Guo-Yue Niu

Interests

Research

Hydrometeorology; ecohydrology; land surface modeling; distributed hydrological modeling; global and regional climate modeling; the role of plant and microbial processes in climate change; application of remote sensing to modeling, etc

Teaching

Niu's teaching interests are hydrometeorology, ecohydrology, physical climatology, and physical hydrology, etc.

Courses

Scholarly Contributions

Chapters

• Volkmann, T. H., Sengupta, A., Pangle, L. A., ... Niu, G. .., & Troch, P. A. (2018). Controlled Experiments of Hillslope Coevolution at the Biosphere 2 Landscape Evolution Observatory: Toward Prediction of Coupled Hydrological, Biogeochemical, and Ecological Change. In Hydrology of Artificial and Controlled Experiments(pp 25-74). doi:http://dx.doi.org/10.5772/intechopen.72325
• Niu, G. (2012). Earth System Model, Modeling the Land Component of. In Climate Change Modeling Methodology(pp 139-168). New York: Springer.

Journals/Publications

• Dwivedi, R., Meixner, T., McIntosh, J. C., Ferre, P., Eastoe, C. J., Niu, G., Minor, R. L., Barron-Gafford, G. A., & Chorover, J. (2019). Hydrologic functioning of the deep critical zone and contributions to streamflow in a high-elevation catchment: Testing of multiple conceptual models. HYDROLOGICAL PROCESSES, 33(4), 476-494.
• Elshall, A. S., Ye, M., Niu, G., & Barron-Gafford, G. A. (2019). Bayesian inference and predictive performance of soil respiration models in the presence of model discrepancy. GEOSCIENTIFIC MODEL DEVELOPMENT, 12(5), 2009-2032.
• Huo, X., Gupta, H., Niu, G., Gong, W., & Duan, Q. (2019). Parameter Sensitivity Analysis for Computationally Intensive Spatially Distributed Dynamical Environmental Systems Models. JOURNAL OF ADVANCES IN MODELING EARTH SYSTEMS, 11(9), 2896-2909.
• Ma, X., Jin, J., Liu, J., & Niu, G. (2019). An improved vegetation emissivity scheme for land surface modeling and its impact on snow cover simulations. CLIMATE DYNAMICS, 53(9-10), 6215-6226.
• Wang, Y., Broxton, P., Fang, Y., Behrangi, A., Barlage, M., Zeng, X., & Niu, G. (2019). A Wet-Bulb Temperature-Based Rain-Snow Partitioning Scheme Improves Snowpack Prediction Over the Drier Western United States. GEOPHYSICAL RESEARCH LETTERS.
• Yuan, R., Chang, L., Gupta, H., & Niu, G. (2019). Climatic forcing for recent significant terrestrial drying and wetting. ADVANCES IN WATER RESOURCES, 133.
• 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.
• Elshall, A. S., Ye, M., Pei, Y., Zhang, F., Niu, G., & Barron-Gafford, G. A. (2018). Relative model score: a scoring rule for evaluating ensemble simulations with application to microbial soil respiration modeling. STOCHASTIC ENVIRONMENTAL RESEARCH AND RISK ASSESSMENT, 32(10), 2809-2819.
• Pelletier, J. D., Barron-Gafford, G. A., Gutierrez-Jurado, H., Hinckley, E. S., Istanbulluoglu, E., McGuire, L. A., Niu, G., Poulos, M. J., Rasmussen, C., Richardson, P., Swetnam, T. L., & Tucker, G. E. (2018). Which way do you lean? Using slope aspect variations to understand Critical Zone processes and feedbacks. EARTH SURFACE PROCESSES AND LANDFORMS, 43(5), 1133-1154.
• Tian, L., Jin, J., Wu, P., & Niu, G. (2018). Assessment of the Effects of Climate Change on Evapotranspiration with an Improved Elasticity Method in a Nonhumid Area. SUSTAINABILITY, 10(12).
• Tian, L., Jin, J., Wu, P., & Niu, G. (2018). Assessment of the Effects of Climate Change on Evapotranspiration with an Improved Elasticity Method in a Nonhumid Area. sustainability, 10(12), 4589. doi:doi:10.3390/su10124589
• Tian, L., Jin, J., Wu, P., & Niu, G. (2018). Quantifying the Impact of Climate Change and Human Activities on Streamflow in a Semi-Arid Watershed with the Budyko Equation Incorporating Dynamic Vegetation Information. WATER, 10(12).
• Tian, L., Jin, J., Wu, P., & Niu, G. (2018). Quantifying the Impact of Climate Change and Human Activities on Streamflow in a Semi-Arid Watershed with the Budyko Equation Incorporating Dynamic Vegetation Information. Water, 10(12), 1781. doi:https://doi.org/10.3390/w10121781
• Wang, P., Niu, G., Fang, Y., Wu, R., Yu, J., Yuan, G., Pozdniakov, S. P., & Scott, R. L. (2018). Implementing Dynamic Root Optimization in Noah-MP for Simulating Phreatophytic Root Water Uptake. WATER RESOURCES RESEARCH, 54(3), 1560-1575.
• van, d., Troch, P. A., Booij, M. J., Niu, G., Volkmann, T., & Pangle, L. A. (2018). Effects of differential hillslope-scale water retention characteristics on rainfall-runoff response at the Landscape Evolution Observatory. HYDROLOGICAL PROCESSES, 32(13), 2118-2127.
• Fang, Y., Zhang, X., Corbari, C., Mancini, M., Niu, G., & Zeng, W. (2017). Improving the Xin'anjiang hydrological model based on mass-energy balance. HYDROLOGY AND EARTH SYSTEM SCIENCES, 21(7), 3359-3375.
• Fang, Y., Zhang, X., Niu, G., Zeng, W., Zhu, J., & Zhang, T. (2017). Study of the Spatiotemporal Characteristics of Meltwater Contribution to the Total Runoff in the Upper Changjiang River Basin. WATER, 9(3).
• Ma, N., Niu, G., Xia, Y., Cai, X., Zhang, Y., Ma, Y., & Fang, Y. (2017). A Systematic Evaluation of Noah-MP in Simulating Land-Atmosphere Energy, Water, and Carbon Exchanges Over the Continental United States. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 122(22), 12245-12268.
• Brunke, M. A., Broxton, P., Pelletier, J., Gochis, D., Hazenberg, P., Lawrence, D. M., Leung, L. R., Niu, G., Troch, P. A., & Zeng, X. (2016). Implementing and Evaluating Variable Soil Thickness in the Community Land Model, Version 4.5 (CLM4.5). JOURNAL OF CLIMATE, 29(9), 3441-3461.
• Ma, N., Szilagyi, J., Niu, G., Zhang, Y., Zhang, T., Wang, B., & Wu, Y. (2016). Evaporation variability of Nam Co Lake in the Tibetan Plateau and its role in recent rapid lake expansion. JOURNAL OF HYDROLOGY, 537, 27-35.
• Niu, G. (2016). Implementing and evaluating variable soil thickness in the Community Land Model version 4.5 (CLM4. 5). Journal of Climate, 29(9), 3441-3461.
• Niu, G. (2016). Multiresponse modeling of an unsaturated zone isotope tracer experiment at the Landscape Evolution Observatory. Hydrol. Earth Syst. Sci.,, 20, 4061–4078. doi:doi:10.5194/hess-20-4061-2016
• Niu, G. (2016). Testing the hybrid‐3‐D hillslope hydrological model in a controlled environment. Water Resources Research, 52, 1089–1107. doi:doi:10.1002/2015WR018106
• Pelletier, J. D., Broxton, P. D., Hazenberg, P., Zeng, X., Troch, P. A., Niu, G., Williams, Z., Brunke, M. A., & Gochis, D. (2016). A gridded global data set of soil, intact regolith, and sedimentary deposit thicknesses for regional and global land surface modeling. JOURNAL OF ADVANCES IN MODELING EARTH SYSTEMS, 8(1), 41-65.
• Scudeler, C., Pangle, L., Pasetto, D., Niu, G., Volkmann, T., Paniconi, C., Putti, M., & Troch, P. (2016). Multiresponse modeling of variably saturated flow and isotope tracer transport for a hillslope experiment at the Landscape Evolution Observatory. HYDROLOGY AND EARTH SYSTEM SCIENCES, 20(10), 4061-4078.
• Barlage, M., Tewari, M., Chen, F., Miguez-Macho, G., Yang, Z., & Niu, G. (2015). The effect of groundwater interaction in North American regional climate simulations with WRF/Noah-MP. CLIMATIC CHANGE, 129(3-4), 485-498.
• 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. A., McIntosh, J. C., Meixner, T., Niu, G., Papuga, S. A., Pelletier, J. D., Rasmussen, C. R., & Troch, P. A. (2015). Critical Zone Services: Expanding Context, Constraints, and Currency beyond Ecosystem Services. VADOSE ZONE JOURNAL, 14(1).
• Niu, G. (2015). A gridded global data set of soil, immobile regolith, and sedimentary deposit thicknesses for regional and global land surface modeling. Journal of Advances in Modeling Earth Systems.
• Pangle, L. A., DeLong, S. B., Abramson, N., Adams, J., Barron-Gafford, G. A., Breshears, D. D., Brooks, P. D., Chorover, J., Dietrich, W. E., Dontsova, K., Durcik, M., Espeleta, J., Ferre, T. P., Ferriere, R., Henderson, W., Hunt, E. A., Huxman, T. E., Millar, D., Murphy, B., , Niu, G., et al. (2015). The Landscape Evolution Observatory: A large-scale controllable infrastructure to study coupled Earth-surface processes. GEOMORPHOLOGY, 244, 190-203.
• Pangle, L. A., Delong, S. B., Abramson, N., Adams, J., Barron-Gafford, G. A., Breshears, D. D., Brooks, P. D., Chorover, J. D., Dietrich, W. E., Dontsova, K. M., Durcik, M., Espleta, J., Ferre, P. A., Ferriere, R. H., Henderson, W., Hunt, E. A., Huxman, T. E., Millar, D., Murphy, B., , Niu, G., et al. (2015). The Landscape Evolution Observatory: A large-scale controllable infrastructure to study Earth-surface processes.. Geomorphology, 244, 190-203.
• Pasetto, D., Niu, G., Pangle, L., Paniconi, C., Putti, M., & Troch, P. A. (2015). Impact of sensor failure on the observability of flow dynamics at the Biosphere 2 LEO hillslopes. ADVANCES IN WATER RESOURCES, 86, 327-339.
• Shao Aimei, ., Qiu Chongjian, ., & Niu Guo-Yue, . (2015). A piecewise modeling approach for climate sensitivity studies: Tests with a shallow-water model. JOURNAL OF METEOROLOGICAL RESEARCH, 29(5), 735-746.
• Cai, X., Yang, Z., David, C. H., Niu, G., & Rodell, M. (2014). Hydrological evaluation of the Noah-MP land surface model for the Mississippi River Basin. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 119(1), 23-38.
  More info

