Donald C Slack

Interests

Teaching

Soil and Water Resources Engineering; Watershed Engineering and Hydrology; Research Methods; Engineering Design

Research

Water Resources Engineering; Irrigation Engineering; Water Management; Biofuels Production and Utilization

Courses

Scholarly Contributions

Journals/Publications

• Slack P,E., D. C., & Reyes-Esteves, R. G. (2019). Modeling Approaches for Determining Appropriate Depth of Subsurface Drip Irrigation Tubing in Alfalfa. Journal of Irrigation and Drainage Engineering - ASCE. doi:https://doi.org/10.1061/(ASCE)IR.1943-4774.0001409
• Slack P,E., D. C., & Reyes-Esteves, R. G. (2019). Modeling of soil water to insure a suitable depth and spacing of subsurface drip irrigation tubing for alfalfa. Revista Ingenieria Agricola y Biosystemas. doi:http://dx.doi.org/10.5154/r.inagbi.2018.02.002
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  A major design issue in the implementation of a Subsurface Drip Irrigation (SDI) system is the determination of the appropriate depth of placement of the drip emitters to allow basic farming operations with heavy equipment while still providing adequate water to the crop. In this study, the program HYDRUS-2D is used to determine the wetting pattern from a subsurface drip emitter installed at 50 cm for three soils typically found in Southern CA, a Sandy Clay Loam (SCL), a Clay Loam (CL) and a Loam (L). The model was used to determine the extent of wetting above, and laterally from the drip tubing after twelve hours of irrigation the day of the cut with an irrigation frequency of every three days. The vertical “rise” of water above the drip line after this irrigation period was 27 cm, 30 cm and 22 cm for the SCL, CL, and L, respectively. Then classical soil mechanics theory was utilized to calculate the increase in stress on soil at any depth due to a load on the surface from a typical farm tractor. The minimum depth placement to avoid failure would be 40 cm below the surface in an SCL soil. A similar analysis for the CL and L yielded minimum installation depths of 35 cm and 40 cm respectively. This type of analysis is helpful in determining the depth of placement of subsurface drip irrigation lines to ensure adequate trafficability of soil irrigated with subsurface drip irrigation systems.
• Slack P,E., D. C., Chaibandit, K., & Khonyai, S. (2017). Evaluation of the Water Footprint of Sugarcane in Eastern Thailand.. Engineering Journal, 21(5), 193-201. doi:DOI:10.4186/ej.2017.21.5.193.
• Slack P,E., D. C., Reyes Esteves, R., Espejel, A., Oyorsaval, B., & Ma, Y. (2017). Subsurface Drip Irrigation: A Technology for Safer Irrigation of Vegetable Crops. Engineering and Applied Science Research, 44(2), 111-114.
• Sanchez-Choen, I., Padilla, G. D., Valle, M. V., Slack P,E., D. C., Heilman, P., & Sandoval, A. P. (2015). A Decision Support System for Rainfed Agricultural Areas of Mexico. Computers and Electronics in Agriculture, 10.
• Martinez-Cruz, T. E., Slack P,E., D. C., Ottman, M., & Ogden, K. L. (2014). THE WATER USE OF SWEET SORGHUM AND DEVELOPMENT OF CROP COEFFICIENTS. Irrigation and Drainage, 12.
• Ogden, K. L., Slack, D. C., Ottman, M. J., & Cruz, T. E. (2014). The Water Use of Sweet Sorghum and Development of Crop Coefficients. Irrigation and Drainage.
• Nearing, G. S., Moran, M. S., Thorp, K. R., D., C., & Slack, D. C. (2010). Likelihood parameter estimation for calibrating a soil moisture model using radar bakscatter. Remote Sensing of Environment, 114(11), 2564-2574.
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  Abstract: Land surface model parameter estimation can be performed using soil moisture information provided by synthetic aperture radar imagery. The presence of speckle necessitates aggregating backscatter measurements over large (>100. m × 100. m) land areas in order to derive reliable soil moisture information from imagery, and a model calibrated to such aggregated information can only provide estimates of soil moisture at spatial resolutions required for reliable speckle accounting. A method utilizing the likelihood formulation of a probabilistic speckle model as the calibration objective function is proposed which will allow for calibrating land surface models directly to radar backscatter intensity measurements in a way which simultaneously accounts for model parameter- and speckle-induced uncertainty. The method is demonstrated using the NOAH land surface model and Advanced Integral Equation Method (AIEM) backscatter model calibrated to SAR imagery of an area in the Southwestern United States, and validated against in situ soil moisture measurements. At spatial resolutions finer than 100. m × 100. m NOAH and AIEM calibrated using the proposed radar intensity likelihood parameter estimation algorithm predict surface level soil moisture to within 4% volumetric water content 95% of the time, which is an improvement over a 95% prediction confidence of 10% volumetric water content by the same models calibrated directly to soil moisture information derived from synthetic aperture radar imagery at the same scales. Results suggest that much of this improvement is due to increased ability to simultaneously estimate NOAH parameters and AIEM surface roughness parameters. © 2010 Elsevier Inc.
• Silvestry-Rodriguez, N., Bright, K. R., Slack, D. C., Uhlmann, D. R., & Gerba, C. P. (2008). Silver as a residual disinfectant to prevent biofilm formation in water distribution systems. APPLIED AND ENVIRONMENTAL MICROBIOLOGY, 74(5), 1639-1641. doi:https://doi.org/10.1128/AEM.02237-07
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  Biofilms can have deleterious effects on drinking water quality and may harbor pathogens. Experiments were conducted using 100 microg/liter silver to prevent biofilm formation in modified Robbins devices with polyvinyl chloride and stainless steel surfaces. No significant difference was observed on either surface between the silver treatment and the control.
• Slack, D. C., Garcia, G., Roth, R., Hoenig, S., Segovia, R., Soto, R., & Frayre, A. (2008). Engineered Conservation Structures using Discarded Tires. American Society of Agricultural and Biological Engineers - Conference on 21st Century Watershed Technology: Improving Water Quality and Environment 2008, 163-170.
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  Abstract: We have developed ways to utilize discarded tires for erosion control structures, bank stabilization and slope stabilization and erosion control in the form of engineered structures for each of these applications. This paper also describes and illustrates a channel erosion control structure made entirely from individual tires used to form side walls, wing walls and aprons for the structure. In this case the individual tires were bound together with nylon rope to form a flexible structure. This structure was built in a ephemeral wash (gully) on a local ranch near Tucson, AZ and has been in place for over ten years. More recently, engineers from the University of Arizona and the Technological Institute of Nogales (ITN) collaborated on a demonstration project on the ITN campus to construct retaining walls for steep slopes and use arrays of individual tires for slope stabilization on gentler slopes. For the retaining wall, tires were stacked on a footing and anchored to the footing via a reinforcing rod running up through the tires. The individual columns of tires were then tied together and the rod anchored to the soil mass behind the wall using a "duck-bill" anchor and cable.
• Gerba, C. P., Slack, D. C., Uhlmann, D. R., Bright, K. R., & Silvestry-Rodriguez, N. (2007). Inactivation of Pseudomonas aeruginosa and Aeromonas hydrophila by silver in tap water. JOURNAL OF ENVIRONMENTAL SCIENCE AND HEALTH PART A - TOXIC/HAZARDOUS SUBSTANCES & ENVIRONMENTAL ENGINEERING, 42(11), 1579-1584. doi:https://doi.org/10.1080/10934520701517689
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  This study was conducted to assess the efficacy of silver as a secondary disinfectant to replace or reduce the level of chlorine utilized in water distribution systems. Pseudomonas aeruginosa and Aeromonas hydrophila are opportunistic pathogens present in drinking water and have been associated with waterborne disease. After 8 hours of exposure to 100 microg/L of silver, there was a >6-log10 reduction in P. aeruginosa in tap water at room temperature at pH7 and a 5.55-log10 reduction in the presence of 3 mg/L humic acid. Similar reductions were observed at pH9. At 4 degrees C, reductions greater than 4-log10 were observed after 24 hours. For A. hydrophila, a >6-log10 reduction occurred at both pH7 and pH9 within nine hours. The World Health Organization has determined that this amount of silver could be used for water disinfection without health risks. Furthermore, silver shows promise as a secondary disinfectant, even in the presence of organic matter in concentrations that would reduce the effectiveness of free chlorine.
• Martin, E. C., Slack, D. C., Tanksley, K. A., & Basso, B. (2006). Effects of fresh and composted dairy manure applications on alfalfa yield and the environment in Arizona. Agronomy Journal, 98(1), 80-84.
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  Abstract: The Unified Animal Feeding Operation Strategy requires that field application of animal waste, a common fertilization and disposal practice, may not exceed crop nutrient needs. Additional guidelines set forth by the Arizona Department of Environmental Quality state that animal waste applications on agricultural fields in designated Confined Animal Feeding Operations (CAFOs) must be made in a manner such that the total N applied to the field cannot exceed the uptake from the crop grown. Because alfalfa is grown year round and can take up large quantities of N, many operators of CAFOs apply animal waste to their production alfalfa fields as method of waste disposal. In this research, fresh and composted dairy manure was applied to plots in a production alfalfa (Medicago sativa L.) field to determine the impact on alfalfa yield, soil nitrogen (N), phosphorus (P), and electrical conductivity (EC) levels and the potential for nitrate (NO3) and phosphate (PO4) leaching. Unfertilized plots were maintained as controls. Fresh and composted manure was applied to fertilized plots after each harvest at a rate intended to replace N removed from the previous cutting. After 1.5 yr and 13 cuttings, soil analysis down to 150 cm depth showed no significant difference in soil N between treatments. At study end, NO3-N made up 1.1% of total N in the fertilized plots bet only 0.6% in control plots. Changes in soil N were not significant. Soil P content increased in fertilized plots but remained stable in control plots. Final soil PO4 measurements were 16, 99, and 116 kg ha-1 in the control, manure-treated, and compost-treated plots, respectively. Leachate from three drainage lysimeters contained no detectable NO3 or PO4 from any of the treatments. LSD showed no difference in EC between the beginning and the end of study, and alfalfa yield did not vary among treatments. © American Society of Agronomy.
