The Effect Of The Spatial Heterogeneity On Topographic Wetness Index

The Topographic Wetness Index (TWI) is frequently used to simulate the ideal soil moisture conditions in a watershed quantitatively, and it is the most commonly used indicator for watershed soil erosion and distributed hydrological model as well. The index that extracted from DEM is an important factor with explicit physical significance in the rainfall-runoff process models. It indicates the tendency of water that accumulates or flow out in different terrain area. Some previous studies discovered that the TWI will change with the resolution of DEM from which it was derived, so the TWI depends on the DEM scale. This paper explored the resolution effects of the TWI and the influence of terrain heterogeneity of DEM grids among different resolution DEMs for two different terrain regions. The research area was located in Xiyuan and Neitian watersheds. The terrain in Xiyuan watershed is rather complicated than the easy grade topography of Neitian watershed. A fine-resolution DEM with 2.5m grid size was used to research the scale dependency of TWI values when converting DEM with coarse resolution using GIS, mathematical statistics and comparative analysis and other methods. The results indicated significant correlations between the terrain heterogeneity indices and the TWI variance in Xiyuan watershed. So, these indices can be used to evaluate the impact of the terrain heterogeneity on the TWI for the topography fluctuation area. By extrapolating the fitted curve for the terrain heterogeneity indices and TWI variance, the regression models can improve the accuracy of the TWI derived from a coarse resolution DEM. The effect discussed in this study is helpful in providing a more accurate data for the TWI applications.