, D. D., Mcintosh, J. C., Rasmussen, C., Brooks, P. D., Barron-Gafford, G. A., Gallery, R. E., Ferre, P. A., Meixner, T., Niu, Y., Papuga, S. A., Schaap, M. G., & Troch, P. A. (2014, September). The Catalina-Jemez CZO: Transformative Behavior of Energy, Water and Carbon in the Critical Zone II. Interactions between Long and Short Term Processes that Control Delivery of Critical Zone Services.. National Critical Zone Observatory All-Hands Meeting.

Poster Presentations

• Dontsova, K. M., Lewis, J., Arthur, J., Taylor, S., Beal, S., Simunek, J., Brusseau, M. L., Meixner, T., & Kadoya, W. (2019, December). Integrative Approach to Quantifying Fate of Munitions Constituents on Training Ranges.. 2019 SERDP & ESTCP Symposium. Washington, DC: Strategic Environmental Research and Development Program (SERDP).
• Fitzsimmons, K., Meixner, T., Abrell, L. M., & Mclain, J. E. (2018, February). Might recycled water solve the problem of toxin-producing algae?. University of Arizona Water Resources Research Center 2018 Annual Conference. Tucson, Arizona: University of Arizona Water Resources Research Center.
• Meixner, T., Meixner, T., Leavitt, S. W., Leavitt, S. W., Morino, K., & Morino, K. (2016, December). Water sources over time for a semi-arid river- Implications for water resources. AGU Fall Meeting. San Francisco, CA: American Geophysical Union.
• Meixner, T., Leavitt, S. W., & Morino, K. (2016, December). Water sources over time for a semi-arid river- Implications for water resources. AGU Fall Meeting. San Francisco, CA: American Geophysical Union.

Others

• Meixner, T. (2016, February). The Role of Climate Change and the California Drought of 2012–???. American Bar Association Section of Environment, Energy, and Resources Science and Technology Newsletter.
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  Short rewview of infromation about cliate change and groundwater in California in light of the drought.
• Bytnerowicz, A., Fenn, M. E., McNulty, S. G., Yuan, F., Pourmokhtarian, A., Driscoll, C., & Meixner, T. (2013). Interactive effects of air pollution and climate change on forest ecosystems in the United States-current understanding and future scenarios.