  This study evaluates regional-scale hydrological simulations of the newly developed community Noah land surface model (LSM) with multiparameterization options (Noah-MP). The model is configured for the Mississippi River Basin and driven by the North American Land Data Assimilation System Phase 2 atmospheric forcing at 1/8 degrees resolution. The simulations are compared with various observational data sets, including the U.S. Geological Survey streamflow and groundwater data, the AmeriFlux tower micrometeorological evapotranspiration (ET) measurements, the Soil Climate Analysis Network (SCAN)-observed soil moisture data, and the Gravity Recovery and Climate Experiment satellite-derived terrestrial water storage (TWS) anomaly data. Compared with these observations and to the baseline Noah LSM simulations, Noah-MP shows significant improvement in hydrological modeling for major hydrological variables (runoff, groundwater, ET, soil moisture, and TWS), which is very likely due to the incorporation of some major improvements into Noah-MP, particularly an unconfined aquifer storage layer for groundwater dynamics and an interactive vegetation canopy for dynamic leaf phenology. Noah-MP produces soil moisture values consistent with the SCAN observations for the top two soil layers (0-10cm and 10-40cm), indicating its great potential to be used in studying land-atmosphere coupling. In addition, the simulated groundwater spatial patterns are comparable to observations; however, the inclusion of groundwater in Noah-MP requires a longer spin-up time (34 years for the entire study domain). Runoff simulation is highly sensitive to three parameters: the surface dryness factor (), the saturated hydraulic conductivity (k), and the saturated soil moisture ((max)).
• Chen, F., Barlage, M., Tewari, M., Rasmussen, R., Jin, J., Lettenmaier, D., Livneh, B., Lin, C., Miguez-Macho, G., Niu, G., Wen, L., & Yang, Z. (2014). Modeling seasonal snowpack evolution in the complex terrain and forested Colorado Headwaters region: A model intercomparison study. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 119(24), 13795-13819.
• Fehmi, J. S., Niu, G., Scott, R. L., & Mathias, A. (2014). Evaluating the effect of rainfall variability on vegetation establishment in a semidesert grassland. ENVIRONMENTAL MONITORING AND ASSESSMENT, 186(1), 395-406.
  More info

  Of the operations required for reclamation in arid and semi-arid regions, establishing vegetation entails the most uncertainty due to reliance on unpredictable rainfall for seed germination and seedling establishment. The frequency of successful vegetation establishment was estimated based on a land surface model driven by hourly atmospheric forcing data, 7 years of eddy-flux data, and 31 years of rainfall data at two adjacent sites in southern Arizona, USA. Two scenarios differing in the required imbibition time for successful germination were evaluated-2 or 3 days availability of sufficient surface moisture. Establishment success was assumed to occur if plants could germinate and if the drying front in the soil did not overtake the growth of seminal roots. Based on our results, vegetation establishment could be expected to fail in 32 % of years. In the worst 10-year span, six of ten plantings would have failed. In the best 10-year span, only one of ten was projected to fail. Across all assessments, at most 3 years in a row failed and 6 years in a row were successful. Funding for reclamation seeding must be available to allow reseeding the following year if sufficient amount and timing of rainfall does not occur.
• Niu, G. -., Pasetto, D., Scudeler, C., Paniconi, C., Putti, M., Troch, P. A., DeLong, S. B., Dontsova, K., Pangle, L., Breshears, D. D., Chorover, J., Huxman, T. E., Pelletier, J., Saleska, S. R., & Zeng, X. (2014). Incipient subsurface heterogeneity and its effect on overland flow generation - insight from a modeling study of the first experiment at the Biosphere 2 Landscape Evolution Observatory. HYDROLOGY AND EARTH SYSTEM SCIENCES, 18(5), 1873-1883.
  More info

  Evolution of landscape heterogeneity is controlled by coupled Earth system dynamics, and the resulting process complexity is a major hurdle to cross towards a unified theory of catchment hydrology. The Biosphere 2 Landscape Evolution Observatory (LEO), a 334.5 m(2) artificial hillslope built with homogeneous soil, may have evolved into heterogeneous soil during the first experiment driven by an intense rainfall event. The experiment produced predominantly seepage face water outflow, but also generated overland flow, causing superficial erosion and the formation of a small channel. In this paper, we explore the hypothesis of incipient heterogeneity development in LEO and its effect on overland flow generation by comparing the modeling results from a three-dimensional physically based hydrological model with measurements of total mass change and seepage face flow. Our null hypothesis is that the soil is hydraulically homogeneous, while the alternative hypothesis is that LEO developed downstream heterogeneity from transport of fine sediments driven by saturated subsurface flow. The heterogeneous case is modeled by assigning saturated hydraulic conductivity at the LEO seepage face (K-sat,(sf)) different from that of the rest (K-sat). A range of values for K-sat, K-sat,K-sf, soil porosity, and pore size distribution is used to account for uncertainties in estimating these parameters, resulting in more than 20 000 simulations. It is found that the best runs under the heterogeneous soil hypothesis produce smaller errors than those under the null hypothesis, and that the heterogeneous runs yield a higher probability of best model performance than the homogeneous runs. These results support the alternative hypothesis of localized incipient heterogeneity of the LEO soil, which facilitated generation of overland flow. This modeling study of the first LEO experiment suggests an important role of coupled water and sediment transport processes in the evolution of subsurface heterogeneity and on overland flow generation, highlighting the need of a coupled modeling system that integrates across disciplinary processes.
• Niu, G., Paniconi, C., Troch, P. A., Scott, R. L., Durcik, M., Zeng, X., Huxman, T., & Goodrich, D. C. (2014). An integrated modelling framework of catchment- scale ecohydrological processes: 1. Model description and tests over an energy- limited watershed. ECOHYDROLOGY, 7(2), 427-439.
  More info

  The interactions between atmospheric, hydrological, and ecological processes at various spatial and temporal scales are not fully represented in most ecohydrological models. This first of a two-part paper documents a fully integrated catchment-scale ecohydrological model consisting of a three-dimensional physically based hydrological model and a land surface model. This first part also presents a first application to test the model over an energy-limited catchment (8.4km(2)) of the Sleepers River watershed in Vermont. The physically based hydrological model (CATchment HYdrology, CATHY) describes three-dimensional subsurface flow in variably saturated porous media and surface routing on hillslopes and in stream channels, whereas the land surface model (LSM), an augmented version of Noah LSM with multiple parameterization schemes (NoahMP), accounts for energy, water, and carbon flux exchanges between various land surface elements and the atmosphere. CATHY and NoahMP are coupled through exchanges of water fluxes and states. In the energy-limited catchment of the Sleepers River watershed, where snowmelt runoff generation is the dominant hydrologic flux, the coupled CATHY/NoahMP model at both 90 and 30-m surface grid resolutions, with minimal calibration, performs well in simulating the observed snow accumulation, and melt and subsequent snowmelt discharge. The Nash-Sutcliffe model efficiency of daily discharge is above 0.82 for both resolutions. The simulation at 90-m resolution shows a marginal improvement over that at 30-m resolution because of more elaborate calibration of model parameters. The coupled CATHY/NoahMP also shows a capability of simulating surface-inundated area and distributed surface water height, although the accuracy of these simulations needs further evaluation. The CATHY/NoahMP model is thus also a potentially useful research tool for predicting flash flood and lake dynamics under climatic change. Copyright (c) 2013 John Wiley & Sons, Ltd.
• Niu, G., Troch, P. A., Paniconi, C., Scott, R. L., Durcik, M., Zeng, X., Huxman, T., Goodrich, D., & Pelletier, J. (2014). An integrated modelling framework of catchment- scale ecohydrological processes: 2. The role of water subsidy by overland flow on vegetation dynamics in a semi- arid catchment. ECOHYDROLOGY, 7(2), 815-827.
  More info

  In water-limited regions, surface water and carbon fluxes are strongly controlled by soil water availability, which may be highly variable at very small spatial scales (e.g. metres) because of variations in terrain, soils, and vegetation conditions and to processes of water redistribution along hillslopes. This second of a two-part paper first evaluates the performance of a newly developed ecohydrological model over a small semi-arid experimental catchment (792ha) in southeastern Arizona. Secondly, it investigates the effects of soil properties on water subsidy resulting from lateral overland flow re-infiltration and on overall ecohydrological response. With optimized parameters, the model shows a higher ability to simulate surface energy and water fluxes than CO2 fluxes at all temporal scales. The model simulates observed CO2 fluxes fairly well at diurnal scales during the main growing seasons and the interannual variability of these fluxes in response to soil moisture variations from drought years to wet years. However, the model reproduces less well carbon assimilation in spring and positive CO2 flux pulses following early monsoon rain events, suggesting a need for further development of the model's representations of multiple plant species and soil carbon decomposition. The model simulates soil moisture at 5cm much better than at 15cm mainly because of heterogeneous soil properties. Through five numerical experiments with varying saturated hydraulic conductivity values, it is revealed that the discharge at the outlet of this semi-arid catchment is essentially attributed to lateral overland flow that is generated mainly by infiltration-excess runoff. Subsurface flow plays a minor role in this semi-arid catchment with a very deep groundwater table (>100m). The model produces wetter soils in lowland areas along stream rills and channels through re-infiltration of lateral overland flow. This water subsidy provides plants with favourable conditions to produce more leaves, CO2, and ET fluxes in lowland areas. Re-infiltration of overland flow over complex terrain may play a role in buffering climatic impacts in a warming climate with fewer but more intense rainfall events in the Southwestern United States. Copyright (c) 2013 John Wiley & Sons, Ltd.
• Zhang, X., Niu, G., Elshall, A. S., Ye, M., Barron-Gafford, G. A., & Pavao-Zuckerman, M. (2014). Assessing five evolving microbial enzyme models against field measurements from a semiarid savannah-What are the mechanisms of soil respiration pulses?. GEOPHYSICAL RESEARCH LETTERS, 41(18), 6428-6434.
  More info

  Soil microbial respiration pulses in response to episodic rainfall pulses (the "Birch effect") are poorly understood. We developed and assessed five evolving microbial enzyme models against field measurements from a semiarid savannah characterized by pulsed precipitation to understand the mechanisms to generate the Birch pulses. The five models evolve from an existing four-carbon (C) pool model to models with additional C pools and explicit representations of soil moisture controls on C degradation and microbial uptake rates. Assessing the models using techniques of model selection and model averaging suggests that models with additional C pools for accumulation of degraded C in the dry zone of the soil pore space result in a higher probability of reproducing the observed Birch pulses. Degraded C accumulated in dry soil pores during dry periods becomes immediately accessible to microbes in response to rainstorms, providing a major mechanism to generate respiration pulses. Explicitly representing the transition of degraded C and enzymes between dry and wet soil pores in response to soil moisture changes and soil moisture controls on C degradation and microbial uptake rates improve the models' efficiency and robustness in simulating the Birch effect. Assuming that enzymes in the dry soil pores facilitate degradation of complex C during dry periods (though at a lower rate) results in a greater accumulation of degraded C and thus further improves the models' performance. However, the actual mechanism inducing the greater accumulation of labile C needs further experimental studies.
• Dai, Y., Shangguan, W., Duan, Q., Liu, B., Suhua, F. u., & Niu, G. (2013). Development of a china dataset of soil hydraulic parameters using pedotransfer functions for land surface modeling. Journal of Hydrometeorology, 14(3), 869-887.
  More info

