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How to Calculate Topographic Wetness Index using ArcGIS Pro | Flood Mapping

The Topographic Wetness Index (TWI) is a commonly used index in hydrology and geomorphology to quantify the effects of topography on the location and extent of wet areas in a landscape. It is particularly useful for identifying potential areas of soil saturation, runoff generation, and the spatial distribution of soil moisture. TWI is derived from a Digital Elevation Model (DEM) and is based on the concept that water flow and accumulation are influenced by the shape and steepness of the terrain. Calculation of TWI The TWI is calculated using the formula: In a/tan b where: • a = upslope contributing area per unit width (often derived from flow accumulation) • b = local slope angle (calculated from the DEM) Steps to Calculate TWI 1. Obtain DEM: o Acquire a high-resolution DEM of the area of interest. 2. Calculate Slope: o Use GIS software to calculate the slope from the DEM. This gives the steepness of each grid cell. 3. Calculate Flow Accumulation: o Compute the flow accumulation, which represents the upslope contributing area. This can be done using flow direction and flow accumulation tools in GIS. 4. Compute TWI: o Use the TWI formula to compute the index for each grid cell. This can be done using the Raster Calculator in GIS software by applying the formula above.