Effects of Digital Elevation Models (DEM) Spatial Resolution on Hydrological Simulation

Digital Elevation Model is one of the most important data for watershed modeling whit hydrological models that it has a significant impact on hydrological processes simulation. Several studies by the Soil and Water Assessment Tool (SWAT) as useful Tool have indicated that the simulation results of this model is very sensitive to the quality of topographic data. The aim of this study is evaluating the spatial resolution effect of three type's digital elevation model such as ASTER (30 m), SRTM (90 m) and GTOPO30 (1000 m) on the uncertainty of results for flow and total nitrogen simulation. With increasing spatial resolution of 30 to 1,000 m physiographic characteristics such as the number HRU reduced but the average slope and the average minimum and maximum elevation increased. Furthermore, the channel drawing is heavily affected by the spatial resolution of DEM. The Best results of monthly calibration and validation are obtained in Shirgah station for ASTER digital elevation model. R2 and NS coefficient obtained 0.71 and 0.68 for during calibration period and 0.70 and 0.54 during validation period, respectively. Finally, calculated relative error of SRTM and GTOPO30 simulation results compared with ASTER. The results shows that the model overestimated flow and nitrate by increasing spatial resolution 30 to 90m and underestimated these two parameters by increasing spatial resolution 90 to 1000m. The results of this study showed that the accuracy simulation of discharge and total nitrate with the ASTER with the highest spatial resolution presented the best simulation compared to SRTM and GTOPO30 which this is due to the improvement of important physiographic properties, such as slope length and gradient and thus better simulation of hydrological processes.