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Geographic Information Systems and Models

Background GIS SWAT APEX Technology Transfer Opportunities

The latest hydrologic/water quality computer models developed by ARS (USDA-Agricultural Research Service) and Texas AgriLife Research are used to address the overall ecosystem management of an entire basin or watershed. This process involves both field or farm scale models and watershed or basin scale models:

Whole farm/small watershed model - APEX (Agricultural Policy Environmental Extender)

Basin scale model - SWAT (Soil and Water Assessment Tool)


The State Conservationist of NRCS in Texas recognized the benefit these tools could have in addressing water quality/quantity issues in the state. To facilitate the use of this new technology, he located a staff of NRCS employees at the Blackland Research Center to work directly with scientists and researchers. This group now known as the Water Resource Assessment Team (WRAT) provides feedback for model improvement. These improvements are related to input and output as the models are applied in small watersheds over the State. The WRAT also assemble or develop GIS (Geographic Information System) data required as input to the models. Substantial amounts of reimbursable funding have been provided from partnerships with others to carry out this work.

A multi-agency effort has been underway since 1992 to adapt these models to assess current problems and the effects conservation practices can have in abating the problems associated with watershed management. AgriLife has assisted ARS with databases and GIS interfaces for the models and USDA-NRCS has helped with technology transfer, support and application of the models within NRCS and for many of their clients and partners. The multi-agency effort allows pooling of technical resources, funding, in-kind services and orderly transfer of technology

Geographic Information Systems (GIS)

The WRATs have experience with several GISs including the US Corps of Engineers' (USACE) Geographic Resources Analysis Support System (GRASS), Blackland GRASS, ESRI's Arcview & ArcGis, and ERDAS's Imagine. Simulations using SWAT have been performed in UNIX on SUN workstations and in Windows on desktop PCs. APEX simulations are performed in the Windows environment on desktop PCs. Relational databases used by NRCS include INFORMIX and Access.

Most of the developmental work involving GIS at the ARS/AgriLife laboratory has been with a base scale of 1:250,000 which is readily available for all of the United States. These GIS layers were the foundation for the HUMUS (Hydrologic Unit Model for the United States) project , a cooperative effort between NRCS, ARS, and AgriLife at the Temple, TX laboratory. The purpose of the HUMUS project was to assist in the Resource Conservation Act (RCA) assessment of the status and condition of water resources of the nation under current and projected management conditions. SWAT model technology was originally developed for the HUMUS project.

The need for assessments of smaller areas with a high level of detail requires that greater detail of GIS databases be available. The HUMUS project as an example, uses the STATSGO soils geographic database (1:250,000 scale base) as one of the GIS layers in simulating entire river basins. STATSGO polygons represent soils associations that may include 20-30 individual soil series. Soils information for many of the WRAT projects have been developed from CBMS (computer based mapping system 1:24,000 scale) and SSURGO data that provide much more detail in the GIS layer and model input. A link from the spatial data to the relational soils database provides soil properties for each soil to the SWAT model input.

The integration of the water quality model and GIS reduces significantly the time to prepare input data for models and simplifies model operation. As GIS layers become readily available, the effort to simulate current versus projected management will involve reduced time frames and personnel.

SWAT Model

Soil and Water Assessment Tool - Official SWAT Web Site

SWAT is a basin-scale, continuous time water quality model integrated with a GIS to extract input data to simulate basin hydrology and conditions. Development of SWAT involved combining a routing procedure to the SWRRB simulation model. This allows loadings at sub-basin outlets to be routed through the stream network on a real time basis to the receiving reservoir or point of interest. Integration of GIS and SWAT eased the task of providing input for hundreds of sub-basins and multiple simulations.

SWAT has a unique feature that allows the output of other model runs to be imported at stream routing nodes throughout the watershed simulation. A simulation using very detailed data for a small subbasin of the watershed can be integrated into a general assessment of the entire watershed above a reservoir. This can indicate the targeted basin's effects on loadings at a basin outlet or reservoir. SWAT can handle other features such as point sources of water inflow/outflow and can accommodate irrigation diversions, return flows, wastewater treatment outfalls, and other municipal or industrial permitted uses. To be a realistic simulation of the watershed, the model must handle both nonpoint sources and all permitted point sources as well as water transfers in or out of the basin. Thus predicted stream flow can be compared to measured stream gage records.

Initially, SWAT was run on the small watersheds in much the same manner as for the HUMUS Project. As more detailed GIS data was developed, the subbasins were reduced in size to work toward more reasonable representation of actual conditions.

SWAT and GIS integrated as a modeling tool can guide management decisions regarding runoff, sediment, nutrient and pesticide loadings for small watersheds. This tool allows assessment or evaluation of effects from a watershed based on hydrologic and hydraulic boundaries consistent with basic principles and standards for planning treatment alternatives in water resource projects.

