Reducing hillslope size in digital elevation models at various scales and the effects on slope gradient estimation

Several models simulate watershed areas by delineating hillslopes. Hillslope size depends on the length of stream tributaries, which are affected by the drainage area threshold (DAT). There is no universal approach to identify the appropriate DAT. Therefore, a method to derive the DAT and a series of steps to delineate a watershed into smaller sizes were proposed in this study, and the impact of hillslope size on slope gradient estimation was investigated. The DAT obtained in this study was smaller than that obtained using other methods, resulting in a shorter length of the tributaries. Dividing these tributaries into equal short segments and using them to delineate the study area reduced the size of the hillslope. The results revealed that the shorter the length of the tributaries, the smaller the hillslope size. The accuracy of gradient estimation increased when the size of the hillslope was reduced.