Assessment of Different Representations of Spatial Variability on SWAT Model Performance

River basin management requires a spatially distributed representation of basin hydrology and nutrient transport processes. To accomplish this, the Soil and Water Assessment Tool (SWAT) watershed model was enhanced to simulate water flow across discretized landscape units. The model structure more closely reflects the complex controls on infiltration, runoff generation, run-on, and subsurface flow without requiring large computational resources or detailed parameterization. Four landscape delineation methods were compared: lumped, hydrologic response units (HRUs) or hydrotope, catena, and grid. The lumped method using dominant soil and land use and the HRU delineation do not consider landscape position when computing runoff. The catena method routes flow across a representative catena with divide, hillslope, and valley units. The distributed method divides the watershed into cells (1 ha each) for routing. All methods were calibrated and validated for the USDA-ARS Brushy Creek watershed (17.3 km2) near Riesel, Texas. The calibration results indicate that measured flow at the basin outlet is similar (daily N-S around 0.65) for all four models, or conversely, the new models (catena and grid) do as well as the existing models (lumped and HRU based) in predicting daily flow at the basin outlet. The advantage of the catena and grid models is that the impacts of spatial changes in land use and BMPs on the hillslope valley continuum can now be more realistically assessed.