Marc Nelson

TMDL Data Requirements for Agricultural Watersheds

This paper describes the effect of DEM data resolution on predictions from the nSWAT model for total maximum daily load (TMDL) development. Measured hydrologic, nmeteorological, watershed characteristics, and water quality data from Lincoln Lake watershed n(near Lincoln, AR) were used in the simulation. The effect of input data resolution was evaluated nby running seven scenarios at increasing DEM grid sizes (30 x 30 m, 100 x 100 m, 150 x 150 m, n200 x 200 m, 300 x 300 m, 500 x 500 m, and 900 x 900 m). The model was calibrated on an nannual basis for discharge, NO3-N, and total P using 30 x 30 m DEM data. The predicted output nat the calibrated scale was used to evaluate output accuracy for the remaining input resolutions. nResults of this study showed that DEM resolution affects the watershed delineation, stream nnetwork and sub-basin classification in the SWAT model. Lower values of discharge, NO3-N, nand total P were predicted at the 5 coarsest DEM resolutions, probably due to decrease in ncomputed total watershed area. The model predicted higher values of all three parameters at 100 nm DEM resolution, when the total watershed area increased compared to the base 30 m nresolution.