• Jia, X., Larson, D., Slack, D., & Walworth, J. (2005). Electrokinetic control of nitrate movement in soil. Engineering Geology, 77(3-4 SPEC. ISS.), 273-283.
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  Abstract: Nitrate contamination of surface and groundwater has become a serious concern in many agricultural areas throughout the world. Nitrate fertilization is widely used to increase crop yields and quality, but nitrates are highly soluble in water with low retention by soils. Appropriate nitrate application practices can minimize losses, but untimely rainfall can foil management intent. A small dc electrical input has previously been reported to influence nitrate movement in wet soil. Two sets of lysimeter experiments were conducted to evaluate the effect of parallel electrodes on pH and nitrate distribution in field soils subjected to an electrical input. In one experiment, drip irrigation with injected nitrate application was used to grow Sudan grass; in the other, nitrate distribution in wet soil was observed after application of varying levels of electrical current. Increased nitrate concentration, reduced sodium and calcium concentration and lower pH values near the anode, the expected results, were not attained consistently in the test soils with an electrical input of 100-700 mA. The evaluation is being continued with simpler, more controlled conditions in soil column laboratory experiments. © 2004 Elsevier B.V. All rights reserved.
• Salazar, R., Stone, J., Yakowitz, D., & Slack, D. (2005). Multicriteria analysis in an irrigation district in Mexico. Journal of Irrigation and Drainage Engineering, 131(6), 514-524.
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  Abstract: The Alto Rio Lerma Irrigation District, located in the state of Guanajuato in Mexico, is an agricultural area whose sustainability depends partially upon groundwater withdrawal for crop irrigation. Because of high pumping demands and current land-management practices, groundwater levels have declined severely, resulting in aquifer overdraft. In order to analyze economic, environmental, and water use problems in this region, 12 potential cropping patterns were generated for different groundwater withdrawals using linear programming. Then, simulation of the agricultural system was performed using GLEAMS to estimate the amounts of water, nitrate, and pesticides in both runoff and percolation for each cropping pattern. Pumping costs and an aquifer exploitation coefficient account for the economic and environmental impacts of aquifer overdraft. Finally, the Range of Value Method (multicriteria method) was applied to rank and identify the best cropping pattern. The results show the best alternative for effectively balancing environmental with economic considerations was the farming practice, consisting of land leveling, growing vegetables such as red tomato, and controlled groundwater withdrawals to preserve aquifer sustainability. Journal of Irrigation and Drainage Engineering © ASCE.
• Jia, X., Martin, E. C., & Slack, D. C. (2004). Temperature adjustment for reference evapotranspiration calculation in central Arizona. Journal of Irrigation and Drainage Engineering, 130(5), 384-390.
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  Abstract: A key component in the calculation of reference crop evapotranspiration (ET,) is the weather data. If the weather data have been collected from a station under nonreference conditions, the data itself may contain errors, which will in turn yield inaccurate ET, estimates. It was proposed by Allen in 1996 that data used for evapotranspiration be scrutinized by comparing daily minimum temperature (Tmin) and the daily average dew point temperature (Tdew). If the difference between Tmin and Tdew is greater than 3°C, then the site is considered to be arid (nonreference) and adjustments are recommended for temperature and dew point data. In Arizona, normal weather conditions often occur where Tmin and Tdew do not approach each other. This study examined the appropriateness of applying the conditions set forth by Allen to temperature data collected in central Arizona. Two weather stations were set up in a 35.5 ha alfalfa field in central Arizona to measure dry bulb and wet bulb temperatures. Additionally, plant temperature data were collected to verify field conditions. Daily data were taken for 1.5 years at the University of Arizona's Maricopa Agricultural Center. Of the 611 days of data collected, the difference between Tmin and Tdew was greater than 3°C on 329 days, indicating that these data were not taken under reference conditions. Among these data, 178 days were verified as nonreference but 151 were verified as actually being under reference conditions. Making adjustments for these days (151 days) resulted in a 47 mm decrease in ET, estimation, which mostly occurred during the summer. © ASCE.
• Martin, E. C., Jannusch, J., & Slack, D. C. (2004). Application of animal manure/compost in an oat/corn rotation to assure compliance for CAFO nitrogen management in Arizona. ASAE Annual International Meeting 2004, 2847-2858.
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  Abstract: This study was initiated to develop and assess the management of the application of animal waste and compost to an oat/corn rotation in Arizona. Unlike many Eastern states, Arizona uses nitrogen as the limiting nutrient in animal waste applications. In this project, nutrient analysis for nitrogen, phosphorus and electroconductivity were conducted on soil, manure/compost, and plant samples. There were three treatments replicated four times in this study. Treatment 1: inorganic nitrogen sources; Treatment 2: manure; Treatment 3: composted manure. All management decisions are made to maximize yield while assuring that the amount of nitrogen applied was equivalent to the nitrogen removed in the crop. In the first year, there was little or no difference in crop yields. In the second year, yield differences began to show. The soil nitrate-N content was remaining relatively low and equal between treatments. However soil phosphorus content has significantly increased in the manure and compost plots. According to the ruling, if the level of P becomes high enough, then P will become the limiting nutrient. Thus far, we have not reached that point; however, P levels are being monitored closely.
• Ojeda-Bustamante, W., Sifuentes-Ibarra, E., Slack, D. C., & Carrillo, M. (2004). Generalization of irrigation scheduling parameters using the growing degree days concept: Application to a potato crop. Irrigation and Drainage, 53(3), 251-261.
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  Abstract: An accurate irrigation scheduling methodology is necessary in crops with high water stress sensitivity and production cost. This involves the estimation of the scheduling parameters related with crop water requirements during phenological stages under different irrigation application systems. This paper presents several models to represent the parameters of irrigation scheduling based on growing degree days (GDD) such as: management allowed soil water depletion (MAD), rooting depth (Rd) and crop coefficient (K c). The proposed models were applied accurately to schedule irrigation in two commercial fields of potato under two irrigation methods: surface and sprinkler irrigation. Results show that the model predicted irrigations in a consistent and logical manner. The proposed models are versatile, feasible and easily implemented in irrigation scheduling computer programs. Copyright © 2004 John Wiley & Sons, Ltd.
• Rawahy, S., Larson, D. L., Walworth, J., & Slack, D. C. (2003). Effect of an electrical input with drip irrigation of nitrate distribution in soil. Applied Engineering in Agriculture, 19(1), 55-58.
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  Abstract: Soluble chemicals applied to soils have been identified as major sources of surface and groundwater contamination. This research examined the application of a small dc electrical input to attract and retain nitrates in the root zone of drip irrigated barley in small lysimeter trials. Nitrate content near the anode was higher and pH lower, as desired, during a portion of the test. However, the electrical input seemed to have little effect on nitrate distribution at other times, perhaps due to careful management of water and nitrate inputs. These results indicate a potential benefit of an electrical input to the control of nitrate transport in soil.
• Martin, E. C., Oliveira, A. D., Folta, A. D., Pegelow, E. J., & Slack, D. C. (2001). Development and testing of a small weighable lysimeter system to assess water use by shallow-rooted crops. Transactions of the American Society of Agricultural Engineers, 44(1), 71-78.
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  Abstract: A mobile weighing system was designed in 1994 to weigh small lysimeters to measure water use by shallow-rooted crops. Using a reconditioned pesticide sprayer, a hoist, and a weighmeter, small lysimeters were weighed in the 1995, 1996, and 1997 growing seasons. The lysimeters were constructed of 4.8-mm hot-rolled steel and were 0.91×1.02×0.61 m in size with an internal area of 0.929 m2. The weight of a lysimeter containing only moist soil was 1,110.27 kg. At the beginning of each season of use, several tests were conducted on the weighing system to determine if it was sensitive enough to give adequate results of crop water use. In 1995, tests were conducted on linearity, repeatability, thermal shift, and creep errors. The values were ±0.062 kg, ±0.12 kg, -0.007 kg/° C, and-0.242 kg, respectively. The terminal linearity was 0.082 kg (1996) and 0.043 kg (1997). The measurement uncertainty decreased as the number of lifts increased. The results showed that the system was capable of producing adequate results for determining crop water use.
• Eid, N., Elshorbagy, W., Larson, D., & Slack, D. (2000). Electro-migration of nitrate in sandy soil. Journal of Hazardous Materials, 79(1-2), 133-149.
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  PMID: 11040391;Abstract: Migration of nitrate to groundwater has become a serious threat in many agricultural areas. This paper presents the results of experimental laboratory tests studying the nitrate gradient developed in response to an electrical potential. Two systems were tested; the first had no flow (closed system) and the second had flow opposite to the direction of the electrical current. A solution of sodium nitrate in sandy soil was used in both systems. The tests showed that the electro-kinetic process effectively concentrated and retained nitrate close to the anode. The movement of NO3/- through the soil column was significantly influenced by the development of a pH gradient. Statistical analysis was performed to determine best-fit equations relating the nitrate gradient to the electrical input and pH gradient. A simple one-dimensional finite difference model was used to predict the pH gradient developed during the electro-kinetic process. The experimental measurements closely agreed with the predicted spatial and temporal distribution of the nitrate gradient for both closed and open system configurations. (C) 2000 Elsevier Science B.V.
