An improved Topographic Wetness Index considering topographic position

Abstract

The Topographic Wetness Index (TWI) is frequently used to simulate the soil moisture conditions in a watershed quantitatively, and it is the most commonly used indicator for static soil moisture content as well. So it plays an important role in the research of soil erosion and distributed hydrological model in watersheds, etc. Additionally, the index was generally extracted from Digital Elevation Model (DEM), and the formula is: w = ln (As/ tanβ), where As means the specific catchment area (catchment area divided by the cell width in slope direction); β means the local slope in the steepest down slope direction of the terrain in degrees. Then tanβ is the local slope angle of the specific grid, which is used to replace approximately the local hydraulic gradient under steady state conditions. However, TWI only has the physical significance based on the runoff flowing by gravity, doesen't consider other factors. For example, as for some cells with the same catchment area and slope, the soil moisture content varies in different aspects and terrain positions. This result was directly caused by the differences in temperature, heat and the soil physical and chemical properties which were induced by the solar radiation variance. We classified the aspect into shady slope, sunny slope, and semi-shady and half-sunny slope. And the terrain position was graded by a function of Relative Position Index (RPI). According to the rules: the soil moisture content of (1) shady slope being greater than that of sunny slope, (2) the terrain position increasing from top to bottom, we assigned a weight to the topographic position (i.e. aspect and terrain position) corresponding. Then we revised the simple TWI by the topographic position, and acquired the improved one capable of representing the soil moisture content more accurate. In the work, the DEM with the resolution of 40 m in our experimental area Xiamen City, Fujian Province, P. R. China, was derived from 1:10000 map scale Digital Line Graphic (DLG), and TWI was computed with the improved method introduced above. The result shows that considering topographic position in the simple TWI can make it more clear on physical significance; and the improved TWIs can also provide a scientific basis for predicting the spatial distribution of the soil moisture content, simulating the runoff and evaluating the dispersion of the point pollution sources.