C. Santhi

SWAT: Model Use, Calibration, and Validation

SWAT (Soil and Water Assessment Tool) is a comprehensive, semi-distributed river basin model that requires a large number of input parameters, which complicates model parameterization and calibration. Several calibration techniques have been developed for SWAT, including manual calibration procedures and automated procedures using the shuffled complex evolution method and other common methods. In addition, SWAT-CUP was recently developed and provides a decision-making framework that incorporates a semi-automated approach (SUFI2) using both manual and automated calibration and incorporating sensitivity and uncertainty analysis. In SWAT-CUP, users can manually adjust parameters and ranges iteratively between autocalibration runs. Parameter sensitivity analysis helps focus the calibration and uncertainty analysis and is used to provide statistics for goodness-of-fit. The user interaction or manual component of the SWAT-CUP calibration forces the user to obtain a better understanding of the overall hydrologic processes (e.g., baseflow ratios, ET, sediment sources and sinks, crop yields, and nutrient balances) and of parameter sensitivity. It is important for future calibration developments to spatially account for hydrologic processes; improve model run time efficiency; include the impact of uncertainty in the conceptual model, model parameters, and measured variables used in calibration; and assist users in checking for model errors. When calibrating a physically based model like SWAT, it is important to remember that all model input parameters must be kept within a realistic uncertainty range and that no automatic procedure can substitute for actual physical knowledge of the watershed.

A modeling approach to evaluate the impacts of water quality management plans implemented in a watershed in Texas

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 preBMP 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. 2005 Elsevier Ltd. All rights reserved.

APPLICATION OF A WATERSHED MODEL TO EVALUATE MANAGEMENT EFFECTS ON POINT AND NONPOINT SOURCE POLLUTION

A Total Maximum Daily Load (TMDL) program has been initiated in the North Bosque River Watershed in Texas, USA, where point and nonpoint sources of pollution are of a concern. The Soil and Water Assessment Tool (SWAT), which had been validated for flow and sediment and nutrient transport, was applied to quantify the effects of Best Management Practices (BMPs) related to dairy manure management and municipal wastewater treatment plant effluent. Results are presented for the period from 1960 through 1998 for three sites along the North Bosque River. Results are presented as annual time–weighted concentrations (average of the daily load divided by daily flow over a year) and annual flow–weighted concentrations (total cumulative load divided by total cumulative flow over a year). The wastewater treatment plant BMPs resulted in greater improvement in time–weighted instream soluble phosphorus concentrations than dairy BMPs. On the other hand, dairy BMPs made greater differences in flow–weighted concentrations. This study showed that SWAT could be a useful tool for studying the effects of alternative management scenarios for pollution control from point and nonpoint sources in large watersheds.

A GIS-BASED REGIONAL PLANNING TOOL FOR IRRIGATION DEMAND ASSESSMENT AND SAVINGS USING SWAT

Regional planning for irrigated agriculture requires a thorough understanding of the hydrological processes and spatial and temporal variations associated with hydrological factors such as rainfall, soils, and crops grown in different units of the region. The objective of this study was to improve the capabilities of a basin-scale hydrologic simulation model for regional planning of irrigated agriculture. In this study, a Geographical Information System (GIS) based hydrological model, Soil and Water Assessment Tool (SWAT), was configured as a regional planning tool with a canal irrigation capability for estimating irrigation demand. The tool was capable of simulating hydrological processes associated with soil-plant-water interactions and capable of capturing the spatial and temporal variability of the major factors, which are important in regional planning. The tool was applied to the irrigation districts in the Lower Rio Grande Valley in Texas. It was validated for crop evapotranspiration and canal conveyance efficiency and applied to analyzing the demand and potential water savings of alternative water conservation measures. Estimated potential water savings were 234.2, 65.9, and 194.0 Mm3 for conservation measures related to on-farm management improvements, replacing sugarcane with corn, and improving canal conveyance efficiency, respectively. Results indicated that on-farm management measures might be as beneficial as improving canal conveyance systems. The planning tool (with hydrological modeling and GIS capabilities) and estimations made would be useful for regional planners and irrigation district managers. The tool could be used for other irrigation systems as well.

An integrated modeling approach for estimating the water quality benefits of conservation practices at the river basin scale.

The USDA initiated the Conservation Effects Assessment Project (CEAP) to quantify the environmental benefits of conservation practices at regional and national scales. For this assessment, a sampling and modeling approach is used. This paper provides a technical overview of the modeling approach used in CEAP cropland assessment to estimate the off-site water quality benefits of conservation practices using the Ohio River Basin (ORB) as an example. The modeling approach uses a farm-scale model, Agricultural Policy Environmental Extender (APEX), and a watershed scale model (the Soil and Water Assessment Tool [SWAT]) and databases in the Hydrologic Unit Modeling for the United States system. Databases of land use, soils, land use management, topography, weather, point sources, and atmospheric depositions were developed to derive model inputs. APEX simulates the cultivated cropland, Conserve Reserve Program land, and the practices implemented on them, whereas SWAT simulates the noncultivated land (e.g., pasture, range, urban, and forest) and point sources. Simulation results from APEX are input into SWAT. SWAT routes all sources, including APEXs, to the basin outlet through each eight-digit watershed. Each basin is calibrated for stream flow, sediment, and nutrient loads at multiple gaging sites and turned in for simulating the effects of conservation practice scenarios on water quality. Results indicate that sediment, nitrogen, and phosphorus loads delivered to the Mississippi River from ORB could be reduced by 16, 15, and 23%, respectively, due to current conservation practices. Modeling tools are useful to provide science-based information for assessing existing conservation programs, developing future programs, and developing insights on load reductions necessary for hypoxia in the Gulf of Mexico.

