Effect of Watershed Subdivision on Swat Flow , Sediment , and Nutrient Predictions ( 1 )

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 flow, sediment, and nutrients over 30 years for four Iowa watersheds ranging in size from 2,000 to 18,000 km2 with the Soil and Water Assessment Tool (SWAT) model. The results of the analysis indicated that variation in the total number of subwatersheds had very little effect on streamflow. However, the opposite result was found for sediment, nitrate, and inorganic P; the optimal threshold subwatershed sizes, relative to the total drainage area for each watershed, required to adequately predict these three indicators were found to be around 3, 2, and 5 percent, respectively. Decreasing the size of the subwatersheds below these threshold levels does not significantly affect the predicted levels of these environmental indicators. These threshold subwatershed sizes can be used to optimize input data preparation requirements for SWAT analyses of other watersheds, especially those within a similar size range. The fact that different thresholds emerged for the different indicators also indicates the need for SWAT users to assess which indicators should have the highest priority in their analyses. (KEY TERMS: modeling; flow; sediment; nutrients; watershed subdivision; sensitivity analysis.) Jha, Manoj, Philip W. Gassman, Silvia Secchi, Roy Gu, and Jeff Arnold, 2004. Effect of Watershed Subdivision on SWAT Flow, Sediment, and Nutrient Predictions. Journal of the American Water Resources Association (JAWRA) 40(3):811-825.