  Abstract: The objective of this study is to develop a dataset of the soil hydraulic parametersassociated with two empirical soil functions (i.e., a water retention curve and hydraulicconductivity) using multiple pedotransfer functions (PTFs). The dataset is designed specifically for regional land surface modeling for China. The authors selected 5 PTFs to derive the parameters in the Clapp and Hornberger functions and the van Genuchten and Mualemfunctions and 10 PTFs for soil water contents at capillary pressures of 33 and 1500 kPa. The inputs into the PTFs include soil particle size distribution, bulk density, and soil organic matter. The dataset provides 12 estimated parameters and their associated statistical values. The dataset is available at a 30 3 30 arc second geographical spatial resolution and with seven vertical layers to the depth of 1.38 m. The dataset has several distinct advantages even though the accuracy is unknown for lack of in situ and regional measurements. First, this dataset utilizes the best available soil characteristics dataset forChina. The Chinese soil characteristics dataset was derived by using the 1:1 000 000 SoilMap of China and 8595 representative soil profiles. Second, this dataset represents the first attempt to estimate soil hydraulic parameters using PTFs directly for continental China at a high spatial resolution. Therefore, this dataset should capture spatial heterogeneity better than existing estimates based on lookup tables according to soil textureclasses. Third, the authors derived soil hydraulic parameters using multiple PTFs to allow flexibility for data users to use the soil hydraulic parameters most preferable to or suitable for their applications. © 2013 American Meteorological Society.
• David, C. H., Maidment, D. R., Niu, G., Yang, Z., Habets, F., & Eijkhout, V. (2011). River Network Routing on the NHDPlus Dataset. JOURNAL OF HYDROMETEOROLOGY, 12(5), 913-934.
• David, C. H., Maidment, D. R., Niu, G., Yang, Z., Habets, F., & Eijkhout, V. (2011). River network routing on the NHDPlus dataset. Journal of Hydrometeorology, 12(5), 913-934.
  More info

  Abstract: The mapped rivers and streams of the contiguous United States are available in a geographic information system (GIS) dataset called National Hydrography Dataset Plus (NHDPlus). This hydrographic dataset has about 3 million river and water body reaches along with information on how they are connected into networks. The U.S. Geological Survey (USGS) National Water Information System (NWIS) provides streamflow observations at about 20 thousand gauges located on the NHDPlus river network. Ariver network model called Routing Application for Parallel Computation of Discharge (RAPID) is developed for the NHDPlus river network whose lateral inflow to the river network is calculated by a land surface model. A matrix-based version of the Muskingum method is developed herein, which RAPID uses to calculate flow and volume of water in all reaches of a river network with many thousands of reaches, including at ungauged locations. Gauges situated across river basins (not only at basin outlets) are used to automatically optimize the Muskingum parameters and to assess river flow computations, hence allowing the diagnosis of runoff computations provided by land surface models. RAPID is applied to the Guadalupe and San Antonio River basins in Texas, where flow wave celerities are estimated at multiple locations using 15-min data and can be reproduced reasonably with RAPID. This river model can be adapted for parallel computing and although the matrix method initially adds a large overhead, river flow results can be obtained faster than with the traditional Muskingum method when using a few processing cores, as demonstrated in a synthetic study using the upper Mississippi River basin. © 2011 American Meteorological Society.
• Hua, S. u., Yang, Z., Niu, G., & Wilson, C. R. (2011). Parameter estimation in ensemble based snow data assimilation: A synthetic study. Advances in Water Resources, 34(3), 407-416.
  More info

  Abstract: Estimating erroneous parameters in ensemble based snow data assimilation system has been given little attention in the literature. Little is known about the related methods' effectiveness, performance, and sensitivity to other error sources such as model structural error. This research tackles these questions by running synthetic one-dimensional snow data assimilation with the ensemble Kalman filter (EnKF), in which both state and parameter are simultaneously updated. The first part of the paper investigates the effectiveness of this parameter estimation approach in a perfect-model-structure scenario, and the second part focuses on its dependence on model structure error. The results from first part research demonstrate the advantages of this parameter estimation approach in reducing the systematic error of snow water equivalent (SWE) estimates, and retrieving the correct parameter value. The second part results indicate that, at least in our experiment, there is an evident dependence of parameter search convergence on model structural error. In the imperfect-model-structure run, the parameter search diverges, although it can simulate the state variable well. This result suggest that, good data assimilation performance in estimating state variables is not a sufficient indicator of reliable parameter retrieval in the presence of model structural error. The generality of this conclusion needs to be tested by data assimilation experiments with more complex structural error configurations. © 2010 Elsevier Ltd.
• MingXing, L., ZhuGuo, M., & Guo-Yue, N. (2011). Modeling spatial and temporal variations in soil moisture in China. CHINESE SCIENCE BULLETIN, 56(17), 1809-1820.
• Niu, G., Yang, Z., Mitchell, K. E., Chen, F., Ek, M. B., Barlage, M., Kumar, A., Manning, K., Niyogi, D., Rosero, E., Tewari, M., & Xia, Y. (2011). The community Noah land surface model with multiparameterization options (Noah-MP): 1. Model description and evaluation with local-scale measurements. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 116.
• Niu, G., Yang, Z., Mitchell, K. E., Chen, F., Ek, M. B., Barlage, M., Kumar, A., Manning, K., Niyogi, D., Rosero, E., Tewari, M., & Xia, Y. (2011). The community Noah land surface model with multiparameterization options (Noah-MP): 1. Model description and evaluation with local-scale measurements. Journal of Geophysical Research D: Atmospheres, 116(12).
  More info

  Abstract: This first paper of the two-part series describes the objectives of the community efforts in improving the Noah land surface model (LSM), documents, through mathematical formulations, the augmented conceptual realism in biophysical and hydrological processes, and introduces a framework for multiple options to parameterize selected processes (Noah-MP). The Noah-MP's performance is evaluated at various local sites using high temporal frequency data sets, and results show the advantages of using multiple optional schemes to interpret the differences in modeling simulations. The second paper focuses on ensemble evaluations with long-term regional (basin) and global scale data sets. The enhanced conceptual realism includes (1) the vegetation canopy energy balance, (2) the layered snowpack, (3) frozen soil and infiltration, (4) soil moisture-groundwater interaction and related runoff production, and (5) vegetation phenology. Sample local-scale validations are conducted over the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) site, the W3 catchment of Sleepers River, Vermont, and a French snow observation site. Noah-MP shows apparent improvements in reproducing surface fluxes, skin temperature over dry periods, snow water equivalent (SWE), snow depth, and runoff over Noah LSM version 3.0. Noah-MP improves the SWE simulations due to more accurate simulations of the diurnal variations of the snow skin temperature, which is critical for computing available energy for melting. Noah-MP also improves the simulation of runoff peaks and timing by introducing a more permeable frozen soil and more accurate simulation of snowmelt. We also demonstrate that Noah-MP is an effective research tool by which modeling results for a given process can be interpreted through multiple optional parameterization schemes in the same model framework. Copyright © 2011 by the American Geophysical Union.
• Rosero, E., Gulden, L. E., Yang, Z., De, G., Niu, G., & Kaheil, Y. H. (2011). Ensemble Evaluation of Hydrologically Enhanced Noah-LSM: Partitioning of the Water Balance in High-Resolution Simulations over the Little Washita River Experimental Watershed. JOURNAL OF HYDROMETEOROLOGY, 12(1), 45-64.
• Rosero, E., Gulden, L. E., Yang, Z., G., L., Niu, G., & Kaheil, Y. H. (2011). Ensemble evaluation of hydrologically enhanced Noah-LSM: Partitioning of the water balance in high-resolution simulations over the little washita river experimental watershed. Journal of Hydrometeorology, 12(1), 45-64.
  More info

  Abstract: The ability of two versions of the Noah land surface model (LSM) to simulate the water cycle of the Little Washita River experimental watershed is evaluated. One version that uses the standard hydrological parameterizations of Noah 2.7 (STD) is compared another version that replaces STD's subsurface hydrology with a simple aquifer model and topography-related surface and subsurface runoff parameterizations (GW). Simulations on a distributed grid at fine resolution are compared to the long-term distribution of observed dailymean runoff, the spatial statistics of observed soilmoisture, and locally observed latent heat flux. The evaluation targets the typical behavior of ensembles ofmodels that use realistic, near-optimal sets of parameters important to runoff. STD and GW overestimate the ratio of runoff to evapotranspiration. In the subset of STD and GW runs that best reproduce the timing and the volume of streamflow, the surface-to-subsurface runoff ratio is overestimated and simulated streamflow is much flashier than observations. Both models' soil columns wet and dry too quickly, implying that there are structural shortcomings in the formulation of STD that cannot be overcome by adding GW's increased complexity to the model. In its current formulation, GW extremely underestimates baseflow's contribution to total runoff and requires a shallow water table to function realistically. In the catchment (depth to water table .10 m), GW functions as a simple bucket model. Because model parameters are likely scale and site dependent, the need for even "physically based" models to be extensively calibrated for all domains on which they are applied is underscored. © 2011 American Meteorological Society.
• Su, H., Yang, Z., Niu, G., & Wilson, C. R. (2011). Parameter estimation in ensemble based snow data assimilation: A synthetic study. ADVANCES IN WATER RESOURCES, 34(3), 407-416.
• Yang, Z., Niu, G., Mitchell, K. E., Chen, F., Ek, M. B., Barlage, M., Longuevergne, L., Manning, K., Niyogi, D., Tewari, M., & Xia, Y. (2011). The community Noah land surface model with multiparameterization options (Noah-MP): 2. Evaluation over global river basins. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 116.
• Yang, Z., Niu, G., Mitchell, K. E., Chen, F., Ek, M. B., Barlage, M., Longuevergne, L., Manning, K., Niyogi, D., Tewari, M., & Xia, Y. (2011). The community Noah land surface model with multiparameterization options (Noah-MP): 2. Evaluation over global river basins. Journal of Geophysical Research D: Atmospheres, 116(12).
  More info

  Abstract: The augmented Noah land surface model described in the first part of the two-part series was evaluated here over global river basins. Across various climate zones, global-scale tests can reveal a model's weaknesses and strengths that a local-scale testing cannot. In addition, global-scale tests are more challenging than local- and catchment-scale tests. Given constant model parameters (e. g., runoff parameters) across global river basins, global-scale tests are more stringent. We assessed model performance against various satellite and ground-based observations over global river basins through six experiments that mimic a transition from the original Noah LSM to the fully augmented version. The model shows transitional improvements in modeling runoff, soil moisture, snow, and skin temperature, despite considerable increase in computational time by the fully augmented Noah-MP version compared to the original Noah LSM. The dynamic vegetation model favorably captures seasonal and spatial variability of leaf area index and green vegetation fraction. We also conducted 36 ensemble experiments with 36 combinations of optional schemes for runoff, leaf dynamics, stomatal resistance, and the β factor. Runoff schemes play a dominant and different role in controlling soil moisture and its relationship with evapotranspiration compared to ecological processes such as β the factor, vegetation dynamics, and stomatal resistance. The 36-member ensemble mean of runoff performs better than any single member over the world's 50 largest river basins, suggesting a great potential of land-based ensemble simulations for climate prediction. Copyright © 2011 by the American Geophysical Union.
• Hua, S. u., Yang, Z., Dickinson, R. E., Wilson, C. R., & Niu, G. (2010). Multisensor snow data assimilation at the continental scale: The value of Gravity Recovery and Climate Experiment terrestrial water storage information. Journal of Geophysical Research D: Atmospheres, 115(10).
  More info

