Topographic Wetness Index derived for Madagascar

The Topographic Wetness Index (TWI) is a proxy for soil moisture that can be derived from Digital Elevation Models (DEMs). Previously, TWI has been linked to patterns of plant species richness (Sørensen et al., 2006) and biomass (Xu et al., 2015). TWI also represents the influence of topography, with catenary variation (Sørensen et al., 2006) and other soil properties significant to plants such as the distribution of organic matter (Pei et al., 2010) and soil carbon (Sumfleth and Duttmann, 2008). TWI is a physically-based model of catchment parameters derived from slope that calculates water supply from the contributing hillslope by routing it as drainage down the channel network through all cells in a DEM (Beven and Kirkby, 1979). The TWI quantifies the balance between water accumulation and drainage at a local catchment scale, using the following equation: TWI = ln(upslope area / tan(slope)).