The Ogallala Aquifer has experienced a continuous decline in water levels due to decades of irrigation pumping with minimal recharge. Corn is one of the major irrigated crops in the semi-arid Northern High Plains (NHP) of Texas. Selection of less water-intensive crops may provide opportunities for groundwater conservation. Modeling the long-term hydrologic impacts of alternative crops can be a time-saving and cost-effective alternative to field-based experiments. A newly developed management allowed depletion (MAD) irrigation scheduling algorithm for Soil and Water Assessment Tool (SWAT) was used in this study. The impacts of irrigated farming, dryland farming, and continuous fallow on water conservation were evaluated. Results indicated that simulated irrigation, evapotranspiration, and crop yield were representative of the measured data. Approximately 19%, 21%, and 32% reductions in annual groundwater uses were associated with irrigated soybean, sunflower, and sorghum, respectively, as compared to irrigated corn. On average, annual soil water depletion was more than 52 mm for dryland farming scenarios. In contrast, only 18 mm of soil water was lost to evaporation annually, for the long-term continuous fallow simulation. The fallow scenario also showed 31 mm of percolation for aquifer recharge.
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The application of poultry litter to agricultural land has become a topic of interest for policy makers due to public concern about its effects on water quality. The Soil and Water Assessment Tool (SWAT) version 2005 is designed to assess nonpoint and point sources of pollution. In this study, six subwatersheds in Texas (HUC‐8; 12070101) are used to evaluate the model's ability to simulate water quality at a small scale. Each of these subwatersheds randomly received poultry litter rates of 0.0 to 13.4 Mg ha-1. Monthly and daily data from 2002 were used for calibration purposes, while 2000, 2001, 2003, and 2004 were used for validation. The SCS runoff curve number for moisture condition II (CN2) and the soil evaporation compensation factor (ESCO) parameters were found to be more sensitive than the surface runoff lag time (SURLAG) and initial soil water content expressed as a fraction of field capacity (FFCB). The monthly and daily runoff model simulations for the six subwatersheds resulted in calibration Nash‐Sutcliffe efficiency (NSE) values of 0.59 and 0.53 and validation NSE values 0.82 and 0.80, respectively. The monthly and daily R2 runoff values for the six subwatersheds resulted in calibration values of at least 0.60 and 0.53 and validation R2 values of 0.86 and 0.81, respectively. The observed trends included SWAT's overestimation of runoff in the dry periods and underestimation in the wet periods. The monthly NSE and R2 values for sediment and nutrient losses were generally above 0.4 and 0.5, respectively. Paired t‐tests for the monthly manually adjusted parameter simulation of sediment, organic N and P, NO3-N, and soluble P for the 2000‐2004 period losses showed that their respective SWAT means were not significantly different from the measured values ( = 0.05), except for NO3-N losses for the Y10 subwatershed (p‐value 0.042). The control subwatershed's measured and simulated water quality results were significantly different ( = 0.05) from the treated subwatersheds, most likely due to the amount of inorganic N present. Almost all of the subwatersheds that had poultry litter applied resulted in higher sediment, organic N, organic P, and soluble P losses than the control subwatershed upon averaging the monthly validation values. High NO3-N losses may have been a function of poultry litter and commercial fertilizers being applied before a large rainfall event occurred. The subwatersheds that received smaller amounts of commercial fertilizer and/or poultry litter lost more sediment, organic N, and organic P than the subwatersheds that received the higher litter and/or fertilizer treatments. Overall, the SWAT simulated the hydrology and the water quality constituents at the subwatershed scale more adequately when all of the data were used to simulate the model, as evidenced by statistical measures.
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Several best management practices (BMPs) have been implemented through Water Quality Management Plans (WQMPs) in the West Fork Watershed of Trinity River Basin in Texas, USA, where nonpoint source pollution is a serious concern. Major sources of pollution are sediment erosion and nutrients. The objective of this study was to evaluate the long-term impact of implementation of WQMPs on nonpoint source pollution at the farm level and watershed level using a modeling approach. The Soil and Water Assessment Tool watershed model was applied to quantify the impacts of implementing WQMPs on sediment and nutrients. A pre- BMP scenario representing conditions of the watershed prior to the implementation of WQMPs, and a post-BMP scenario representing the conditions of the watershed after implementation of WQMPs were simulated to estimate the reductions in nonpoint source pollution due to WQMP implementation. The results are presented as percentage reductions in sediment and nutrient loadings, at the farm level and at two locations within the watershed. The results revealed that (a) the benefits of the WQMPs were greater (up to 99%) at the farm level and (b) the benefits due to WQMPs were 1e2% at the watershed level. Watershed level benefits are tangible as the WQMP implementation area is very small compared to the watershed area. An additional scenario was evaluated to show the possible impacts of expanding the current BMP effort on load reductions. This study showed that a modeling approach can be used to estimate the impacts of water quality management programs in large watersheds.
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The design of institutions that maximizes water’s beneficial use in the face of growing demands for scarce andrand om supplies is the central policy issue in dry places. Information on water’s economic value enables decision makers to make informed choices on water development, conservation, allocation, and use when growing demands for all uses are made in the face of increased scarcity. Conceptually correct and empirically accurate estimates of the economic value of water are essential for rational allocation of scarce water across locations, uses, users, andtime periods. This review article raises several issues that must be considered in deriving accurate estimates of the economic value of water. These include establishing common denominators for water values in quantity, time, location and quality; identifying the point of view from which values are measured; distinguishing the period of adjustment over which values are estimated; and accounting for the difference between total, average, and incremental values of water. We llustrate values of water for agricultural use, based on a recent drought policy analysis of the Rio Grande Basin.
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