  Abstract: This investigation establishes a multisensor snow data assimilation system over North America (from January 2002 to June 2007), toward the goal of better estimation of snowpack (in particular, snow water equivalent and snow depth) via incorporating both Gravity Recovery and Climate Experiment (GRACE) terrestrial water storage (TWS) and Moderate Resolution Imaging Spectroradiometer (MODIS) snow cover fraction (SCF) information into the Community Land Model. The different properties associated with the SCF and TWS observations are accommodated through a unified approach using the ensemble Kalman filter and smoother. Results show that this multisensor approach can provide significant improvements over a MODIS-only approach, for example, in the Saint Lawrence, Fraser, Mackenzie, Churchill & Nelson, and Yukon river basins and the southwestern rim of Hudson Bay. At middle latitudes, for example, the North Central and Missouri river basins, the inclusion of GRACE information preserves the advantages (compared with the open loop) shown in the MODIS-only run. However, in some high-latitude areas and given months the open loop run shows a comparable or even better performance, implying considerable room for refinements of the multisensor algorithm. In addition, ensemble-based metrics are calculated and interpreted domainwide. They indicate the potential importance of accurate representation of snow water equivalent autocovariance in assimilating TWS observations and the regional and/or seasonal dependence of GRACEs capability to reduce ensemble variance. These analyses contribute to clarifying the effects of GRACEs special features (e.g., a vertical integral of different land water changes, coarse spatial and temporal resolution) in the snow data assimilation system. Copyright 2010 by the American Geophysical Union.
• Rosero, E., Yang, Z., Wagener, T., Gulden, L. E., Yatheendradas, S., & Niu, G. (2010). Quantifying parameter sensitivity, interaction, and transferability in hydrologically enhanced versions of the Noah land surface model over transition zones during the warm season. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 115.
• Su, H., Yang, Z. -., Dickinson, R. E., Wilson, C. R., & Niu, G. (2010). Multisensor snow data assimilation at the continental scale: The value of Gravity Recovery and Climate Experiment terrestrial water storage information. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 115.
• Bin, Y., Li-Liang, R., Chen, X. i., Yun, Z., Wan-Chang, Z., Cong-Bin, F., & Guo-Yue, N. (2009). Development of a large-scale hydrological model TOPX and its coupling with regional integrated environment modeling system RIEMS. CHINESE JOURNAL OF GEOPHYSICS-CHINESE EDITION, 52(8), 1954-1965.
• Chen, J. L., Wilson, C. R., Tapley, B. D., Yang, Z. L., & Niu, G. Y. (2009). 2005 drought event in the Amazon River basin as measured by GRACE and estimated by climate models. Journal of Geophysical Research B: Solid Earth, 114(5), B05404.
  More info

  Abstract: [1] Satellite gravity measurements from the Gravity Recovery and Climate Experiment (GRACE) provide new quantitative measures of the 2005 extreme drought event in the Amazon river basin, regarded as the worst in over a century. GRACE measures a significant decrease in terrestrial water storage (TWS) in the central Amazon basin in the summer of 2005, relative to the average of the 5 other summer periods in the GRACE era. In contrast, data-assimilating climate and land surface models significantly underestimate the drought intensity. GRACE measurements are consistent with accumulated precipitation data from satellite remote sensing and are also supported by in situ water-level data from river gauge stations. This study demonstrates the unique potential of satellite gravity measurements in monitoring large-scale severe drought and flooding events and in evaluating advanced climate and land surface models. Copyright 2009 by the American Geophysical Union.
• Jiang, X., Niu, G., & Yang, Z. (2009). Impacts of vegetation and groundwater dynamics on warm season precipitation over the Central United States. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 114.
• Jiang, X., Niu, G., & Yang, Z. (2009). Impacts of vegetation and groundwater dynamics on warm season precipitation over the Central United States. Journal of Geophysical Research D: Atmospheres, 114(6), D06109.
  More info

  Abstract: We investigated the impacts of vegetation and groundwater dynamics on warm season precipitation by using the Weather Research and Forecasting (WRF) model coupled with a modified Noah land surface model (LSM). The modified Noah LSM was augmented with an interactive canopy model and a simple. groundwater model (SIMGM). A series of experiments performed shows that incorporating vegetation and groundwater dynamics into the WRF model can improve the simulation of summer precipitation in the Central United States. The enhanced model produces more precipitation in response to an increase in the latent heat flux. The advantage of incorporating the two components into the model becomes more discernable after 1 month. The model results suggest that the land-atmosphere feedback is an important mechanism for summer precipitation over the Central United States. Vegetation growth and groundwater dynamics play a significant role in enhancing the persistence of intraseasonal precipitation in regional climate models. Their combined effects act to favor a stronger land-atmosphere feedback during the summer season. The simulated diurnal cycle of precipitation is improved by the WRF model with the augmented Noah LSM. Moreover, we found that the coupling between the soil moisture and the lifting condensation level (LCL) is enhanced by adding the two components to the WRF model. The impact of groundwater is significant when the soil moisture is relatively dry. This study suggests that incorporating vegetation and groundwater dynamics into a regional climate model would be especially beneficial for seasonal precipitation forecast in the transition zones. Copyright 2009 by the American Geophysical Union.
• Rosero, E., Yang, Z., Gulden, L. E., Niu, G., & Gochis, D. J. (2009). Evaluating Enhanced Hydrological Representations in Noah LSM over Transition Zones: Implications for Model Development. JOURNAL OF HYDROMETEOROLOGY, 10(3), 600-622.
• Rosero, E., Yang, Z., Gulden, L. E., Niu, G., & Gochis, D. J. (2009). Evaluating enhanced hydrological representations in Noah LSM over transition zones: Implications for model development. Journal of Hydrometeorology, 10(3), 600-622.
  More info

  Abstract: The authors introduce and compare the performance of the unified Noah land surface model (LSM) and its augments with physically based, more conceptually realistic hydrologic parameterizations. Forty-five days of 30-min data collected over nine sites in transition zones are used to evaluate (i) their benchmark, the standard Noah LSM release 2.7 (STD); (ii) a version equipped with a short-term phenology module (DV); and (iii) one that couples a lumped, unconfined aquifer model to the model soil column (GW). Their model intercomparison, enhanced by multiobjective calibration and model sensitivity analysis, shows that, under the evaluation conditions, the current set of enhancements to Noah fails to yield significant improvement in the accuracy of simulated, high-frequency, warm-season turbulent fluxes, and near-surface states across these sites. Qualitatively, the versions of DV and GW implemented degrade model robustness, as defined by the sensitivity of model performance to uncertain parameters. Quantitatively, calibrated DV and GW show only slight improvement in the skill of the model over calibrated STD. Then, multiple model realizations are compared to explicitly account for parameter uncertainty. Model performance, robustness, and fitness are quantified for use across varied sites. The authors show that the least complex benchmark LSM (STD) remains as the most fit version of the model for broad application. Although GWtypically performs best when simulating evaporative fraction (EF), 24-h change in soil wetness (ΔW30), and soil wetness, it is only about half as robust as STD, which also performs relatively well for all three criteria. GW's superior performance results from bias correction, not from improved soil moisture dynamics. DV performs better than STD in simulating EF and ΔW30 at the wettest site, because DV tends to enhance transpiration and canopy evaporation at the expense of direct soil evaporation. This same model structure limits performance at the driest site, where STD performs best. This dichotomous performance suggests that the formulations that determine the partitioning of LE flux need to be modified for broader applicability. Thus, this work poses a caveat for simple "plug and play" of functional modules between LSMs and showcases the utility of rigorous testing during model development. © 2009 American Meteorological Society.
• Yong, B., Ren, L., Chen, X., Zhang, Y., Zhang, W., Fu, C., & Niu, G. (2009). Development of a large-scale hydrological model TOPX and its coupling with regional integrated environment modeling system RIEMS. Chinese Journal of Geophysics (Acta Geophysica Sinica), 52(8), 1954-1965.
  More info

  Abstract: On the basis of the improved SIMTOP runoff parameterization scheme and the calculating method of three-layer soil moisture balance in Xinanjiang model, this paper developed a simple but highly-efficient large-scale hydrological model (TOPX), which can provide the function of scaling transformation on topographic index. Although the TOPX model has less data input and minimum parameters for calibrating, it can better describe the two-dimensional hydrological processes. Then TOPX was coupled with the Regional Integrated Environment Modeling System (RIEMS) to enforce its ability of numerical simulation for the runoff in large-scale watershed. The results of the offline test performed at Youshui River catchment indicate that the TOPX model produced better simulation effect of daily runoff in small sized catchments and it can describe the various hydrological processes of watershed. The offline test of Jinghe basin shows that TOPX model has the better ability of distributed simulation at large-scale and it can capture the major characteristics of land-surface hydrological processes at regional-scale. Then the coupling model of RIEMS and TOPX was still online tested in Jinghe basin. By means of the scale transformation scheme on topographic index and the yielding and runoff routing theory, the coupling model uses the meteorological data simulated by a regional climate model to drive the hydrological model for predicting the daily runoff at large-scale watershed. A further analysis revealed that the accuracy of the distributed rainfall data simulated by the regional climate model (RIEMS) is the crucial factor to affect the modeled runoff in the coupling model (RIEMS-TOPX).
• Yong, B., Zhang, W., Niu, G., Ren, L., & Qin, C. (2009). Spatial statistical properties and scale transform analyses on the topographic index derived from DEMs in China. COMPUTERS & GEOSCIENCES, 35(3), 592-602.
• Yong, B., Zhang, W., Niu, G., Ren, L., & Qin, C. (2009). Spatial statistical properties and scale transform analyses on the topographic index derived from DEMs in China. Computers and Geosciences, 35(3), 592-602.
  More info

  Abstract: The topographic index (TI), frequently used in approximately characterizing the spatial distribution of variable source areas within a watershed, has been widely applied in topography-based land-surface process schemes coupled in regional or global climatic models. The TI concept, however, was originally developed for studying hill-slope scale hydrological processes and was most commonly used in simulations from small- to medium-sized watersheds. It is still questionable whether the TI computed from coarse-resolution digital elevation models (DEMs) for large-scale hydrology and climate studies can effectively reflect the spatial distribution of soil moisture, surface saturation, and runoff generation processes in most areas. In this study, we first proposed an improved multiple flow direction algorithm (IMFD) for accurately computing the TI distribution. We then evaluated the IMFD algorithm quantitatively by using four types of artificial mathematical surfaces. Subsequently, we conducted statistical analyses on the TI distributions computed with IMFD from 90×90 m2 and 1000×1000 m2 resolution DEM blocks sampled from across the whole of China. We found there are linear relationships between the mean TI values computed from the two different resolution DEMs in three sampled blocks of different sizes, i.e., 0.1°×0.1°, 0.5°×0.5° and 1°×1°. Systematic analyses further suggested that the forms of these linear relationships are evidently affected by the algorithm used for the TI computation, while the size, location, and number of the selected TI samples have minor effects on them. Finally, we investigated the influence of DEM resolution on the spatial statistical properties of TI. From the viewpoint of terrain discretization and smoothing effects, we also discussed the mechanism and the reasons causing the similarity on TI at different spatial resolutions. © 2008 Elsevier Ltd. All rights reserved.
• Zhang, X., Wang, X., Yang, Z., Niu, G., & Xie, Z. (2009). Simulations of seasonal variations of stable water isotopes in land surface process model CLM. Chinese Science Bulletin, 54(10), 1765-1772.
  More info

