Re-conceptualizing the soil and water assessment tool (SWAT) model to predict runoff from variable source areas

• Published in: Journal of hydrology (Amsterdam) Vol. 348; no. 3; pp. 279 - 291
• Main Authors: Easton, Zachary M., Fuka, Daniel R., Walter, M. Todd, Cowan, Dillon M., Schneiderman, Elliot M., Steenhuis, Tammo S.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 15.01.2008; [Amsterdam; New York]: Elsevier; Elsevier Science
• Subjects: Curve-number (CN); Distributed model; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Freshwater; Geochemistry; high water table; hydrochemistry; hydrologic models; Hydrology; Hydrology. Hydrogeology; losses from soil; Mineralogy; Non-point source pollution; nonpoint source pollution; overland flow; phosphorus; Pollution, environment geology; prediction; runoff; Saturation excess; Silicates; Soil and Water Assessment Tool model; streams; Variable source area (VSA); variable source areas; Water geochemistry; Water quality; water table; watershed hydrology; watersheds; New York; Appalachians; United States; Catskill Mountains; USA, New York; Saturation excess; Distributed model; Non-point source pollution; Curve-number (CN); Variable source area (VSA); Water quality; ground water; pollution; nonpoint sources; North America; saturation; drainage basins; aquifers; hydrochemistry; water content; soils; export; flow; phosphorus; models; rainfall; streams; water table; runoff; hydrodynamics; infiltration; depth; water quality; water resource management; prediction; storms; streamflow
• ISSN: 0022-1694; 1879-2707
• DOI: 10.1016/j.jhydrol.2007.10.008

Many water quality models use some form of the Natural Resources Conservation Services (formerly Soil Conservation Service) curve number (CN) equation to predict storm runoff in ways that implicitly assume an infiltration-excess response to rainfall. Because of this, these models may fail to predict variable source areas (VSAs) correctly, i.e. where runoff is typically generated in rural, humid regions. In this study, the Soil and Water Assessment Tool (SWAT) model was re-conceptualized to distribute overland flow in ways consistent with VSA hydrology by modifying how the CN and available water content were defined; the new modeling approach is called SWAT-VSA. Both SWAT and SWAT-VSA were applied to a sub-watershed in the Cannonsville basin in upstate New York to compare model predictions of integrated and distributed responses, including surface runoff, shallowly perched water table depth, and stream phosphorus loads against direct measures. Event runoff was predicted similarly well for SWAT-VSA and SWAT. However, the distribution of shallowly perched water table depth was predicted better by SWAT-VSA and it is this shallow groundwater that governs VSAs. Event based dissolved phosphorus export from the watershed was also predicted better by SWAT-VSA, presumably because the distribution of runoff source areas was better predicted particularly from areas where manure was applied. This has important consequences for using models to evaluate and guide watershed management because correctly predicting where runoff is generated is critical to locating best management practices to control non-point source pollution.