The APEX Model

EPIC and APEX Models - AgriLife Research Texas A&M System

The Agricultural Policy/Environmental eXtender (APEX) model was developed for use in whole farm/small watershed management (Williams et al., 2000). The model was constructed to evaluate various land management strategies considering sustainability, erosion (wind, sheet, and channel), economics, water supply and quality, soil quality, plant competition, weather, and pests. Management capabilities include irrigation, drainage, furrow diking, buffer strips, terraces, waterways, fertilization, manure management, lagoons, reservoirs, crop rotation and selection, pesticide application, grazing, and tillage. Besides the farm management functions, APEX can be used in evaluating the effects of global climate/CO2 changes; designing environmentally safe, and economical landfill sites; designing biomass production systems for energy; and other spin-off applications. The model operates on a daily time step and is capable of simulating hundreds of years if necessary. Farms may be subdivided into fields, soil types, landscape positions, or any other desirable configuration.

The individual field simulation component of APEX is taken from the Environmental Policy Integrated Climate (EPIC) model. The drainage area considered by EPIC is generally a field-sized area, up to 100 ha (247 acres), where weather, soils, and management systems are assumed to be homogeneous. The major components in EPIC are weather simulation, hydrology, erosion-sedimentation, nutrient cycling, pesticide fate, plant growth, soil temperature, tillage, economics, and plant environment control. Although EPIC operates on a daily time step, the optional Green and Ampt infiltration equation simulates rainfall excess rates at shorter time intervals (0.1 h). The model offers options for simulating several other processes including five PET equations, six erosion/sediment yield equations, and two peak runoff rate equations. EPIC can be used to compare management systems and their effects on nitrogen, phosphorus, pesticides and sediment. The management components that can be changed are crop rotations, tillage operations, irrigation scheduling, drainage, furrow diking, liming, grazing, tree pruning, thinning, and harvest, manure handling, and nutrient and pesticide application rates and timing.

The APEX model was developed to extend the EPIC model capabilities to whole farms and small watersheds. In addition to the EPIC functions, APEX has components for routing water, sediment, nutrients, and pesticides across complex landscapes and channel systems to the watershed outlet. APEX also has groundwater and reservoir components. A watershed can be subdivided as much as necessary to assure that each subarea is relatively homogeneous in terms of soil, land use, management, etc. The routing mechanisms provide for evaluation of interactions between subareas involving surface runoff, return flow, sediment deposition and degradation, nutrient transport, and groundwater flow. Water quality in terms of nitrogen (ammonium, nitrate, and organic), phosphorus (soluble and adsorbed/mineral and organic), and pesticides concentrations may be estimated for each subarea and at the watershed outlet. Commercial fertilizer or manure may be applied at any rate and depth on specified dates or automatically. The GLEAMS pesticide model is used to estimate pesticide fate considering runoff, leaching, sediment transport and decay. Because of routing and subdividing there is no limit on watershed size. However, a practical limit may be about 2500 km2 (965 mi2) because of the detailed crop/management system of APEX. The major uses of APEX have been dairy manure management to maintain water quality in Erath and Hopkins Counties, TX, a national study to assess the effectiveness of filter strips in controlling sediment and other pollutants, and a study on the effects of conservation practices in rangeland. APEX has its own data bases for weather simulation, soils, crops, tillage, fertilizer, and pesticides.

The potential for use of APEX and SWAT computer models for watershed management and nonpoint source pollution assessment is in an upward trend and toward an unlimited use of the technology.

Technology Transfer

The WRAT staff has provided support to several staffs in setting up projects for modeling with APEX and/or SWAT on both UNIX workstations and PC computers. This support has ranged from simply loading all software and databases on the hardware to the other extreme of developing much of the GIS data and step by step assistance in executing the model runs. In most cases the support has been provided by the staffs coming to the Temple laboratory and working along side the WRAT staff for periods of time of a few days up to intermittent visits over several months.


Detects of very small concentrations of pesticides in several drinking water sources in the State have accentuated the need for computer modeling tools. These issues require a fast but scientific analysis with realistic alternative solutions to maintain public water quality. It is also essential that the technology used for these analyses be uniformly applied across the vast ranges of ecosystems, climate and needs of the landscape of the State. These issues exist far beyond the boundaries of Texas, they can and should be applied nationwide and even worldwide.

The primary functions that can be provided by the WRAT staff to other staffs within NRCS or other agencies can be summarized as:

Training and application support in GIS (Arcview and ArcGIS)

Training and application support in APEX and SWAT.

Turnkey analysis of water quality/water quantity problems on a watershed basis.