  Abstract: In this study, we simulated and analyzed the monthly variations of stable water isotopes in different reservoirs at Manaus, Brazil, using the Community Land Model (CLM) that incorporates stable isotopic effects as a diagnostic tool for understanding stable water isotopic processes, filling the observational data gaps and predicting hydrometeorological processes. The simulation results show that the δ18O values in precipitation, vapor and surface runoff have distinct seasonality with the marked negative correlations with corresponding water amount. Compared with the survey results by the International Atomic Energy Agency (IAEA) in co-operation with the World Meteorological Organization (WMO), the simulations by CLM reveal the similar temporal distributions of the δ18O in precipitation. Moreover, the simulated amount effect between monthly δ18O and monthly precipitation amount, and MWL (meteoric water line) are all close to the measured values. However, the simulated seasonal difference in the δ18O in precipitation is distinctly smaller than observed one, and the simulated temporal distribution of the δ18O in precipitation displays the ideal bimodal seasonality rather than the observed single one. These mismatches are possibly related to the simulation capacity and the veracity in forcing data. © 2009 Science in China Press and Springer-Verlag GmbH.
• Zhang, X., Yang, Z., Niu, G., & Wang, X. (2009). Stable water isotope simulation in different reservoirs of Manaus, Brazil, by Community Land Model incorporating stable isotopic effect. INTERNATIONAL JOURNAL OF CLIMATOLOGY, 29(5), 619-628.
• Zhang, X., Yang, Z., Niu, G., & Wang, X. (2009). Stable water isotope simulation in different reservoirs of Manaus, Brazil, by community land model incorporating stable isotopic effect. International Journal of Climatology, 29(5), 619-628.
  More info

  Abstract: The daily and monthly variations of stable water isotopes in different reservoirs at Manaus, Brazil, are simulated and inter-compared in an equilibrium year, using the Community Land Model (CLM) involving the stable isotopic effects as a diagnostic tool for an in-depth understanding of the hydrometeorological processes. On the daily scale, the δ18O in precipitation, vapour and surface runoff have clear seasonality, with marked negative correlations with the corresponding water amount. However, the δ18O in surface dew displays marked positive correlation with dew amount. On the diurnal time scale, the δ18O in precipitation displays an unclear diurnal variation and an unmarked correlation with the precipitation amount. However, the δ18O in vapour keeps consistency with specific humidity. On the monthly time scale, the δ18O in precipitation and surface runoff displays distinct bimodal seasonality, with two maxima in January and in July, and two minima in April and in October; Vapor displays a similar bimodal pattern, two maxima appear in January and August, and two minima in April and November. The amount effect simulated on the monthly time scale has consistency with the actual survey result at the Manaus station, from 1965 to 1990, set up by International Atomic Energy Agency (IAEA)/World Meteorological Organization (WMO). In addition, the slope (7.49) and the intercept (6.25) of the simulated meteoric water line (MWL) are all smaller than those of the actual mean MWL. However, compared with the annual MWL, the simulated MWL lies within the variation range of actual MWLs. Copyright © 2008 Royal Meteorological Society.
• Gulden, L. E., Rosero, E., Yang, Z., Wagener, T., & Niu, G. (2008). Model performance, model robustness, and model fitness scores: A new method for identifying good land-surface models. Geophysical Research Letters, 35(11).
  More info

  Abstract: We introduce three metrics for rigorous evaluation of land-surface models (LSMs). This framework explicitly acknowledges perennial sources of uncertainty in LSM output. The model performance score (ζ) quantifies the likelihood that a representative model ensemble will bracket most observations and be highly skilled with low spread. The robustness score (ρ) quantifies the sensitivity of performance to parameter and/or data error. The fitness score (φ) combines performance and robustness, ranking models' suitability for broad application. We demonstrate the use of the metrics by comparing three versions of the Noah LSM. Using time-varying ζ for hypothesis testing and model development, we show that representing short-term phenological change improves Noah's simulation of surface energy partitioning and subsurface water dynamics at a semi-humid site. The least complex version of Noah is most fit for broad application. The framework and metrics presented here can significantly improve the confidence that can be placed in LSM predictions. Copyright 2008 by the American Geophysical Union.
• Gulden, L. E., Rosero, E., Yang, Z., Wagener, T., & Niu, G. (2008). Model performance, model robustness, and model fitness scores: A new method for identifying good land-surface. GEOPHYSICAL RESEARCH LETTERS, 35(11).
• Gulden, L. E., Yang, Z., & Niu, G. (2008). Sensitivity of biogenic emissions simulated by a land-surface model to land-cover representations. ATMOSPHERIC ENVIRONMENT, 42(18), 4185-4197.
• Gulden, L. E., Yang, Z., & Niu, G. (2008). Sensitivity of biogenic emissions simulated by a land-surface model to land-cover representations. Atmospheric Environment, 42(18), 4185-4197.
  More info

  Abstract: We evaluate the sensitivity of biogenic emissions simulated by a land-surface model (LSM) to different representations of land-cover vegetation. We drive the community land model on a 0.1° grid over Texas, USA, from 1993 to 1998 using bilinearly interpolated North American Regional Reanalysis data. Two land-cover datasets provide the starting point for analysis: (1) a satellite-derived vegetation and soil-color database and (2) a vegetation-distribution dataset derived from ground surveys. These datasets help us to qualitatively characterize the uncertainty in land-cover representations. We systematically vary the datasets to examine the sensitivity of modeled emissions to variation in representation of bare-soil fraction, vegetation-type distribution, and phenology. Different datasets' representation of vegetation-type distribution leads to simulated mean statewide total biogenic emissions that vary by a factor of 3. Variation in specified bare-soil fraction causes simulated statewide average emissions that vary by a factor of 1.7. Scaling leaf area index values within reasonable bounds causes a near-linear change in simulated emissions. Differences in simulated values are the largest for major metropolitan regions and for eastern and central Texas, where biogenic emissions are the highest and where tropospheric ozone pollution is a significant concern. Changing bare-soil fraction alters simulated vegetation temperature and consequently indirectly affects modeled emissions (≤16% of inherent emissions capacity). Our estimates of the model sensitivity to land-cover representation are consistent with those for other regions. © 2008 Elsevier Ltd. All rights reserved.
• Hua, S. u., Yang, Z., Niu, G., & Dickinson, R. E. (2008). Enhancing the estimation of continental-scale snow water equivalent by assimilating MODIS snow cover with the ensemble Kalman filter. Journal of Geophysical Research D: Atmospheres, 113(8).
  More info

  Abstract: High-quality continental-scale snow water equivalent (SWE) data sets are generally not available, although they are important for climate research and water resources management. This study investigates the feasibility of a framework for developing such needed data sets over North America, through the ensemble Kalman filter (EnKF) approach, which assimilates the snow cover fraction observed by the Moderate Resolution Imaging Spectroradiometer (MODIS) into the Community Land Model (CLM). We use meteorological forcing from the Global Land Data Assimilation System (GLDAS) to drive the CLM and apply a snow density-based observation operator. This new operator is able to fit the observed seasonally varying relationship between the snow cover fraction and the snow depth. Surface measurements from Canada and the Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) estimates (in particular regions) are used to evaluate the assimilation results. The filter performance, including its ensemble statistics in different landscapes and climatic zones, is interpreted. Compared to the open loop, the EnKF method more accurately simulates the seasonal variability of SWE and reduces the uncertainties in the ensemble spread. Different simulations are also compared with spatially distributed climatological statistics from a re-gridded data set, which shows that the SWE estimates from the EnKF are most improved in the mountainous west, the northern Great Plains, and the west and east coast regions. Limitations of the assimilation system are analyzed, and the domain-wide innovation mean and normalized innovation variance are assessed, yielding valuable insights (e.g., about the misrepresentation of filter parameters) as to implementing the EnKF method for large-scale snow properties estimation. Copyright 2008 by the American Geophysical Union.
• Oleson, K. W., Niu, G. -., Yang, Z. -., Lawrence, D. M., Thornton, P. E., Lawrence, P. J., Stöckli, R., Dickinson, R. E., Bonan, G. B., Levis, S., Dai, A., & Qian, T. (2008). Improvements to the community land model and their impact on the hydrological cycle. Journal of Geophysical Research G: Biogeosciences, 113(1).
  More info

  Abstract: The Community Land Model version 3 (CLM3) is the land component of the Community Climate System Model (CCSM). CLM3 has energy and water biases resulting from deficiencies in some of its canopy and soil parameterizations related to hydrological processes. Recent research by the community that utilizes CLM3 and the family of CCSM models has indicated several promising approaches to alleviating these biases. This paper describes the implementation of a selected set of these parameterizations and their effects on the simulated hydrological cycle. The modifications consist of surface data sets based on Moderate Resolution Imaging Spectroradiometer products, new parameterizations for canopy integration, canopy interception, frozen soil, soil water availability, and soil evaporation, a TOPMODEL-based model for surface and subsurface runoff, a groundwater model for determining water table depth, and the introduction of a factor to simulate nitrogen limitation on plant productivity. The results from a set of offline simulations were compared with observed data for runoff, river discharge, soil moisture, and total water storage to assess the performance of the new model (referred to as CLM3.5). CLM3.5 exhibits significant improvements in its partitioning of global evapotranspiration (ET) which result in wetter soils, less plant water stress, increased transpiration and photosynthesis, and an improved annual cycle of total water storage. Phase and amplitude of the runoff annual cycle is generally improved. Dramatic improvements in vegetation biogeography result when CLM3.5 is coupled to a dynamic global vegetation model. Lower than observed soil moisture variability in the rooting zone is noted as a remaining deficiency. © 2008 by the American Geophysical Union.
• Stöckli, R., Lawrence, D. M., Niu, G. -., Oleson, K. W., Thornton, P. E., Yang, Z. -., Bonan, G. B., Denning, A. S., & Running, S. W. (2008). Use of FLUXNEt in the community land model development. Journal of Geophysical Research G: Biogeosciences, 113(1).
  More info

  Abstract: The Community Land Model version 3 (CLM3.0) simulates land-atmosphere exchanges in response to climatic forcings. CLM3.0 has known biases in the surface energy partitioning as a result of deficiencies in its hydrological and biophysical parameterizations. Such models, however, need to be robust for multidecadal global climate simulations. FLUXNET now provides an extensive data source of carbon, water and energy exchanges for investigating land processes, and it encompasses a global range of ecosystem-climate interactions. Data from 15 FLUXNET sites are used to identify and improve model deficiencies. Including a prognostic aquifer, a bare soil evaporation resistance formulation and numerous other changes in the model result in a significantly improved soil hydrology and energy partitioning. Terrestrial water storage increased by up to 300 mm in warni climates and decreased in cold climates. Nitrogen control of photosynthesis is revealed as another missing process in the model. These improvements increase the correlation coefficient of hourly and monthly latent heat fluxes from a range of 0.5-0.6 to the range of 0.7-0.9. RMSE of the simulated sensible heat fluxes decrease by 20-50%. Primary production is overestimated during the wet season in mediterranean and tropical ecosystems. This might be related to missing carbon-nitrogen dynamics as well as to site-specific parameters. The new model (CLM3.5) with an improved terrestrial water cycle should lead to more realistic land-atmosphere exchanges in coupled simulations. FLUXNET is found to be a valuable tool to develop and validate land surface models prior to their application in computationally expensive global simulations. Copyright 2008 by the American Geophysical Union.
• Su, H., Yang, Z., Niu, G., & Dickinson, R. E. (2008). Enhancing the estimation of continental-scale snow water equivalent by assimilating MODIS snow cover with the ensemble Kalman filter. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 113(D8).
• Gulden, L. E., Rosero, E., Yang, Z., Rodell, M., Jackson, C. S., Niu, G., Yeh, P. J., & Famiglietti, J. (2007). Improving land-surface model hydrology: Is an explicit aquifer model better than a deeper soil profile?. Geophysical Research Letters, 34(9).
  More info

