Abstract: Taking Erhai basin, Yunnan province as an example, this paper investigated the effect of sub division level on simulations of runoff and sediment load under SWAT (Soil and Water Assessment Tool) framework, and determined the optimal sub division scheme.Results showed that the annual average runoff depth increased with the number of the sub watershed，but the sensitivity was relatively small，and the relative deviation was only 0.89%.In contrast，the annual average sediment load decreased with the number of the sub watershed，and the sensitivity was relatively large，the relative deviation being 21.21%.Considering the response of both the annual average runoff depth and sediment load to the sub division scheme, a threshold value of 137 was proposed as the optimal sub division level in Erhai basin, and the average area of the sub watershed accounts for 0.73% of the total basin area.Above this critical value, the simulated runoff depth and sediment load were stabilized, and their accuracy could not be improved further.According to our results, the method, using ratio of a subwatershed to total basin area to determine the optimal number of sub watershed in previous studies, cannot describe the real condition in a basin.Our results have implication for hydrological simulations and non point source pollution simulations in other similar complex topographical watershed.

Key words: SWAT model, Erhai basin, Optimal sub division of watershed