• Eid, N., Slack, D., & Larson, D. (2000). Nitrate electromigration in sandy soil: Closed system response. Journal of Irrigation and Drainage Engineering, 126(6), 389-397.
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  Abstract: Contamination of surface and subsurface waters has become a major problem related to agricultural practices. Nitrate, one of the principal plant fertilizers, is the chemical having the greatest effect. This research utilized closed system laboratory experiments to evaluate the effectiveness of an electrokinetic process in concentrating and retaining nitrates close to the anode and studied the effect of selected electrokinetic system design parameters on the performance of the process. The tests demonstrated that an electrokinetic process can effectively concentrate and retain nitrate close to the anode in saturated sandy soil.
• Elshorbagy, W. E., Eid, N. M., Larson, D., & Slack, D. (2000). Electrochemical remediation of nitrate-contaminated sandy soil. Geoengineering in arid lands. Developments in arid regions research 1., 625-631.
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  Abstract: Migration of nitrate to ground water has become a serious threat in many agricultural areas. This paper presents the results of experimental laboratory tests studying the nitrate gradient developed in response to an electrical potential. Two systems were tested; the first had no flow (closed system) and the second had flow opposite to the direction of the electrical current. A solution of sodium nitrate in sandy soil was used in both systems. The tests showed that the electrokinetic process effectively concentrated and retained nitrate close to the anode. The movement of NO3- through the soil column was significantly influenced by the development of a pH gradient. Statistical analysis was performed to determine best-fit equations relating the nitrate gradient to the electrical input and pH gradient. A simple one-dimensional finite difference model was used to predict the pH gradient developed during the electrokinetic process. The experimental measurements closely agreed with the predicted spatial and temporal distribution of the nitrate gradient for both closed and open system configurations.
• Rawahy, S. A., Larson, D. L., Walworth, J., & Slack, D. C. (2000). Effect of an Electrical Input with Drip Irrigation on Nitrate Distribution in Soil. 2000 ASAE Annual International Meeting, Technical Papers: Engineering Solutions for a New Century, 2, 2245-2261.
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  Abstract: Soluble chemicals applied to soils have been identified as major sources of surface and groundwater contamination. This research examined the application of a small dc electrical input to attract and retain nitrates in the root zone of drip irrigated barley in small lysimeter trials. Nitrate content near the anode was higher and pH lower, as desired, during a portion of the test. However, the electrical input seemed to have little effect on these conditions at other times, a result not fully understood.
• Eid, N., Larson, D., Slack, D., & Kiousis, P. (1999). Nitrate electromigration in sandy soil in the presence of hydraulic flow. Journal of Irrigation and Drainage Engineering, 125(1), 7-11.
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  Abstract: Chemicals applied to agricultural lands inevitably move below the plant root zone and may contaminate the underlying ground-water reservoirs. Laboratory open system soil column experiments were conducted to evaluate electromigration as a process for concentrating and retaining the nitrates close to the anode in soil subjected to hydraulic flow. Electromigration was found to be an effective means for concentrating and retaining nitrate close to the anode in saturated sandy soil at low flow rates. However, for a given electrical input, the effect was reduced as the hydraulic flow rate increased, being indiscernible for higher flow rates.
• Mishra, A., & Slack, D. C. (1998). Effect of flow variability and sedimentation in a canal reach on turnouts, flow rate and lateral spill. Water and Energy International, 55(3), 41-54.
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  Abstract: Flow fluctuation in a canal reach occurs due to variation of flow rate at the headworks, by gate settings and by varying the water demands. These flow variations ultimately get transmitted to turnouts flow rate and lateral spill. The degree of non-uniformity in flow deliveries amongst the turnouts because of flow variability in a reach (due to various possible reasons) and sedimentation in a reach has been studied in a simple hypothetical canal system. Simulations of hydraulic parameters have been done using the model CANALMAN developed at Utah State University, USA. Structural and operational improvements which can bring reduction in the non-uniformity in flow distribution have also been discussed.
• Martin, E. C., Hla, A. K., Waller, P. M., & Slack, D. C. (1997). Heat unit-based crop coefficient for grapefruit trees. Applied Engineering in Agriculture, 13(4), 485-489.
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  Abstract: The onset and rate of sap moving up the branches of grapefruit (Citrus paradisi Macfadyen) trees were monitored hourly using portable sap flow sensors at Waddell, Arizona. Hourly reference evapotranspiration (ETo) estimates were calculated using data from a nearby weather station. Crop water use was estimated from soil moisture measurements using a neutron probe. These data were used to first delineate the upper and lower temperature threshold values for the determination of heat units. A heat unit-based crop coefficient was then derived from a correlation of the crop coefficient with heat units over a crop year. The heat unit-based crop coefficient was found to be similar to crop coefficients derived by other researchers.
• Oliveira, A. S., Martin, E. C., Slack, D. C., & Pagelow, E. J. (1997). Use of the subsurface drip irrigation in the determination of lettuce water requirement in an arid environment. Paper - American Society of Agricultural Engineers, 2.
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  Abstract: Winter head lettuce water use was measured using a weighing lysimeter system at the Maricopa Agricultural Center of the University of Arizona. Crop coefficient curve based on the FAO Penman method, FAO Penman-Monteith method, and Hargreaves method was developed with accumulated growing-degree day as normalizing factor.
• Sanchez-Cohen, I., Lopes, V. L., Slack, D. C., & Fogel, M. M. (1997). Water balance model for small-scale water harvesting systems. Journal of Irrigation and Drainage Engineering, 123(2), 123-127.
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  Abstract: This paper describes a simple, few-parameters simulation model for a water harvesting-strip farming system. The model includes runoff threshold functions for different soil types, and for different initial soil water contents. The evapotranspiration component of the model accounts for dynamic root water extraction using regression equations that correlate canopy cover with root growth as a function of time. The model is simple to operate and uses readily available inputs. It can work with several crops when leaf area indices and crop stage coefficients are provided. The agreement between simulation outputs and field-observed data indicates that the model accurately describes the water balance within the system. The model is then recommended for obtaining preliminary design of water harvesting systems. A FORTRAN77 source code has been written for the simulation procedure.
• Silva, B. d., & Slack, D. C. (1997). Diurnal and seasonal variation of CWSI with cotton in northeast of Brazil. Paper - American Society of Agricultural Engineers, 2.
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  Abstract: A field experiment was conducted with a cotton crop subjected to two different irrigation treatments. The objective was to evaluate diurnal and seasonal variation of Crop Water Stress Index - CWSI based on the energy balance and its feasibility in irrigation water management. Measurements of canopy and air temperature, net radiation, wind speed, and water vapor pressure were made daily at 9h30 and 13h30, and many diurnal cycles at hourly interval between 6 h to 17 h. Porometric measurements were also used to evaluate the crop resistance. It was concluded that irrigation should be made when the CWSI reaches 0.3.
• Silva, B. d., Slack, D. C., Oliveira, A. S., & Netto, J. A. (1997). Energy balance in a vineyard field under seriarid condition in northeast of Brazil. Paper - American Society of Agricultural Engineers, 2.
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  Abstract: Results of a field experiment with table grapes in Petrolina, Brazil, are presented in this paper. Net radiometers, solarimeters, soil heat flux plates, psychrometes, leaf area meter, porometer, and tensiometers were user in order to obtain the components of the energy balance, the actual evapotranspiration, according to Bowen Ratio Method, and the soil water balance. Some daily cycles of the components of energy balance at 10 min interval were presented.
• Thoreson, B. P., Slack, D. C., Satyal, R. P., & Neupane, R. S. (1997). Performance-based maintenance for irrigation systems. Journal of Irrigation and Drainage Engineering, 123(2), 100-105.
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  Abstract: A framework for determining the effect of maintenance events on irrigation system flows is described. Standard definitions for corrective and preventive maintenance are presented and two maintenance objectives and six classifications are established. Maintenance activities and decision criteria common to many irrigation systems are suggested. A format for describing these and other maintenance activities is proposed. A methodology for setting decision levels for maintenance activities is presented. Maintenance cost is compared with income lost as a result of less than maximum production because water supplied was insufficient for crop requirements. This comparison demonstrates that maintenance decision levels should be set so that maximum evapotranspiration can be achieved. Budget request forms and report forms are presented with examples of actual maintenance events showing the expected and actual impact on system flows.
