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 <em>et al</em>., 2006) and biomass (Xu <em>et al.</em>, 2015). TWI also represents the influence of topography, with catenary variation (Sørensen <em>et al</em>., 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: <em>TWI = ln(upslope area / tan(slope))</em>.