The Soil and Water Assessment Tool, or SWAT, was developed by the USDA-Agricultural Research Service. While it is quite data intensive, when resources allow it can provide a high degree of precisions, delineating factors down to the sub-field level. The model is considered a physical process based one, as it simulates interactions that occur on the watershed scale between factors such as weather, water, and land management to simulate functions such as erosion and other transportation mechanisms.
There are two degrees of precision within the SWAT model outputs. The first is the subbasin level, which relies on more homogenous data inputs and delineates a singular receiving water body. The second level of precision is the hydrologic response unit (HRU), which is a smaller area of a subbasin and can be delineated based on similar land-use, soils and topographic slope. They can be further delineated to individual fields and subfields based on agronomic practices. Furthermore, SWAT can simulate both upland and in-channel Processes.
Successful application of SWAT requires a high level of data intensity. Data inputs include (from Giri et al. 2012):
- Digital Elevation Model (DEM)
- Cropland data layer
- Soil Characteristics
- Stream networks and daily stream flow data
- Climatic data
- Management practices and operations
There is some flexibility as to what level of inputs are required, as SWAT has some estimation capabilities, but the more detailed the data, the more accurate the outputs.
SWAT instructional video series
This video series, created by Purdue University in collaboration with Texas A&M, takes users through a step-by-step process of using ArcSWAT, from download to evaluation. ArcSWAT is a GIS-compatible version of the software. Access the series HERE.
Heartland REgional WAter Quality Monitoring Group
The Heartland Regional Water Coordination Initiative – a collaboration between the EPA Region 7 land grant colleges (Nebraska, Kansas, Iowa, and Missouri) and the USDA – works to build cooperative frameworks across stakeholder groups to lead to better water quality outcomes. One of their initiatives is to assist planning groups in applying watershed scale models to the planning efforts. On their website, you can access a number of webcasts to assist in the use and application of some of these models including SWAT. You can explore the webcast series HERE.
Mississquoi Bay SWAT application Case study
In the Missisquoi Bay of Lake Champlain in Northern Vermont, the SWAT model has been extensively applied and serves as a useful case study for the application and scope of the SWAT model. Despite significant investment in phosphorus reduction at both the state and federal level, this basin remained a high contributor to overall phosphorus levels in Lake Champlain. As such, they hired Stone Environmental, Inc to identify critical source areas within the basin. In addition to an extensive report which you can access HERE (This file is rather large, so if you have issues downloading it, the executive summary can be accessed here) Stone has produced an interactive map which shows critical source areas across multiple scales, from the sub-watershed to the field scale. The interactive map can be accessed HERE.