• Cairo, G., Larson, D., & Slack, D. (1996). Electromigration of nitrates in soil. Journal of Irrigation and Drainage Engineering - ASCE, 122(5), 286-290.
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  Abstract: This project evaluated electroreclamation utilizing subsurface horizontal drainage tubing and parallel electrodes to concentrate and remove nitrates from soil. Nitrate concentration in saturated soil tended to increase from anode to cathode after the application of a small direct-current electrical field as the nitrate solution moved toward the cathode. When soil moisture content dropped below the saturation level, the nitrate began to migrate toward the anode.
• Guerra, A. F., & Slack, D. C. (1996). Aerodynamic resistance: Effect of the methods used to specify the vegetated surface roughness parameters. Pesquisa Agropecuaria Brasileira, 31(10), 673-681.
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  Abstract: The purposes of this study were to determine the surface roughness parameters for momentum (z0m) and heat (z0h) for bermudagrass and to examine the effect of using different criteria for selecting these surface roughness parameters on the calculation of the aerodynamic resistances. Analysis of the data collected over bermudagrass suggests that z0m, is virtually constant at wind speeds greater than 2 m/s, but increases rapidly as wind speeds decrease below 2 m/s. For wind speeds larger than 2 m/s it resulted in an average z0m of 0.0006 m and an average z0h of 0.00015 m, which is about 1/7.6 of z0m. The use of the different criteria to specify the surface roughness parameters resulted in considerable differences in the calculation of the aerodynamic resistances.
• Akhand, N. A., Larson, D. L., & Slack, D. C. (1995). Canal irrigation allocation planning model. Transactions - American Society of Agricultural Engineers, 38(2), 545-550.
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  Abstract: A water allocation model was developed to recommend allocation of irrigation water to different crop fields in a canal-based irrigation project. Model components are an irrigation scheduling program to predict irrigation water demands, a crop response model to compute crop yields, and a canal delivery model to check the physical feasibility of water delivery. Allocation constraints are irrigation water demand, irrigation water availability, canal delivery capacity, minimum irrigation limitations, and crop response model limitations. -from Authors
• Cohen, I. S., Lopes, V. L., Slack, D. C., & Yanez, C. H. (1995). Assessing risk for water harvesting systems in arid environments. Journal of Soil & Water Conservation, 50(5), 446-449.
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  Abstract: Climate variability is perhaps the most common and unpredictable problem that farmers in dryland regions have to face year by year. This fact has established the need for developing methodologies for coping with water scarcity. One of the most widely used strategies has been the implementation of water harvesting systems. The efficiency of the system is strongly associated with its expensiveness and therefore, risk assessment should be an integral part of any project involving water harvesting systems. This paper focuses on a review of methodologies that have been used for assessing risk in small scale water harvesting systems. A simulation-modeling stochastic approach is discussed highlighting the concept of risk behavior. -from Authors
• Hussein, I. A., & Slack, D. C. (1994). Fruit diameter and daily fruit growth rate of three apple cultivars on rootstock-scion combinations. HortScience, 29(2), 79-81.
• Jalali-Farahani, H., Slack, D. C., Kopec, D. M., Matthias, A. D., & Brown, P. W. (1994). Evaluation of resistances for bermudagrass turf crop water stress index models. Agronomy Journal, 86(3), 574-581.
• Akhand, N. A., Larson, D. L., & Slack, D. C. (1993). Irrigation allocation planning model. Management of Irrigation and Drainage Systems : Integrated Perspectives, 842-849.
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  Abstract: An irrigation allocation model using multiperiod linear programming was developed to recommend canal water allocation among different irrigated fields to maximize net annual benefits for given conditions. These conditions include irrigation water demands for each field, crop response functions, water supply quantities, canal carrying capacity, water cost, crop values and minimum irrigation limitations for surface irrigation systems. The model was validated using the soils, crops, canal description and management data of the Maricopa Agricultural Center (MAC), University of Arizona.
• Cairo, G., Larson, D. L., & Slack, D. C. (1993). Electro-osmotic removal of nitrates from soils. Management of Irrigation and Drainage Systems : Integrated Perspectives, 629-635.
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  Abstract: Nitrate migration to ground and surface waters is a serious U.S. contamination threat. Electro-osmosis may be a more effective means for soil reclamation than leaching or chemical treatment. A field lysimeter was constructed to evaluate use of electro-osmosis to concentrate nitrates in and remove nitrates from soils. Tests are being conducted using different electrode materials and electrical voltages.
• Fox, F. A., Slack, D. C., Clark, L. J., & Scherer, T. F. (1993). AZSCHED - software for real-time irrigation scheduling. Management of Irrigation and Drainage Systems : Integrated Perspectives, 891-898.
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  Abstract: A user-friendly computer based scheduling program was developed to minimize the amount of data which needs to be input by the user by utilizing a network of twenty automated weather stations in Arizona. Irrigation scheduling is performed using a soil water balance approach. The Modified Penman equation is used to calculate reference evapotranspiration and crop coefficients are based on growing-degree-days. These methods make this model highly adaptable to a variety of climatological conditions without loss of accuracy. Accuracy can be compromised through the use of inaccurate inputs by the user. The sensitivity of the model to different user inputs under Arizona conditions are presented, showing that the model is most sensitive to the inputs used for irrigation efficiency and applied irrigation volume. Recommendations for rational use of this method require either measurement of irrigation volumes applied or periodic correction of the soil water balance through field measurements.
• Jalali-Farahani, H., Slack, D. C., Kopec, D. M., & Matthias, A. D. (1993). Crop water stress index models for bermudagrass turf: A comparison. Agronomy Journal, 85(6), 1210-1217.
• Ojeda, W., Copeland, S. M., & Slack, D. C. (1993). CPIVOT v 2.0 center pivot analysis and design software. Management of Irrigation and Drainage Systems : Integrated Perspectives, 883-890.
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  Abstract: A micro-computer based software package is presented which can be used for hydraulic design and/or analysis of center pivot irrigation systems. The software is written in C and requires a computer of 640 Kilobytes or more of RAM to operate. It can be used to select required system flow rate and lateral size and will select nozzle size and location using either a fixed spacing between nozzles, a variable spacing or a combination of both. The analysis option provides pressure distribution along the lateral, application uniformity, application depth along the lateral and application rate versus time at any point along the lateral. Each of the outputs of the analysis may be presented in tabular and/or graphical form. The software is designed with 'pull-down' menus which make it easy to use with a mouse or interaction with the user can be via the keyboard. Results of an example analysis and design problem are presented.
• Thoreson, B., Slack, D. C., & Molden, D. J. (1993). Availability analysis of irrigation systems for improved management. Management of Irrigation and Drainage Systems : Integrated Perspectives, 1154-1161.
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  Abstract: Availability of an irrigation system is defined as the ratio of time of successful system operation to total desired operation time. Operation is successful when the flow rate of water supplied exceeds a given percentage of the lesser of crop water requirements or design flow rate. Thus, availability provides adequacy and dependability information. Furthermore, this ratio can be decomposed into unavailabilities indicating the percentage of time low flow was caused by various problems. This is valuable information for improving water delivery performance. Over 12 seasons availability varied from 0.0 to 0.90 in two Nepal irrigation systems. Canal capacity less than crop water requirements was the greatest reason for unavailability during the monsoon season. Lack of water in the source was the greatest reason during other seasons.
• Ottoni, T., Matthias, A., Guerra, A., & Slack, D. (1992). Comparison of three resistance methods for estimating heat flux under stable conditions. Agricultural and Forest Meteorology, 58(1-2), 1-18.
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  Abstract: Aerodynamic resistance to sensible heat transfer (rah) is an important parameter in bulk resistance models of energy exchange between the Earth's surface and the atmosphere. To estimate sensible heat flux density (H) over irrigated fields, it is often necessary to evaluate rah under stable atmospheric conditions. This is because irrigated fields are often cooler than air temperature as a result of net radiative cooling at night and advective influences. Sensible heat fluxes are generally small at night, but may become a relatively large component of the surface energy budget during advective conditions. The purpose of this paper was to compare H modeled for stable conditions over an extensive area of irrigated bermudagrass (Cynodon dactylon) cv. Midiron) using three rah methods from the literature, and to determine the sensitivities of the methods to the various parameter inputs. In addition to comparing modeled H values with each other, comparisons were made with H values measured by eddy correlation over the bermudagrass. The three methods studied were expressed as functions of the bulk Richardson number (RiB). The methods differ mainly in the stability correction function, and the added resistance term (kB-1, where k is von Kármán's constant and B-1 is a non-dimensional bulk parameter), which arises as a result of dissimilarities between heat and momentum transfer mechanisms near surface obstacles. Application of the methods to the bermudagrass data resulted in marked differences between the various modeled H values, and between modeled and measured H values. The modeled H values were more sensitive to surface emissivity and temperature inputs. © 1992.
