Topography index (wetness index) is a crucial kind of input data,which links the saturation deficit of a basin with the conception of variable contributing area.According to the definition of topography index,procedures of calculating the index were presented.Solutions to problems of processing grids with zero slope degree and expurgated deviations induced by the involvement of channels in computation were introduced.The index was derived from the DEM of the Lianghekou Basin with a solution of 60 m in this method,and then imported into TOP-MODEL with a suitable data set.The result of daily runoff simulation turns out to be acceptable with about 70% prediction precision.