Abstract: The objective of this research is to better reflect the watershed land surface characteristics of computing units in distributed hydrological model. According to selected indexes, the terrain in the studied watershed was classified. On the basis of the classification and the division unit area function of ArcGIS, the optimal drainage area threshold of the studied area in the upstream of Yangtze River is obtained by using improved fitness index method, then the relationship between drainage area threshold and river network density could be determined. Results show that by using improved fitness index method, the simulated river network could reflect the actual river system and is reasonably close to the reality. According to the relationship, the optimal drainage area threshold could be obtained quickly given the blue line network density.

Key words: Digital Elevation Model(DEM), terrain classification, improved fitness index method, river network density, optimal drainage area threshold