• Kopec, D. M., Brown, P. W., Mancino, C. F., & Slack, D. C. (1990). Developing crop coefficients for desert turfgrass calibrating reference ET with turf water use. ASAE Publication, 181-185.
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  Abstract: A program is underway by the University of Arizona to empirically determine water use rates for cool and warm season turf grasses, and to compare these water use rates with an atmospheric (reference) demand for water vapor in order to calculate crop coefficients (Kc) values for desert turfs. These Kc values will be used in the future with near real time evapotranspiration (ET) estimates to calculate soil moisture replacement values available for use by turf managers. This type of public program is being developed to promote water conservation among turfgrowers and clientele in Arizona. This paper deals with the protocol for developing and evaluating turfgrass Kc values.
• Scherer, T. F., Fox Jr., F., Slack, D. C., & Clark, L. (1990). Near real time irrigation scheduling using heat unit based crop coefficients. Array, 544-551.
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  Abstract: An interactive computer program (SCHEDPEN) which utilizes growing-degree-day based crop coefficients for irrigation scheduling was developed and tested on cotton during 1988 and 1989 at two locations in Arizona. The program can directly accept weather data files from Arizona's automated agricultural weather station network (AZMET) via phone access. At both locations the program was compared to two other methods of irrigation scheduling and while yield and water use were not significantly different among the three methods, some algorithmic errors were found in SCHEDPEN which have been corrected. The program continues to be developed for additional crops and incorporation of additional management information.
• Scherer, T. F., Slack, D. C., Clark, L., & Fox, F. (1990). Comparison of three irrigation scheduling methods in the arid southwestern US. ASAE Publication, 287-291.
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  Abstract: During the 1988 and 1989 growing seasons, three irrigation scheduling methods were compared at two locations in Arizona. Cotton was used as the test crop in a randomized complete block statistical design with three treatments and three replications. The three methods were; a soil water balance model based on historic consumptive use curves; a soil water balance model based on the Modified Penman Equation using daily weather from the Arizona Meteorological Network (AZMET); and infrared thermometry using the Crop Water Stress Index (CWSI). Yield results at both locations and both years show there was no significant statistical difference between the three methods. However, both years there was a significant difference at one location in the total amount of irrigation water applied by one of the scheduling methods. Although one method used more water than the others, all three methods of irrigation scheduling could be used by growers to improve their irrigation water management.
• Slack, D. C., Kopec, D. M., Brown, P. W., Matthias, A. D., & Jalali-Farahani, H. (1990). Evaluation and comparison of crop water stress index models for irrigation scheduling of Bermuda grass turf. ASAE Publication, 725-730.
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  Abstract: A simple two-parameter regression model expressing canopy-air temperature difference for a nonwater-stressed vegetative surface as a function of vapor pressure deficit is used to evaluate an empirical crop-water-stress-index (CWSI) for irrigation scheduling of Bermuda grass turf. The present study compares two threshold CWSIs of 0.20 and 0.40 for initiating irrigation. A third treatment using a 'critical temperature' is also being evaluated. Whenever irrigation is indicated by any of the methods, the treatment lysimeter is irrigated with three liters of water (equivalent to 12.5mm). Soil moisture is monitored with the neutron probe and drainage is collected to complete a water balance.
• Slack, D. C., Killen, M. A., Berglund, E. R., & Onstad, C. A. (1988). Application of the Green-Ampt-Mein-Larson infiltration model to taconite tailings. Transactions of the American Society of Agricultural Engineers, 31(5), 1455-1461.
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  Abstract: Experimental results show that the coarse and medium fractions of the tailings material have porosities which are much higher than that of natural soils of similar particle size resulting in higher internal drainage and higher infiltration capacities. The finer fractions have high bulk densities with low infiltration capacities. The W.H.Green-G.Ampt-R.G.Mein-C.L.Larson (GAML) model predicted infiltration rates and infiltrated volumes with reasonable accuracy under the experimental conditions studied. The model is quite sensitive to the degree of saturation encountered under field conditions. However, this can be estimated depending on material size and properties. Best agreement between observed and predicted infiltration was obtained when the model was 'fitted' using degree of saturation as the fitting parameter. The model should prove to be a valuable tool for predicting runoff and seepage from such materials.
• Clanton, C. J., & Slack, D. C. (1987). HYDRAULIC PROPERTIES OF SOILS AS AFFECTED BY SURFACE APPLICATION OF WASTEWATER.. Transactions of the American Society of Agricultural Engineers, 30(3), 683-687.
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  Abstract: Wastewater was applied to three soils to determine the sealing effect or change in hydraulic properties. Saturated conductivities of disturbed soils were measured weekly under laboratory conditions over a 40-wk period. For the Lester clay loam and Waukegan silt loam, saturated conductivities of the wastewater seal. In Hubbard loamy sand, an immediate seal was formed due to the application of wastewater.
• Killen, M. A., & Slack, D. C. (1987). Green-Ampt-model to predict surge irrigation phenomena.. J. IRRIG. & DRAIN. ENGNG. (ASCE), 113(4 , Nov. 1987), 575-584.
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  Abstract: Considers the effects of intermittent wetting on instantaneous infiltration rates for a silty clay loam and a sandy loam. The Green-Ampt model combined with a simple redistribution model is used to illustrate the effects of reduced wetting front suction due to intermittent wetting on the instantaneous infiltration rate. The effect of various surge cycle times on the volume infiltrated versus time is also predicted by the model. This approach to modelling infiltration under surge irrigation shows considerable potential for determining the effect of surge irrigation on a range of soil textures, and as a design tool for optimizing surge cycle times.
• Almeida, J. A., & Slack, D. C. (1986). USE OF INFRARED THERMOMETRY TO MEASURE CANOPY-AIR TEMPERATURE DIFFERENCE AT PARTIAL COVER.. Paper - American Society of Agricultural Engineers.
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  Abstract: The purpose of this research was to develop and evaluate a method for estimating canopy temperatures under partial canopy situations and to do a sensitivity analysis on Heilman's equation in order to study the sensitivity of the projected canopy temperature to parameters used in its prediction. From this study we have been able to show that the predicted canopy temperature is extremely sensitive to composite and soil temperatures, is only moderately sensitive to canopy cover and emissivity and very insensitive to soil emissivity and sky irradiance. In addition, this study also shows that it might be possible to extract canopy temperature from composite temperatures but also that additional studies are needed in order to more precisely understand the effects of the parameters to which the extracted canopy temperature is sensitive and to improve accuracy of evaluating required parameters.
• Clanton, C. J., Slack, D. C., & Shaffer, M. J. (1986). EVALUATION OF THE NTRM MODEL FOR LAND APPLICATION OF WASTEWATER.. Transactions of the American Society of Agricultural Engineers, 29(5), 1307-1313.
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  Abstract: The Nitrogen Tillage Residue Management (NTRM) model was evaluated as a means of predicting moisture movement, matrix potential and nitrate concentrations following land application of watewater. The mass transport of water and nitrogen and the nitrogen transformations within the soil profile were measured in loamy sand and clay loam soil columns. Moisture movement, matrix potential and nitrate concentrations were compared between the model and experimental results.
• Jalali-Farahani, H., Slack, D. C., Matthias, A. D., & Kopec, D. M. (1986). CROP WATER STRESS PARAMETERS FOR TURFGRASS AND THEIR ENVIRONMENTAL DEPENDABILITY.. Paper - American Society of Agricultural Engineers.
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  Abstract: The empirical and theoretical crop water stress index (CWSI) were evaluated for Bermuda Turfgrass over a range of water-stressed conditions. The empirical CWSI gave inconsistent estimates which were moderately correlated with net radiation. The theoretical CWSI were consistent, but different results were obtained by applying the simple energy balance and Penman-Monteith equations for evapotranspiration.
• Slack, D. C., Jalali-Farahani, H., Kopec, D. M., & Matthias, A. D. (1986). PREDICTING TURFGRASS EVAPOTRANSPIRATION FROM CANOPY TEMPERATURE.. Paper - American Society of Agricultural Engineers.
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  Abstract: The purpose of the research described in this paper was: to estimate evapotranspiration (ET) of Bermuda turfgrass (Cynodon dactylon, Midiron) in Tucson, Arizona using a surface energy balance approach; and to evaluate the method by comparison to daily ET values obtained from non-weighting lysimeters. The surface energy balance equation was solved for the latent heat term to estimate ET from Bermuda Turfgrass over a range of stress conditions. Resulting ET estimates were within 13% of measured ET values using empirically determined aerodynamic resistance values.
• Clanton, C. J., Slack, D. C., & Hansen, B. J. (1985). EFFECT OF WASTEWATER ON SOIL HYDRAULIC PROPERTIES.. Paper - American Society of Agricultural Engineers.
