Examination of the Role of Physical Resolution and Scale on Sediment and Nutrient Yields

Currently, watershed delineation and extraction of stream networks are accomplished with GIS databases of digital elevation maps (DEMs). The most common method for extracting channel networks requires the a-priori specification of a critical source area that is required for channel initiation. There are no established guidelines on how to select the critical source area. The critical source area could be selected by identifying an optimal scale of geomorphologic resolution such that further refinement in spatial scale does not contribute to a significant improvement in predicting design quantities at the watershed outlet. In this study, the Soil and Water Assessment Tool (SWAT) model integrated into the BASINS framework was utilized for this purpose. The BASINS framework allows the user to automatically or manually delineate the watershed based on a DEM. The SWAT model was calibrated and validated at Dreisbach and Smith Fry, two subwatersheds of the Black Creek watershed in the Maumee River basin in northeast Indiana. After calibration, several stream definition values were used to estimate total sediment yield for the Smith Fry (Site 2) subwatershed. This study showed that there was no clear critical source area that could be identified for stream definition in SWAT modeling. However, an optimal stream definition value was suggested from a plot of sediment yield versus number of subbasins.