  Abstract: We use Monte Carlo analysis to show that explicit representation of an aquifer within a land-surface model (LSM) decreases the dependence of model performance on accurate selection of subsurface hydrologic parameters. Within the National Center for Atmospheric Research Community Land Model (CLM) we evaluate three parameterizations of vertical water flow: (1) a shallow soil profile that is characteristic of standard LSMs; (2) an extended soil profile that allows for greater variation in terrestrial water storage; and (3) a lumped, unconfined aquifer model coupled to the shallow soil profile. North American Land Data Assimilation System meteorological forcing data (1997-2005) drive the models as a single column representing Illinois, USA. The three versions of CLM are each run 22,500 times using a random sample of the parameter space for soil texture and key hydrologic parameters. Other parameters remain constant. Observation-based monthly changes in state-averaged terrestrial water storage (dTWS) are used to evaluate the model simulations. After single-criteria parameter exploration, the schemes are equivalently adept at simulating dTWS. However, explicit representation of groundwater considerably decreases the sensitivity of modeled dTWS to errant parameter choices. We show that approximate knowledge of parameter values is not sufficient to guarantee realistic model performance: because interaction among parameters is significant, they must be prescribed as a congruent set. Copyright 2007 by the American Geophysical Union.
• Gulden, L. E., Rosero, E., Yang, Z., Rodell, M., Jackson, C. S., Niu, G., Yeh, P., & Famiglietti, J. (2007). Improving land-surface model hydrology: Is an explicit aquifer model better than a deeper soil profile?. GEOPHYSICAL RESEARCH LETTERS, 34(9).
• Gulden, L. E., Yang, Z., & Niu, G. (2007). Interannual variation in biogenic emissions on a regional scale. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 112(D14).
• Gulden, L. E., Yang, Z., & Niu, G. (2007). Interannual variation in biogenic emissions on a regional scale. Journal of Geophysical Research D: Atmospheres, 112(14).
  More info

  Abstract: Interannual variation in biogenic emissions is not well quantified, especially on regional scales. We use a land surface model augmented with a short-term dynamic phenology scheme to estimate the interannual variation in the emission of biogenic volatile organic compounds (BVOCs) between 1982 and 2004. We use North American Regional Reanalysis data to drive two versions of the National Center for Atmospheric Research Community Land Model (CLM) on a 0.1° grid over eastern Texas. The first version is the standard CLM with prescribed leaf area index (LAI) (i.e., LAI varies seasonally but not interannually); the second version is the standard CLM augmented with a dynamic phenology scheme (CLM-DP) that allows LAI to respond to environmental variation. We calibrate CLM-DP using satellite-derived LAI as our visual constraint. When phenology is prescribed, the domain-mean (domain-maximum) average absolute departure from the monthly mean BVOC flux is 11.7% (70.6%); when phenology is allowed to vary with environmental conditions, it is 22.4% (137.7%). The domain-mean (domain-maximum) average absolute departure from the monthly mean flux is lower during summer: using CLM-DP, it is 15.7% (35.3%); using the standard CLM, it is 7.0% (23.0%). The domain-average, mean-normalized standard deviation of the June-July-August mean BVOC flux is 0.0619 when LAI is prescribed and 0.183 when LAI varies with environmental conditions. Our results imply that interannual variation of leaf biomass density, which is primarily driven by interannual variability of precipitation, is a significant contributor to year-to-year differences in BVOC flux on a regional scale, of at least equal importance to interannual variation of temperature and shortwave radiation. Phenology-driven biogenic emission variability is most pronounced in regions with relatively low emissions: as a grid cell's mean BVOC flux decreases, the mean-normalized standard deviation of BVOC flux tends to increase. BVOC flux is most variable between years in subhumid, sparsely wooded regions where interannual variability of precipitation is relatively large. Copyright 2007 by the American Geophysical Union.
• Li, W., Dickinson, R. E., Fu, R., Niu, G., Yang, Z., & Canadell, J. G. (2007). Future precipitation changes and their implications for tropical peatlands. GEOPHYSICAL RESEARCH LETTERS, 34(1).
• Niu, G., & Yang, Z. (2007). An observation-based formulation of snow cover fraction and its evaluation over large North American river basins. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 112(D21).
• Niu, G., & Yang, Z. (2007). An observation-based formulation of snow cover fraction and its evaluation over large North American river basins. Journal of Geophysical Research D: Atmospheres, 112(21).
  More info

  Abstract: Snow cover strongly interacts with climate through snow albedo feedbacks. However, global climate models still are not adequate in representing snow cover fraction (SCF), i.e., the fraction of a model grid cell covered by snow. Through an analysis of the advanced very high resolution radiometer (AVHRR) derived SCF and the Canadian Meteorological Centre (CMC) gridded snow depth and snow water equivalent (SWE), we found that the SCF-snow depth relationship varies with seasons, which may be approximated by variations in snow density. We then added snow density to an existing SCF formulation to reflect the variations in the SCF-snow depth relationship with seasons. The reconstructed SCF with the gridded snow depth and SWE by employing this snow density-dependent SCF formulation agrees better with the AVHRR-derived SCF than other formulations. The default SCF formulation in the National Center for Atmospheric Research community land model (CLM), driven by observed near-surface meteorological forcings, simulates a smaller SCF and a shallower snow depth than observations. Implementation of the new SCF formulation into the NCAR CLM greatly improves the simulations of SCF, snow depth, and SWE in most North American (NA) river basins. The new SCF formulation increases SCF by 20-40%, decreases net solar radiation by up to 20 W M-2, and decreases surface temperature by up to 4 K in most midlatitude regions in winter and at high latitudes in spring. The new scheme reproduces the observed SCF, snow depth, and SWE in terms of interannual variability and interbasin variability in most NA river basins except for the mountainous Columbia and Colorado River basins. It produces SCF trends similar to that of AVHRR. However, it produces greater decreasing trends in ablation seasons and smaller increasing trends in accumulation seasons than those of the CMC snow depth and SWE. Copyright 2007 by the American Geophysical Union.
• Niu, G., Seo, K., Yang, Z., Wilson, C., Hua, S. u., Chen, J., & Rodell, M. (2007). Retrieving snow mass from GRACE terrestrial water storage change with a land surface model. Geophysical Research Letters, 34(15).
  More info

  Abstract: A reliable snow water equivalent (SWE) product is critical for climate and hydrology studies in Arctic regions. Passive microwave sensors aboard satellites provide a capability of observing global SWE and have produced many SWE datasets. However, these datasets have significant errors in boreal forest regions and where snowpack is deep or wet. The Gravity Recovery and Climate Experiment (GRACE) satellites are measuring changes in terrestrial water storage (TWS), of which snow mass is the primary component in winter Arctic river basins. This paper shows SWE can be derived from GRACE TWS change in regions where the ground is not covered by snow in a summer month if accurate changes in below-ground water storage (including soil water and groundwater) can be provided by a land surface model. Based on gravity change, the GRACE-derived SWE estimates are not affected by the boreal forest canopy and are more accurate in deep snow regions than microwave retrievals. The paper also discusses the uncertainties in the SWE retrievals. Copyright 2007 by the American Geophysical Union.
• Niu, G., Seo, K., Yang, Z., Wilson, C., Su, H., Chen, J., & Rodell, M. (2007). Retrieving snow mass from GRACE terrestrial water storage change with a land surface model. GEOPHYSICAL RESEARCH LETTERS, 34(15).
• Niu, G., Yang, Z., Dickinson, R. E., Gulden, L. E., & Hua, S. u. (2007). Development of a simple groundwater model for use in climate models and evaluation with Gravity Recovery and Climate Experiment data. Journal of Geophysical Research D: Atmospheres, 112(7).
  More info

  Abstract: Groundwater interacts with soil moisture through the exchanges of water between the unsaturated soil and its underlying aquifer under gravity and capillary forces. Despite its importance, groundwater is not explicitly represented in climate models. This paper developed a simple groundwater model (SIMGM) by representing recharge and discharge processes of the water storage in an unconfined aquifer, which is added as a single integration element below the soil of a land surface model. We evaluated the model against the Gravity Recovery and Climate Experiment (GRACE) terrestrial water storage change (ΔS) data. The modeled total water storage (including unsaturated soil water and groundwater) change agrees fairly well with GRACE estimates. The anomaly of the modeled groundwater storage explains most of the GRACE ΔS anomaly in most river basins where the water storage is not affected by snow water or frozen soil. For this reason, the anomaly of the modeled water table depth agrees well with that converted from the GRACE ΔS in most of the river basins. We also investigated the impacts of groundwater dynamics on soil moisture and evapotranspiration through the comparison of SIMGM to an additional model run using gravitational free drainage (FD) as the model's lower boundary condition. SIMGM produced much wetter soil profiles globally and up to 16% more annual evapotranspiration than FD, most obviously in arid-to-wet transition regions. Copyright 2007 by the American Geophysical Union.
• Niu, G., Yang, Z., Dickinson, R. E., Gulden, L. E., & Su, H. (2007). Development of a simple groundwater model for use in climate models and evaluation with Gravity Recovery and Climate Experiment data. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 112(D7).
• Wenhong, L. i., Dickinson, R. E., Rong, F. u., Niu, G., Yang, Z., & Canadell, J. G. (2007). Future precipitation changes and their implications for tropical peatlands. Geophysical Research Letters, 34(1).
  More info

  Abstract: Carbon (C) in tropical peatlands over Southeast Asia and Amazonia, if released to the atmosphere, can substantially increase the growth rate of atmospheric carbon dioxide. Over Southeast Asia, where the most extensive tropical peatlands in the world occur, 11 climate models for the IPCC Fourth Assessment show an overall decrease of rainfall in future dry seasons. Over Amazonia, future rainfall changes in dry seasons are highly uncertain; five models predict increased rainfall, and the remaining models predict the opposite. We have further examined the UKMO-HadCM3, GISS-ER, and GFDL-CM2.1 models. Over Southeast Asia, all three models predict similar decreases of rainfall and evaporative fraction, implying an increase of water table depth and surface dryness during the dry season south of the equator. Such changes would potentially switch peat ecosystems from acting as C sinks to C sources. Over Amazonia, the two models with the best simulations of current rainfall produce conflicting results for the future of peat stability. Copyright 2007 by the American Geophysical Union.
• Niu, G., & Yang, Z. (2006). Assessing a land surface model's improvements with GRACE estimates. Geophysical Research Letters, 33(7).
  More info