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  Abstract: Wastewater was applied to three soils to determine the sealing effect or change in hydraulic properties. Saturated conductivities were measured weekly over a 40-week period. For the Lester clay loam and Waukegan silt loam, saturated conductivities of the soils was less than the saturated conductivities of the wastewater seal. In Hubbard loamy sand, an immediate seal was found due to the application of wastewater. The saturated conductivity continued to decrease for the 12-week period during the wastewater application. When wastewater application terminated, an immediate increase in the saturated conductivity occurred. Following the end of wastewater application, the saturated conductivity became greater than in a normal soil, indicating an improvement in hydraulic properties.
• Slack, D. C., & Moon, J. W. (1985). WATER REQUIREMENTS OF ULTRA-HIGH DENSITY APPLES IN SOUTHEAST ARIZONA.. Paper - American Society of Agricultural Engineers.
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  Abstract: Soil Moisture Balance Techniques were employed using a neutron probe to evaluate mid-season water use by high density 'Granny Smith' Apples in southeast Arizona. Mid-season crop coefficients (ET/ETr) for mature apples were estimated at 1. 57. The annual irrigation requirement for mature apples is about 2300mm.
• Slack, D. C., Berglund, E. R., & Killen, M. A. (1985). INFILTRATION CHARACTERISTICS OF TACONITE TAILING MATERIAL.. Transactions of the American Society of Agricultural Engineers, 28(5), 1482-1486.
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  Abstract: Hydraulic and physical characteristics of taconite tailings materials from several tailings basins in Northeast Minnesota were evaluated using laboratory and field techniques. A particle density of 3,085 g/cm**3 was determined as representative of all materials evaluated. Infiltration capacities ranges from 1. 09 cm/h on fine basin material to more than 12. 7 cm/h on unvegetated coarse dike material. Both hydraulic and physical properties exhibited much less variability than natural soils. Vegetated coarse dike surfaces had significantly higher infiltration capacities than similar unvegetated surfaces. However, there was no significant difference in infiltration capacities between vegetated and unvegetated medium and fine materials within the tailing basins.
• Slack, D. C., Killen, M. A., & Berglund, E. R. (1985). Infiltration characteristics of taconite tailing material.. TRANS. AM. SOC. AGRIC. ENGRS. (GEN. EDN.), 28(5 , Sep.-Oct. 1985, p.1482-1486.).
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  Abstract: Hydraulic and physical characteristics of taconite tailings materials from several tailings basins in Northeast Minnesota were evaluated using laboratory and field techniques. A particle density of 3.085 g/cm SUP 3 was determined as representative of all materials evaluated. Infiltration capacities ranged from 1.09 cm/h on fine basin material to more than 12.7 cm/h on unvegetated coarse dyke material. Both hydraulic and physical properties exhibited much less variability than natural soils. Vegetated coarse dyke surfaces had significantly higher infiltration capacities than similar unvegetated surfaces. However, there was no significant difference in infiltration capacities between vegetated and unvegetated medium and fine materials within the tailing basins. (A)
• Knoch, B. C., Slack, D. C., & Larson, C. L. (1984). PREDICTING DIRECT RECHARGE OF SURFICIAL AQUIFERS.. Transactions of the American Society of Agricultural Engineers, 27(6), 1739-1744.
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  Abstract: A one-dimensional, physically-based computer model was developed for predicting direct groundwater recharge. Although the processes of infiltration and redistribution during frozen soil periods were not modeled, the model is capable of operating during both frozen and nonfrozen soil periods. The model includes submodels for evapotranspiration, soil-water extraction, snowmelt, surface depressional storage, infiltration and redistribution. The model predicts water table level and soil moisture. Water extraction may also be modeled. The model predicted both water table elevations and soil moisture levels with reasonable accuracy over the three-year period modeled.
• Onstad, C. A., Wolfe, M. L., Larson, C. L., & Slack, D. C. (1984). Tilled soil subsidence during repeated wetting.. Transactions - American Society of Agricultural Engineers, 27(3), 733-736.
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  Abstract: A total of 15.2 cm of water was applied without raindrop impact energy to six replicates of four freshly tilled soils to determine its effect on subsidence, random roughness, bulk density, and hydraulic conductivity. The results are described for each variable. -from Authors English
• Slack, D. C., Berglund, E. R., Killen, M. A., Slack, D. C., Berglund, E. R., & Killen, M. A. (1984). INFILTRATION CHARACTERISTICS OF TACONITE TAILING MATERIAL.. Paper - American Society of Agricultural Engineers.
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  Abstract: Hydraulic and physical characteristics were evaluated using field and laboratory techniques for several tailings basins in Northeast Minnesota. A particle density of 3. 085 gm/cm**3 was determined as representative of all materials evaluated. Infiltration capacities ranged from 1. 09 cm/hr on fine basin material to more than 12. 7 cm/hr on unvegetated coarse dike material.
• Slack, D. C., Eshenaur, W. C., & Berkas, T. H. (1984). PREDICTING THE PERFORMANCE OF A WATER-PUMPING HYDRAULIC RAM.. International Journal for Development Technology, 2(4), 261-271.
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  Abstract: This paper reports on research aimed at developing and evaluating a theoretical method of analyzing the performance characteristics of the water-pumping hydraulic ram. Results indicate that analytical solutions of one-dimensional equations, which describe unsteady flow during the drive and pumping cycles, lead to relationships that can be conveniently used for an accurate prediction of hydram characteristics under low drive heads. These relationships, however, do not perform as well under higher drive heads, and the primary reason for which would appear to be the simplifying assumptions that have to be made so that an analytical solution can be found. In particular, the assumption of constant drive and delivery valve loss coefficients is thought to be the primary source of error. An alternative approach for overcoming this difficulty is discussed.
• Scherer, T. F., Slack, D. C., & Eshenaur, W. C. (1983). AUTOMATED DATA COLLECTION FOR IRRIGATION SCHEDULING USING CANOPY - AIR TEMPERATURE DIFFERENCE.. Paper - American Society of Agricultural Engineers.
• IDIKE, F. I., LARSON, C. L., & SLACK, D. C. (1982). MODELING SOIL MOISTURE AND EFFECTS OF BASIN TILLAGE.. TRANS AM SOC AGRIC ENG (GEN ED), V 25(N 5), 1262-1267.
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  Abstract: THE STUDY DESCRIBED WAS CONDUCTED TO DEVELOP A MODEL CAPABLE OF PREDICTING SOIL MOISTURE LEVELS IN A FIELD UNDER CORN PRODUCTION THROUGHOUT THE GROWING SEASON, AND TO DETERMINE BYTHE USE OF THIS MODEL WHETHER BASIN TILLAGE CAN INCREASE SOIL MOISTURE LEVELS IN AREAS WHERE RAINFALL DURING THE GROWING SEASON IS LIMITING.
• Idike, F. I., Larson, C. L., & Slack, D. C. (1982). MODELING EFFECTS OF SURFACE CONFIGURATION ON SOIL MOISTURE STORAGE, EVAPOTRANSPIRATION AND RUNOFF.. Array, 221-234.
• Idike, F. I., Larson, C. L., & Slack, D. C. (1982). Modeling soil moisture and effects of basin tillage.. TRANS. AM. SOC. AGRIC. ENGRS. (GEN. EDN.), 25(5 , Sep.-Oct. 1982, p.1262-1267.).
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  Abstract: A soil-water model was developed and tested to study the potential effects of basin tillage on soil moisture levels at one location in Minnesota and another in south central Texas, U.S.A. The model was found to satisfactorily predict soil moisture levels and, at both locations, application of the model indicated that basin tillage increased soil moisture and decreased crop stress compared with conventional tillage, due to surface storage and infiltration of excess precipitation during the growing season, off-season, or both. (P.M.T.)
• Slack, D. C., Berkas, T. H., & Eshenaur, W. C. (1982). THEORETICAL AND EXPERIMENTAL CHARACTERISTICS OF THE WATER PUMPING HYDRAULIC RAM.. Paper - American Society of Agricultural Engineers.
• Wilson, B. N., Slack, D. C., & Young, R. A. (1982). A comparison of three infiltration models.. Transactions, American Society of Agricultural Engineers, 25(2), 349-356.
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  Abstract: Three physically based infiltration models are compared both in terms of theoretical development and performance. The models are an unmodified Green-Ampt-Mein-Larson (GAML-UNMOD) model, a similar model which was modified to account for entrapped air (GAML-ETA model) and a model which considers both air entrapment and air resistance effects (GAML-ART model). -from Authors
• Zirbel, M. L., Hirschi, M. C., Larson, C. L., Slack, D. C., & Young, R. A. (1982). FIELD MEASUREMENT OF GREEN-AMPT INFILTRATION PARAMETERS.. Paper - American Society of Agricultural Engineers.
• Zirbel, M. L., Larson, C. L., & Slack, D. C. (1982). LABORATORY METHOD FOR EVALUATING GREEN-AMPT INFILTRATION PARAMETERS.. Paper - American Society of Agricultural Engineers.
• Idike, F. I., Larson, C. L., & Slack, D. C. (1981). MODELING EFFECTS OF SURFACE CONFIGURATION ON SOIL MOISTURE STORAGE, EVAPOTRANSPIRATION AND RUNOFF.. Array, 116-117.
• Moore, I. D., Larson, C. L., Slack, D. C., Wilson, B. N., Idike, F., & Hirschi, M. C. (1981). Modelling infiltration: A measurable parameter approach. Journal of Agricultural Engineering Research, 26(1), 21-32.
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  Abstract: Development of a 2-stage, measured parameter, infiltration model is reviewed. The model has been verified against finite difference solutions of the unsaturated flow equation, by laboratory tests and has been evaluated under field conditions. Progress of recent research utilizing this model is discussed. Results of recent research indicate that the effects of air entrapment on hydraulic conductivity must be taken into account. A reasonable approach to this problem is to evaluate relative conductivity as a function of relative moisture content and use this relationship to evaluate hydraulic conductivity at field saturation. Additional factors which have been investigated include air viscous effects and the effects of tillage and surface sealing. While significant progress has been made toward including these effects in the model, additional research is required. An attractive attribute of the model is that all parameters utilized in the model are measurable and thus no parameter fitting is required. The paper discusses how parameters may be evaluated and how the model operates, and presents some examples of model predictions under different conditions of soil composition and rainfall pattern. © 1981.
