Effect of Watershed Subdivision on SWAT Flow, Sediment, and Nutrient Predictions

The size, scale, and number of subwatersheds can affect a watershed modeling process and subsequent results. The objective of this study was to determine the appropriate level of subwatershed division for simulating sediment yield. The Soil and Water Assessment Tool (SWAT) model with a geographic information system interface (AVSWAT) was applied to four Iowa watersheds that varied greatly in drainage area. Annual output was analyzed from each simulation, which was executed for 30 years using climatic data representing the 1970 to 2000 period. The optimal threshold subwatershed size of the total drainage area to adequately predict sediment yield was found to be around 3 percent. Decreasing the size of subwatersheds beyond this level does not significantly affect the computed sediment yield. This threshold subwatershed size can be used to optimize SWAT input data preparation requirements and simplify the interpretation of results without compromising simulation accuracy.