Topographic Wetness Index Computed Using Multiple Flow Direction Algorithm and Local Maximum Downslope Gradient

Topographic wetness index (TWI) can quantify the control of local topography on hydrological processes and indicate the spatial distribution of soil moisture and surface saturation. The formulation is TWI=ln(a/tanβ), where a is the upslope contributing area per unit contour length and β is the local gradient. Many of the existing flow-direction algorithms for computing a use global partition exponent to allocate the flow among downslope neighboring cells, which lead to unrealistic part itioning of flow. For approximating β, the downslope gradient can reflect impacts of local terrain on local drainage better than slope gradient at the point. This paper examines how TWI is impacted by computing a with a new multiple-flow-direction algorithm and approximating β with local maximum downslope gradient. The new multipleflow-direction algorithm is adaptive to local terrain conditions by altering the flow partition exponent based on local maximum downslope gradient. The proposed approach for comp uting TWI was applied to a small catchment (about 60 km) with low relief in the Nenjiang watershed in Northeastern China. The results show that the distribution of TWI by the proposed approach is more adaptive to terrain conditions than widely-used algorithm for TWI.