• Slack, D. C., Geiser, K. M., Stange, K. W., & Allred, E. R. (1981). IRRIGATION SCHEDULING IN SUBHUMID AREAS WITH INFRARED THERMOMETRY.. ASAE Publication, 116-124.
• Wilson, B. N., Slack, D. C., & Larson, C. L. (1981). INFILTRATION MODEL: DEVELOPMENT AND EVALUATION OF ITS PARAMETERS.. Transactions of the American Society of Agricultural Engineers, 24(3), 670-677, 683.
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  Abstract: An infiltration model is developed which considers air viscous forces. The S. E. Buckley-M. C. Leverett soil moisture profile is used at surface ponding and thereafter. A viscous correction factor and an effective gravity correction factor were utilized to simplify the resistance and gravity components respectively.
• Hirschi, M. C., Larson, C. L., & Slack, D. C. (1980). HYDRAULIC CONDUCTIVITY AND DEGREE OF SATURATION DURING INFILTRATION.. Paper - American Society of Agricultural Engineers.
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  Abstract: A field method to measure hydraulic conductivity during infiltration was developed and utilized to measure the hydraulic conductivity at ″field saturation″ . The degree of saturation was also measured and G. S. Campbell's method for estimating relative hydraulic conductivity was evaluated using the collected field data.
• Idike, F. I., Larson, C. L., Slack, D. C., & Young, R. A. (1980). EXPERIMENTAL EVALUATION OF TWO INFILTRATION MODELS.. Transactions of the American Society of Agricultural Engineers, 23(6), 1428-1433.
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  Abstract: Infiltration from constant application rates as predicted by the H. N. Holtan model and the W. H. Green-G. A. Ampt model as modified by R. G. Mein and C. L. Larson (GAML model) was compared to values determined experimentally.
• Moore, I. D., Larson, C. L., & Slack, D. C. (1980). PREDICTING INFILTRATION AND MICRO-RELIEF SURFACE STORAGE FOR CULTIVATED SOILS.. WRRC Bulletin (University of Minnesota) (Water Resources Research Center).
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  Abstract: The paper presents a comprehensive review of the literature on the factors that influence water movement in soils. The object of the study described was to develop a model capable of predicting transient infiltration into tillage affected soils and to test the proposed model against field data. The model consists of three main parts. The first part considers erosion, sedimentation and consolidation of the plow layer, the second, surface storage, and the third, infiltration. The surface relief and the physical properties of the soil are modified by the effects of raindrop impact, which in turn affects the surface storage and infiltration properties of the soil (i. e. surface sealing). In the model runoff is treated as the residual after surface storage and infiltration demands have been satisfied.
• Riggle, F. R., & Slack, D. C. (1980). RAPID DETERMINATION OF SOIL WATER CHARACTERISTIC BY THERMOCOUPLE PSYCHROMETRY.. Transactions of the American Society of Agricultural Engineers, 23(1), 99-103.
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  Abstract: A procedure was developed for determination of the soil water characteristic curve with thermocouple psychrometry. Complete soil water characteristic curves were obtained for a Nicollet clay loam, a Waukegan silt loam, and a Hubbard sand within one week in ambient air conditions. The curves obtained by the psychrometer method compared favorably with those obtained by pressure methods.
• Slack, D. C. (1980). MODELING INFILTRATION UNDER MOVING SPRINKLER IRRIGATION SYSTEMS.. Transactions of the American Society of Agricultural Engineers, 23(3), 596-600.
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  Abstract: A modified R. C. Mein and C. L. Larson model was employed to predict time to surface ponding and corresponding volume of water infiltrated under two types of center pivot irrigation systems. Predictions were made of two soil types with differing surface cover. Results were compared with those observed in the field.
• Slack, D. C., & Riggle, F. R. (1980). EFFECTS OF JOULE HEATING ON THERMOCOUPLE PSYCHROMETER WATER POTENTIAL DETERMINATIONS.. Transactions of the American Society of Agricultural Engineers, 23(4), 877-883.
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  Abstract: Commercial soil thermocouple psychrometers of both the single junction and double junction types were tested to evaluate the magnitude and significance of Joule and resistance heating on water potential determinations near saturation. For water potential determinations about about minus 4 bars a cooling current of 8 mA produced significant heating and yielded unreliable results for both probe types. A cooling current of 3 to 4 mA applied for 15 s provided reliable determinations of water potential up to about minus 0. 5 bars when psychrometer output was read 6 s after cessation of cooling.
• Wilson, B. N., Slack, D. C., & Larson, C. L. (1979). DEVELOPMENT AND EVALUATION OF INFILTRATION MODEL PARAMETERS.. Paper - American Society of Agricultural Engineers.
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  Abstract: An infiltration model is developed which considers air viscous forces. The Buckley-Leverett soil moisture profile is used at surface ponding and thereafter. A viscous correction factor, beta , and an effective gravity correction factor, zeta , were utilized to simplify the resistance and gravity components respectively. These parameters proved to be extremely difficult to evaluate and were very sensitive to values of moisture content upstream of the Buckley-Leverett wetting front.
• Grannes, S. G., Slack, D. C., & Allred, E. R. (1978). UTILIZING AN INFRARED THERMOMETER FOR AIR TEMPERATURE MEASUREMENTS.. Paper - American Society of Agricultural Engineers.
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  Abstract: A system for determining air temperature with an infrared thermometer has been developed. This method utilizes a shielded ″black body″ emitter as an atmospheric temperature reference. The emitter has been shown to approach air temperature to within 0. 13 degree C. This measurement system is especially useful in convective heat exchange studies between exposed surfaces and the atmosphere.
• Idike, F., Larson, C. L., Slack, D. C., & Young, R. A. (1978). EXPERIMENTAL EVALUATION OF TWO INFILTRATION EQUATIONS.. Paper - American Society of Agricultural Engineers.
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  Abstract: Infiltration from constant application rates as predicted by the H. N. Holtan equation and the R. G. Mein-C. L. Larson equation was compared to values determined experimentally. Both equations did a good job of predicting infiltration during latter and middle portions of the experimental runs. The Mein and Larson equation did a good job of predicting time to start of runoff, while the Holtan equation generally underpredicted infiltration during initial stages of the runs.
• Riggle, F. R., & Slack, D. C. (1978). RAPID DETERMINATION OF SOIL WATER CHARACTERISTIC BY THERMOCOUPLE PSYCHROMETRY.. Paper - American Society of Agricultural Engineers.
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  Abstract: A procedure was developed for determination of the soil water characteristic curve with thermocouple psychrometry. Complete soil water characteristic curves were obtained for a Nicollet clay loam, a Waukegan silt loam, and a Hubbard sandy loam within one week in ambient air conditions. The curves obtained by the psychrometer method compared favorably with those obtained by pressure methods.
• Slack, D. C. (1978). MODELING INFILTRATION UNDER MOVING SPRINKLER IRRIGATION SYSTEMS.. Paper - American Society of Agricultural Engineers.
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  Abstract: A modified R. C. Mein-C. L. Larson model for predicting time to surface saturation was employed to predict ponding under two types of center pivot irrigation systems on two agricultural soils in Minnesota. Initial moisture deficit was taken as the difference between fillable porosity and the initial soil water content; hydraulic conductivity in the wetted zone was taken as the unsaturated hydraulic conductivity corresponding to water content at fillable porosity; and the average suction at the wetting front was taken as the area under the suction-relative conductivity curve betwen initial moisture content and saturation. Results of predictions utilizing these parameters were compared with experimental values. The model did a good job of predicting time to surface ponding but did not perform as well as for predicting volume infiltrated at ponding.
• Slack, D. C., Haan, C. T., & Wells, L. G. (1977). MODELING SOIL WATER MOVEMENT INTO PLANT ROOTS.. Transactions of the American Society of Agricultural Engineers, 20(5), 919-927, 933.
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  Abstract: A mathematical model was developed which describes water uptake by a plant as a function of leaf water potential, initial soil water potntial and other soil and plant parameters. The model utilizes two nonlinear partial differential equations for which there are no known analytical solutions. Therefore, to operate the model it was necessary to develop a numerical procedure employing finite difference techniques for the solution of the equations. A computer program, MULTROOT, was developed to solve the numerical model on a digital computer. A review of the literature presented describes the studies made on soil water potential.
• SLACK, D. C., & WALKER, J. N. (1971). DEFLECTION OF PIER FOUNDATIONS SUBJECTED TO LATERAL LOADS. Transactions of the American Society of Agricultural Engineers, 14(1), 85-89.
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  Abstract: The study was conducted to establish a procedure for predicting pier deflections based on the theory of beams on elastic foundations and to establish a procedure for determining the parameters in the developed equation. Three sizes of expander plates were used in three trenches to determine the horizontal coefficient of subgrade reaction. Theory of elasticity was used in relating the coefficient to plate size and depth. A deflection relationship was proposed which describes pier deflection as a function of load, depth, and soil characteristics. Twenty piers of five types were tested with varying loads, depths, and diameters.
• WALKER, J. N., & SLACK, D. C. (1970). PROPERTIES OF GREENHOUSE COVERING MATERIALS. Transactions of the American Society of Agricultural Engineers, 13(5), 682-684.
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  Abstract: A series of physical tests were conducted at the University of Kentucky to determine transverse load carrying capability and impact resistance of glazing materials. The paper summarizes these studies and also summarizes the optical properties of selected glazing materials.