Development of Algorithms for Modeling Onsite Wastewater Systems within SWAT

Onsite wastewater systems (OWSs) are a significant source of nonpoint-source pollution to surface and groundwater in both rural and suburban settings. Methods to quantify their effect are therefore important. The mechanics of OWS biogeochemical processes are well studied. However, tools for their assessment, especially at the watershed scale, are limited. As part of this work, modeling capabilities were developed within the Soil Water Assessment Tool (SWAT) such that OWSs and their subsequent environmental impacts can be evaluated A case study was initiated on the Hoods Creek watershed in North Carolina to test the new SWAT algorithms. Included were: (1) field-scale simulations of groundwater quantity (water table height) and quality (N, P), (2) Monte Carlo evaluations of OWS service life to evaluate suggested calibration parameters, and (3) assessments of watershed-scale pollutant loadings within the model. Results were then analyzed at both the field and watershed scales. The model performed well in predicting both site groundwater table levels (R2 = 0.82 and PBIAS = -0.8%) and NO3-N concentration in the groundwater (R2 = 0.76, PBIAS = 2.5%). However, the performance for PO4-P simulations was less reliable due to difficulty in representing the mobility of soluble P in the soil. An advanced P algorithm is recommended to address the sophisticated physiochemical properties of soil particles and improve the models performance.

An Approach for Estimating Water Quality Benefits of Conservation Practices at the National Level

The United States Department of Agriculture has initiated the Conservation Effects Assessment Project (CEAP) to quantify the environmental benefits of conservation practices at the national scale. This paper provides an overview of the analytical approach being used in the CEAP national assessment to estimate off-site water quality benefits. For the assessment, a sampling and modeling approach is used. The farm-scale model Agricultural Policy/Environmental EXtender (APEX) is used to simulate conservation practices for cultivated cropland. Farmer surveys conducted on a subset of National Resource Inventory sample points provide information on current farming activities and conservation practices for APEX. Output from APEX will be input into the watershed scale model, Soil and Water Assessment Tool (SWAT) in the HUMUS (Hydrologic Unit Modeling for the United States) system for routing the pollutants to the 8-digit watershed outlet. SWAT will be calibrated and validated using the United States Geological Survey’s SPAtially Referenced Regressions On Watershed attributes (SPARROW) model output, streamflow and pollutant data. The HUMUS system simulates in-stream effects for (a) a baseline scenario with conservation practices and (b) an alternative scenario without conservation practices. The off-site water quality benefits of conservation practices currently in use will be determined by comparing outputs for the alternative scenario to the baseline outputs at each 8-digit watershed. Benefits will be reported as reductions in in-stream concentrations and loadings of sediment, nutrients and pesticides, and reductions in the number of days that concentrations exceed human health and ecological thresholds.

Measuring Environmental Benefits of Conservation Practices: The Conservation Effects Assessment Project (CEAP)-A Model Calibration Approach at the National Level

The purpose of the Conservation Effects Assessment Project (CEAP) is to provide people involved with environmental policy issues an accounting of the environmental benefits obtained from United States Department of Agriculture (USDA) conservation program expenditures. There are two main components in CEAP: (1) The national assessment and (2) watershed assessment. This paper will focus on the calibration aspects of modeling component of the national assessment part of the project. The primary focus of the project is on cropland where most of the conservation practices were implemented. Hence field level modeling for CEAP will be conducted by APEX (Agricultural Policy/Environmental eXtender). Outputs from the APEX model runs will be input to the watershed scale model Soil and Water Assessment Tool (SWAT) in HUMUS (Hydrologic Unit Modeling for the United States) setup to route flow, sediment, and pollutants to the outlet of each watershed in different agricultural regions. Environmental benefits at the national scale will be estimated based on the differences in output between the two scenarios “before conservation practices” and “after conservation practices”. In this modeling approach a calibration is necessary to compensate for the uncertainties in input data. The scale of the study, limits manual calibration. Hence an automated calibration procedure is developed to manage the large-scale calibration efforts. The auto-calibration procedure automatically adjusts the input parameters in HUMUS-SWAT so that the annual average stream flow and sediment yield values from HUMUS-SWAT system match to that of SPARROW (SPAtially Referenced Regression On Watershed attributes) model estimates for each 8-digit watershed.