Abstract: As a physical index to express the information of topography in hydrological modeling, topographic index, ln(α/tan β) has a high application value. River Tools and GRIDATB programs are used to compute the spatial distribution of ln(α/tan β) in the small watershed in redsoil hilly region with moderate topography based on DEM. They use D8 and MFD algorithms respectively. The results show that values of ln(α/tan β) computed with the D8 algorithm are more discrete and have rougher pattern than those computed with the MFD algorithm, and the mean values of ln(α/tan β) with D8 algorithm are slightly higher than those with the MFD algorithm. Compared to the D8 algorithm, the variances and skews of the distributions with the MFD algorithm are lower. However, the spatial distribution of ln(α/tan β) values with the D8 algorithm looks similar to that with the MFD algorithm. For some hydrographic models, such as TOPMODEL, the D8 algorithm can be used to compute the spatial distribution of ln(α/tan β) instead of the MFD algorithm.

Key words: topographic index ln(α/tan β), D8 algorithm, MFD algorithm, red-soil hilly region