Proceedings Publications

• Slack P,E., D. C., Didan, K., Nichols, M., & Cameron, D. (2019, September). A Hydrologic Engineering Analysis of a Failed Rangeland Water Control Structure on the Buenos Aires National Wildlife Refuge. In V Congresso Nacional de Riego y Drenaje, COMEII 2019., 15p.
• Slack P,E., D. C., Gerba, C. P., & Esteves, R. R. (2019, September). Application of Microbial Risk Assessment of Escherichia coli in Irrigation Water on a Lettuce Crop.. In V Congresso Nacional de Riego y Drenaje, COMEII 2019., 8p.
• Rodriguez, J., Didan, K., Tong, D., Slack, D. C., & Yitayew, M. (2016, April 4-8, 2016). Downscaling MODIS Evapotranspiration via Area-to-Point Kriging in Wellton-Mohawk Irrigation and Drainage District, Yuma, AZ. In INNOVATION MATCH MX 2015 – 2016. 1er Foro International de Talento Mexicano. Articulanto Conocimento Global., N/A.
• Slack P,E., D. C., & Reyes Esteves, R. (2017, May). Modeling Approaches to Determination of Appropriate Depth of Subsurface Drip Irrigation Tubing in Alfalfa.. In XVIII Congresso Nacional de Especialistas en Irrigacion, 8p.
• Slack P,E., D. C., & Reyes Esteves, R. (2017, November). Modeling of Soil Water to Insure a Suitable Depth and Spacing of Subsurface Drip Irrigation Tubing for Alfalfa. In III Congresso Nacional de Riego y Drenaje COMEII 2017, 12p.
• Slack P,E., D. C., Esteves, R. R., Espejel, A., Oyorsaval, B., & Ma, Y. (2016, August). Subsurface Drip Irrigation: A Technology for Safer Irrigation of Vegetable Crops.. In Sixth Khon Kaen University International Engineering Conference - 2016, 7p.
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  This paper describes a modeling excericse using HYDRUS 2D to determine the appropriate depth to place drip irrigation tubing to avoid wetting the soil surface.
• Livingston, P. A., & Slack P,E., D. C. (2014, Spring). Management of the Schmutzdecke Layer in a Slow Sand Filter to Reuse Drainage Water from a Greenhouse. In 5th KKU International Engineering Conference, 5p.
• Livingston, P. A., & Slack P,E., D. C. (2014, Spring). Pilot-Scale Continuous Conveyor Diffusion Extraction System for Sweet Sorghum. In 5th KKU International Engineering Conference, 5p.
• Slack, D. C., & Livingston, P. A. (2013, April 1-4). Management of the Schmutzdecke Layer of a Slow Sand Filter. In 6th International Conference of TSAE. Thai Society of Agricultural Engineers, 6p.
• Slack, D. C., Espinoza, P., Livingston, P. A., & Husman, S. (2013, April 1-4). A Continuous Growing and Harvesting Cycle for Sweet Sorghum for Ethanol Production. In 6th International Conference of TSAE. Thai Society of Agricultural Engineers, 5p.
• Slack, D. C., Espinoza, P., McCraken, K., Rivera, M., York, G., & Livingston, P. A. (2013, April 1-4). Hydroponic Barley Fodder System. In 6th International Conference of TSAE. Thai Society of Agricultural Engineers, 7p.
• Slack, D. C., Yanes, M., Lau, T., & Livingston, P. A. (2013, April 1-4). Pilot Scale Continuous Conveyor Diffusion Extraction System for Sweet Sorghum. In 6th International Conference of TSAE. Thai Society of Agricultural Engineers, 4.
• , T., Slack, D., Ogden, K., & Ottman, M. (2012, May). THE EFFECT OF WATER STRESS ON SWEET SORGHUM (Sorghum bicolor (L.) Moench) AND THE DEVELOPMENT OF CROP COEFFICIENTS. In 4th KKU International Engineering Conference.
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  5/10/2012
• Chandarana, U., Slack, D., & Momayez, M. (2012, May). A Review: Land Surface Changes Before and After Earthquake. In 4th KKU International Engineering Conference.
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  5/10/2012
• Slack, D. C., Rojas, I., Slack, C., Riley, M., & Orbach, M. (2012, May). Bioconversion of Lignocellulosic Components of Sweet Sorghum Stalks to Ethanol Utilizing Fungi and Yeast. In 4th KKU International Engineering Conference.
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  5/10/2012
• Slack, D., Martinez, C. T., Ogden, K., Ottman, M., & Husman, S. (2012, May). Sweet Sorghum as an Energy Crop. In 4th KKU International Engineering Conference.
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  5/10/2012

Presentations

• Slack P,E., D. C. (2019, November). Water for Arizona and the Southwest US. X Congreso Latino Americano y Del Caribe de Estudiantes de Ingeneria Agricola.. Texcoco de Mora, Mexico, Mexico: Asociacion Latinoamericana y del Caribe de Estudiantes de Ingenieria Agricola.
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  Invited Keynote Speaker
• Slack P,E., D. C. (2019, September). Irrigation Trends and Challenges in Arizona and the SW US. V Congreso Nacional de Riego y Drenaje COMEII 2019. Mazatlan, Sinaloa, Mexico: Mexican College of Engineers in Irrigation (COMEII).
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  Invited Keynote Speaker
• Slack P,E., D. C., & Esteves, R. R. (2019, November). Sustaining Water for Desert Agriculture. IV Congreso Internacional y el XV Congreso Nacional Sobre Recursos Bioticos de Zonas Aridas. Bermajillo, Durango, Mexico: Chapingo Autonomous University, Zonas Aridas Campus.
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  Invited Keynote Speaker
• Slack P,E., D. C. (2017, May). Feeding 9.7 Billion - Not Without Irrigation!. XVIII Congresso Nacional de Especialistas en Irrigacion. Monticello, Mexico: National Association of Irrigation Specialist (Mexico).
• Slack P,E., D. C. (2017, October). Dual Ph.D. Program - ABE-UA and IAUIA - Chapingo. Invited Presentation to the Graduate Program in Agricultural Engineering and Alternative Use of Water at Chapingo Autonomous University. Texcoco de Mora, Mexico, Mexico: Chapingo Autonomous University.
• Rodriguez, J., Didan, K., Yitayew, M., & Slack, D. C. (2016, 04). Downscaling MOIS Evapotranspiration via Area-to-Point Kriging in Wellton-Mohawk Irrigation and Drainage District, Yuma, AZ. INNOVATION MATCH MX 2015 – 2016. 1er Foro International de Talento Mexicano. Articulanto Conocimento Global. April 4-8, 2016, Guadalajara, Mexico. Guadalajara, Mexico: INNOVATION MATCH MX 2015 – 2016.
• Slack P,E., D. C. (2016, August). Sustaining Water for Desert Agriculture. Twelfth International Dryland Development Conference, "Sustainable Development of Drylands in the Post 2015 World". Alexandria, Egypt: International Dryland Development Commission.
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  This was an invited paper and I was a lead speaker in the Plenary Session
• Slack P,E., D. C. (2016, October). Sustaining Water for Desert Agriculture. XII Congresso Nacional Sobre Recursos Bioticos de Zonas Aridas. Bermajio, Durango, Mexico: Universidad Autonoma Chapingo - Unidad Regional Universitaria de Zonas Aridas.
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  This was an invited presentation of the same title that I gave in Alexandria, Egypt but with a slightly different slant and a completely different audience.
• Slack P,E., D. C., Esteves, R. R., Ma, Y., Espejel, A., & Oyorsaval, B. (2016, August). Subsurface Drip Irrigation: A Technology for Safe Irrigation of Vegetable Crops with Recycled Water. Experts Meeting on Regional Action on Climate Change and Future Earth. Alexandria, Egypt: International Dryland Development Commission.
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  This presentation was similar to the one I did in Thailand but with a specific emphasis on using recycled water to irrigate vegetable crops.
• Slack, D. (2011). Irrigated Agriculture after Climate Change. 16th National Congress of the Association of Irrigation Specialists of Mexico. Culiacan, Sinaloa, Mexico: Association of Irrigation Specialists of Mexico.

Poster Presentations

• Gonzalez Cena, J. R., Gonzalez Cena, J. R., Slack, D. C., Slack, D. C., Babaeian, E., Babaeian, E., Tuller, M., & Tuller, M. (2016, November). A Comparison of Sequential and Conventional Flood Irrigation Under Field Conditions. ASA-CSSA-SSSA International Annual Meeting. Phoenix, AZ: Soil Science Society of America (SSSA).
• , J., Mirjat, M., Slack, D., & Tuller, M. (2012, November). Alternating partial soil surface wetting to optimize leaching A model analysis. USCID Fall Conference. Managing Irrigation Systems in Today's Environment. Reno, Nevada: USCID Fall Conference.
• Slack, D. (2011, September). Water Requirements of Wine Grapes in the Sonoita Wine Region of Arizona. University of Arizona Foundation - Board of Trustees and National Leadership Council. Tucson, AZ: UA Foundation, Board of Trustees and National Leadership Council.