Impact of DEM mesh size and soil map scale on SWAT runoff, sediment, and NO3-N loads predictions

The accuracy of agricultural nonpoint source pollution models depends to a great extent on how well model input spatial parameters describe the relevant characteristics of the watershed. It is assumed that reducing the precision of spatial input parameters affects the simulation results of runoff and sediment yield from the entire watershed. However, there may be no significant increase in the accuracy of models, as a result of more precise topographic or soil information, which increase the input data collection and preparation. The objective of this study was to determine the impact of the mesh size of the digital elevation model, DEM (from 20 to 500 m) and the soil map scale (1/25,000; 1/250,000 and; 1/500,000 scale) within the Soil and Water Analysis Tool (SWAT) to simulate runoff, sediment, and NO3-N loads at the outlet of an agricultural watershed. Results of the Lower Walnut Creek (21.8 km2, central Iowa) showed that an upper limit to DEM mesh size of 50 m is required to simulate watershed loads. Decreasing the mesh size beyond this threshold does not substantially affect the computed runoff flux but generated prediction errors for nitrogen and sediment yields. Whatever the DEM mesh size considered, a detailed soil map has to be considered to accurately estimate the loads. Finally, the impacts of DEM mesh size and soil map scale on the modeling results are discussed in respect of the relevant characteristics of the watershed and included in SWAT.