  Abstract: The Gravity Recovery and Climate Experiment (GRACE) satellites have produced an unprecedented data set of terrestrial water storage (TWS) change in large-scale river basins. Recent research has found that monthly variations of soil moisture and snow water simulated by land surface models compared favorably with the GRACE-derived TWS change. Compared to the GRACE data, the standard version of the National Center for Atmospheric Research (NCAR) Community Land Model (CLM) produces a weaker TWS variability in tropical and mid-latitudes but a stronger TWS variation in high latitudes. However, a modified version of CLM that includes more realistic interception, runoff, and frozen soil processes improves the simulation of TWS change in global river basins of various scales. In addition, the modified CLM improves the modeling of evapotranspiration through the improvements in the modeling of TWS variation and runoff in the Amazon River basin. Along this line of research, this paper shows that such GRACE data can be used as a means of evaluating the hydrological schemes in a land surface model. Copyright 2006 by the American Geophysical Union.
• Niu, G., & Yang, Z. (2006). Effects of frozen soil on snowmelt runoff and soil water storage at a continental scale. JOURNAL OF HYDROMETEOROLOGY, 7(5), 937-952.
• Niu, G., & Yang, Z. (2006). Effects of frozen soil on snowmelt runoff and soil water storage at a continental scale. Journal of Hydrometeorology, 7(5), 937-952.
  More info

  Abstract: The presence of ice in soil dramatically alters soil hydrologic and thermal properties. Despite this important role, many recent studies show that explicitly including the hydrologic effects of soil ice in land surface models degrades the simulation of runoff in cold regions. This paper addresses this dilemma by employing the Community Land Model version 2.0 (CLM2.0) developed at the National Center for Atmospheric Research (NCAR) and a simple TOPMODEL-based runoff scheme (SIMTOP). CLM2.0/SIMTOP explicitly computes soil ice content and its modifications to soil hydrologic and thermal properties. However, the frozen soil scheme has a tendency to produce a completely frozen soil (100% ice content) whenever the soil temperature is below O°C. The frozen ground prevents infiltration of snowmelt or rainfall, thereby resulting in earlier- and higher-than-observed springtime runoff. This paper presents modifications to the above-mentioned frozen soil scheme that produce more accurate magnitude and seasonality of runoff and soil water storage. These modifications include 1) allowing liquid water to coexist with ice in the soil over a wide range of temperatures below O°C by using the freezing-point depression equation, 2) computing the vertical water fluxes by introducing the concept of a fractional permeable area, which partitions the model grid into an impermeable part (no vertical water flow) and a permeable part, and 3) using the total soil moisture (liquid water and ice) to calculate the soil matric potential and hydraulic conductivity. The performance of CLM2.0/SIMTOP with these changes has been tested using observed data in cold-region river basins of various spatial scales. Compared to the CLM2.0/SIMTOP frozen soil scheme, the modified scheme produces monthly runoff that compares more favorably with that estimated by the University of New Hampshire-Global Runoff Data Center and a terrestrial water storage change that is in closer agreement with that measured by the Gravity Recovery and Climate Experiment (GRACE) satellites. © 2006 American Meteorological Society.
• Niu, G., Yang, Z., Dickinson, R. E., & Gulden, L. E. (2005). A simple TOPMODEL-based runoff parameterization (SIMTOP) for use in global climate models. Journal of Geophysical Research D: Atmospheres, 110(21), 1-15.
  More info

  Abstract: This paper develops a simple TOPMODEL-based runoff parameterization (hereinafter SIMTOP) for use in global climate models (GCMs) that improves the runoff production and the partitioning of runoff between surface and subsurface components. SIMTOP simplifies the TOPMODEL runoff formulations in two ways: (1) SIMTOP represents the discrete distribution of the topographic index as an exponential function, not as a three-parameter gamma distribution; this change improves the parameterization of the fractional saturated area, especially in mountainous regions. (2) SIMTOP treats subsurface runoff as a product of an exponential function of the water table depth and a single coefficient, not as a product of several parameters that depend on topography and soil properties; this change facilitates applying TOPMODEL-based runoff schemes on global scale. SIMTOP is incorporated into the National Center for Atmospheric Research (NCAR) Community Land Model version 2.0 (CLM 2.0). SIMTOP is validated at a watershed scale using data from the Sleepers River watershed in Vermont, USA. It is also validated on a global scale using the monthly runoff data from the University of New Hampshire Global Runoff Data Center (UNH-GRDC). SIMTOP performs favorably when compared to the baseline runoff formulation used in CLM2.0. Realistic simulations can be obtained using two distinct saturated hydraulic conductivity (Ksat) profiles. These profiles include (1) exponential decay of Ksat with depth (as is typically done in TOPMODEL-based runoff schemes) and (2) the definition of Ksat using the soil texture profile data (as is typically done in climate models) and the concordant reduction of the gravitational drainage from the bottom of the soil column. Copyright 2005 by the American Geophysical Union.
• Boone, A., Habets, F., Noilhan, J., Clark, D., Dirmeyer, P., Fox, S., Gusev, Y., Haddeland, I., Koster, R., Lohmann, D., Mahanama, S., Mitchell, K., Nasonova, O., Niu, G. Y., Pitman, A., Polcher, J., Shmakin, A. B., Tanaka, K., van, d., , Verant, S., et al. (2004). The Rhone-aggregation land surface scheme intercomparison project: An overview. JOURNAL OF CLIMATE, 17(1), 187-208.
• Niu, G., & Yang, Z. (2004). Effects of vegetation canopy processes on snow surface energy and mass balances. Journal of Geophysical Research D: Atmospheres, 109(23), 1-15.
  More info

  Abstract: This paper addresses the effects of canopy physical processes on snow mass and energy balances in boreal ecosystems. We incorporate new parameterizations of radiation transfer through the vegetation canopy, interception of snow by the vegetation canopy, and under-canopy sensible heat transfer processes into the Versatile Integrator of Surface and Atmosphere (VISA) and test the model results against the Boreal Ecosystem-Atmosphere Study (BOREAS) data observed at South Study Area, Old Jack Pine. A modified two-stream radiation transfer scheme that accounts for the three-dimensional geometry of vegetation accurately simulates the transferring of solar radiation through the vegetation canopy when the leaf and stem area index is reduced to match the observed. VISA produces higher-than-observed surface albedo in wintertime. Implementation of a snow interception model that explicitly describes the loading and unloading of snow and the melting and refreezing of snow on the canopy into VISA reduces the fractional snow cover on the canopy and the surface albedo. VISA overestimates the downward sensible heat fluxes from the canopy to the snow surface, which leads to earlier snow ablation and a shallower snowpack than the observed. Explicitly including a canopy heat storage term in the canopy energy balance equation decreases the spuriously large amplitude of the diurnal canopy temperature variation and reduces the excessive daytime sensible heat flux from the canopy downward to the snow surface. Sensitivity tests reveal that the turbulent sensible heat flux below the vegetation canopy strongly depends on the canopy absorption coefficient of momentum. During spring the daytime temperature difference between the snow surface and the vegetation canopy forms a strongly stable atmospheric condition, which results in a larger absorption coefficient of momentum and a weak turbulent sensible heat flux. The modeled excessive downward sensible heat flux from the vegetation canopy to the snow surface is considerably reduced through the stability correction to the canopy absorption coefficient of momentum. Copyright 2004 by the American Geophysical Union.
• Niu, G., & Yang, Z. (2004). Modeling the Continental Hydrology: The Interplay between Canopy Interception and Hill-Slope Runoff. Engineering Construction and Operations in Challenging Environments Earth and Space 2004: Proceedings of the Ninth Biennial ASCE Aerospace Division International Conference, 284-289.
  More info

  Abstract: Considerable progress in continental hydrological research has been made recently that includes sub-grid-scale variability of precipitation, canopy interception, through flow, and hill-slope runoff production. Many such studies have focused on one single process at a time. This work aims to examine the interplay between canopy interception, throughflow and runoff using the NCAR Community Atmosphere Model (CAM2) coupled with NCAR Community Land Model (CLM2). The simulations are compared with the observed runoff from the University of New Hampshire-Global Runoff Data Center (UNH-GRDC) 0.5 degree monthly climatological composite runoff fields. The simulations from the CAM2/CLM2 show an excessive canopy interception and low soil moisture content over the climatologically wet regions, especially in the Amazon basin. A series of experiments are conducted which explicitly account for the sub-grid-scale variability of precipitation and its effects on canopy interception and throughflow. These experiments incorporate changes in the treatment of topography-induced runoff. The results have demonstrated that the interception and runoff processes are intimately coupled, and that their schemes should be changed together to ensure the improvement in the hydrological simulations. The simplified topography-based runoff model (TOPMODEL) and the sub-grid interception scheme proposed in this study, when used together, result in better simulations of runoff. With these proposed schemes, the water budgets show favorable changes in the Amazon basin in that canopy evaporation is reduced, runoff is increased, and soil is wetter.
• Yang, Z., Niu, G., & Zeng, Q. (2004). Impacts of Fractional Snow Cover on Surface Air Temperature in the NCAR Community Atmosphere Model (NCAR-CAM2). Engineering Construction and Operations in Challenging Environments Earth and Space 2004: Proceedings of the Ninth Biennial ASCE Aerospace Division International Conference, 269-275.
  More info

  Abstract: The NCAR Community Atmosphere Model (CAM2) coupled with the Community Land Model (CLM2) shows a significant warm bias in surface air temperature over snow-covered areas in winter An intercomparison of surface albedos from CAM2/CLM2 and MODIS reveals that in the northern mid-latitude grassland and cropland regions, the modeled values are much lower than observations mainly due to shallower-than observed snow depth This underestimated snow depth is related to how snow cover fraction (SCF) is parameterized in the land module (CLM2) Most land surface models relate SCF to grid (or patch) averaged snow depth and surface roughness length This study suggests that the SCF formulation of Yang et al (1997) largely improves the simulations of snow depth, surface albedo and surface air temperature over non-forested regions in winter However, snow ablation is delayed and a cold bias appears in spring, suggesing that the SCF-snow depth relationship may be different for accumulation and ablation seasons Guided by limited watershed-scale measurements, a different SCF-snow depth formulation for ablation season was tested in CLM2 Preliminary modeling results from CAM2/CLM2 indicated that using this SCF scheme for melting period and the SCF scheme of Yang et al (1997) for accumulation period leads to the most encouraging results, such as a significant reduction of wintertime warm bias over snow-covered areas and few undesired effects on the springtime snow depth and air temperature The caution of using these schemes will also be discussed.
• Bowling, L. C., Lettenmaier, D. P., Nijssen, B., Graham, L. P., Clark, D. B., Maayar, M. E., Essery, R., Goers, S., Gusev, Y. M., Habets, F., Van, B., Jin, J., Kahan, D., Lohmann, D., Xieyao, M. a., Mahanama, S., Mocko, D., Nasonova, O., Niu, G., , Samuelsson, P., et al. (2003). Simulation of high-latitude hydrological processes in the Torne-Kalix basin: PILPS Phase 2(e) 1: Experiment description and summary intercomparisons. Global and Planetary Change, 38(1-2), 1-30.
  More info

  Abstract: Twenty-one land-surface schemes (LSSs) participated in the Project for Intercomparison of Land-surface Parameterizations (PILPS) Phase 2(e) experiment, which used data from the Torne-Kalix Rivers in northern Scandinavia. Atmospheric forcing data (precipitation, air temperature, specific humidity, wind speed, downward shortwave and longwave radiation) for a 20-year period (1979-1998) were provided to the 21 participating modeling groups for 218 1/4° grid cells that represented the study domain. The first decade (1979-1988) of the period was used for model spin-up. The quality of meteorologic forcing variables is of particular concern in high-latitude experiments and the quality of the gridded dataset was assessed to the extent possible. The lack of sub-daily precipitation, underestimation of tree precipitation and the necessity to estimate incoming solar radiation were the primary data concerns for this study. The results from two of the three types of runs are analyzed in this, the first of a three-part paper: (1) calibration-validation runs - calibration of model parameters using observed streamflow was allowed for two small catchments (570 and 1300 km2), and parameters were then transferred to two other catchments of roughly similar size (2600 and 1500 km2) to assess the ability of models to represent ungauged areas elsewhere; and 2) reruns - using revised forcing data (to resolve problems with apparent underestimation of solar radiation of approximately 36%, and certain other problems with surface wind in the original forcing data). Model results for the period 1989-1998 are used to evaluate the performance of the participating land-surface schemes in a context that allows exploration of their ability to capture key processes spatially. In general, the experiment demonstrated that many of the LSSs are able to capture the limitations imposed on annual latent heat by the small net radiation available in this high-latitude environment. Simulated annual average net radiation varied between 16 and 40 W/m2 for the 21 models, and latent heat varied between 18 and 36 W/m2. Among-model differences in winter latent heat due to the treatment of aerodynamic resistance appear to be at least as important as those attributable to the treatment of canopy interception. In many models, the small annual net radiation forced negative sensible heat on average, which varied among the models between - 11 and 9 W/m2. Even though the largest evaporation rates occur in the summer (June, July and August), model-predicted snow sublimation in winter has proportionately more influence on differences in annual runoff volume among the models. A calibration experiment for four small sub-catchments of the Torne-Kalix basin showed that model parameters that are typically adjusted during calibration, those that control storage of moisture in the soil column or on the land surface via ponding, influence the seasonal distribution of runoff, but have relatively little impact on annual runoff ratios. Similarly, there was no relationship between annual runoff ratios and the proportion of surface and subsurface discharge for the basin as a whole. © 2003 Elsevier Science B.V. All rights reserved.
• Dai, Y. J., Zeng, X. B., Dickinson, R. E., Baker, I., Bonan, G. B., Bosilovich, M. G., Denning, A. S., Dirmeyer, P. A., Houser, P. R., Niu, G. Y., Oleson, K. W., Schlosser, C. A., & Yang, Z. L. (2003). The Common Land Model. BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY, 84(8), 1013-1023.
• Nijssen, B., Bowling, L. C., Lettenmaier, D. P., Clark, D. B., El, M. M., Essery, R., Goers, S., Gusev, Y. M., Habets, F., van, d., Jin, J. M., Kahan, D., Lohmann, D., Ma, X. Y., Mahanama, S., Mocko, D., Nasonova, O., Niu, G. Y., Samuelsson, P., , Schmakin, A. B., et al. (2003). Simulation of high latitude hydrological processes in the Torne-Kalix basin: PILPS phase 2(e) - 2: Comparison of model results with observations. GLOBAL AND PLANETARY CHANGE, 38(1-2), 31-53.
• Niu, G., & Yang, Z. (2003). The versatile integrator of surface atmospheric processes part 2: Evaluation of three topography-based runoff schemes. Global and Planetary Change, 38(1-2), 191-208.
  More info

  Abstract: Three different schemes of topography-based runoff production [versatile integrator of surface atmospheric processes (VISA)-TOP1, VISA-TOP2, and VISA-TOP3] are described for a land-surface model (LSM) developed for use with a general circulation model (GCM). The schemes' sensitivities to some key parameters are assessed for two catchments using data sets developed for the Project for Intercomparison of Land-Surface Parameterization Schemes (PILPS) Phase 2e. VISA-TOP1 differs from VISA-TOP2 only in how to treat oversaturated soil water from the soil layers. In VISA-TOP1, the oversaturated soil water is thrown out of the soil column; hence, it no longer plays a role in the ensuing soil water budgets. In VISA-TOP2, this oversaturated soil water is recharged back to the unsaturated soil layers above the water table; hence, it continues to involve in the water budgets. Unlike VISA-TOP1 and VISA-TOP2, VISA-TOP3 relaxes its dependence on the topographic parameters. The oversaturated soil water is treated the same in both VISA-TOP2 and VISA-TOP3. All three models reproduce the daily and seasonal cycles of streamflow provided that different values of the saturated hydraulic conductivity decay factor are used. The decay factor controls the timing and partitioning of subsurface runoff. In both VISA-TOP1 and VISA-TOP2, an anisotropic parameter explaining different hydraulic conductivities in the vertical and horizontal directions is critical for using the topographic index in the land-surface model. In the VISA-TOP2 scheme, the topography-controlled subsurface runoff is dominant because the oversaturated water is recharged to upper unsaturated soil layers to raise the water table. The water budgets in all these schemes show dramatically different responses to the decay factor, indicating that the calibrated parameters and the model formulations should not be separated. © 2003 Elsevier Science B.V. All rights reserved.
• Yang, Z., Gochis, D., Shuttleworth, W. J., & Niu, G. (2003). The impact of sea surface temperature on the North American monsoon: A GCM study. Geophysical Research Letters, 30(2), 5-1.
  More info

  Abstract: The NCAR CCM3 was used to simulate the circulation and rainfall patterns of the North American monsoon system (NAMS). When forced with repeated annual cycles of climatological average sea surface temperatures (SSTs), the CCM3 significantly under-represents monsoon rainfall in the southwest United States while simulating excessive precipitation in the tropical eastern Pacific Ocean and the Caribbean Sea. However, when forced with the observed monthly average SSTs from 1979 to 1997, the CCM3 produces an improved simulation of monsoon rainfall in the southwestern U.S., as well as in the eastern tropical Pacific Ocean and the Caribbean Sea. Using the SSTs for 1983 in the Pacific and climatological SSTs elsewhere, the modeled circulation and rainfall distribution resembles that given with observed monthly average SSTs. The simulations are sensitive to the size of the domains over which the Pacific SST anomalies are imposed. Overall, these results suggest that the magnitude and size of winter- and springtime Pacific SST anomalies have a significant influence on summertime rainfall in the southwest U.S., and that these SSTs contribute to the NAMS precipitation climatology in extreme years more than in less extreme years.
• Cassardo, C., Loglisci, N., Gandini, D., Qian, M. W., Niu, G. Y., Ramieri, P., Pelosini, R., & Longhetto, A. (2002). The flood of November 1994 in Piedmont, Italy: a quantitative analysis and simulation. HYDROLOGICAL PROCESSES, 16(6), 1275-1299.
• Yang, Z. L., Dickinson, R. E., Hahmann, A. N., Niu, G. Y., Shaikh, M., Gao, X. G., Bales, R. C., Sorooshian, S., & Jin, J. M. (1999). Simulation of snow mass and extent in general circulation models. HYDROLOGICAL PROCESSES, 13(12-13), 2097-2113.
• Guoyue, N., Carssardo, C., Zhongxiang, H., & Longhetto, A. (1998). Numerical simulation on the response of land surface to severe weather. Acta Meteorologica Sinica, 12(4), 410-424.
  More info

  Abstract: A coupled model of RAMS3b (Regional Atmospheric Modeling System, Version 3b) and LSPM (a land surface process model), in which some basic hydrological processes such as precipitation, evapotranspiration, surface runoff, infiltration and bottom drainage are included, has been established. With the coupled model, we have simulated the response of soil to the severe weather process which caused the disastrous flood in north Italy during 4-7, November, 1994, simultaneously compared with the observation and the original RAMS3b, which has a soil and vegetation parameterization scheme (hereafter, SVP) emphasizing on the surface energy fluxes, while some hydrological processes in the soil are not described clearly. The results show that the differences between coupling LSPM and SVP exist mainly in the response of soil to the precipitation. The soil in the SVP never saturates under the strong input of precipitation, while the newly coupled model seems better, the soil has been saturated for one day or more and causes strong surface runoff, which constitutes the flood. Further sensitivity experiments show that the surface hydrological processes are very sensitive to the initial soil moisture and soil type when we compared the results with a relatively dry case and sandy soil. The coupled model has potentiality for simulation on the interaction between regional climate and land surface hydrological processes, and the regional water resources research concerning desertification, drought and flood.
• Niu, G., Sun, S., & Hong, Z. (1997). Water and heat transport in the desert soil and atmospheric boundary layer in western China. Boundary-Layer Meteorology, 85(2), 179-195.
  More info

  Abstract: In order to understand the exchange and transfer processes of water and energy in the desert soil and the atmospheric boundary layer (ABL), we have developed a coupled model, in which a desert soil model including water movement of both liquid and vapour phase, and an ABL model based on a non-local transilient turbulence closure scheme, are coupled together. With this model, the evolution of potential temperature and specific humidity, the distribution of net radiation among sensible, latent and soil heat fluxes, and the water and heat flux profiles both in the soil and ABL have been simulated. The HEIFE (HEIhe River Basin Field Experiment) observational data are used to calibrate calculation of the water and heat flux both in the soil and the ABL. The sensible and latent heat fluxes warm and moisten the bottom grid box (100 m) of the ABL. In this way the ABL model and the desert soil model are coupled together. The simulated results show that when the flux of water vapour in the soil is neglected, the evaporation rate and the flux profiles of specific humidity in the ABL show great changes, hence the importance of water vapour movement in the desert soil for the calculation of specific humidity in the ABL. In the upper 5 cm of the soil, which is called an active layer, water and heat transport are more effective than in the substrate (soil below 5 cm). © 1997 Kluwer Academic Publishers.

Presentations

• Elshall, A., Ye, M., Niu, G., & Barron-Gafford, G. A. (2015, December). Bayesian multimodel inference of soil microbial respiration models: Theory, application and future prospective. American Geophysical Union Annual Meeting. San Francisco, CA.
• Chang, H. I., Castro, C. L., & Niu, G. (2013, December). Impact of dynamic vegetation on evolution of the North American Monsoon region in a regional climate model.. 46th American Geophysical Union Fall Meeting. San Francisco, CA: American Geophysical Union.

Poster Presentations

• Troch, P. A., Zeng, X., Wang, Y., Van Haren, J. L., Tuller, M., Sibayan, M., Schaap, M. G., Saleska, S. R., Ruiz, J., Rasmussen, C., Pelletier, J. D., Niu, G., Monson, R. K., Meredith, L., Alves Meira Neto, A., Matos, K. A., Maier, R. M., Kim, M., Hunt, E. A., , Harman, C. J., et al. (2017, December). Controlled Experiments of Hillslope Co-evolution at the Biosphere 2 Landscape Evolution Observatory: Toward Prediction of Coupled Hydrological, Biogeochemical, and Ecological change. AGU International Annual Meeting. New Orleans, LA: American Geophysical Union (AGU).