Tutorial: Calculate the Topographic Wetness Index from a DTM

1. Introduction

The Topographic Wetness Index (TWI) is a useful model to estimate where water will accumulate in an area with elevation differences. It is a function of slope and the upstream contributing area:



a = uplope contributing area (m2)
b = slope in radians.

In this tutorial we're going to implement this equation in QGIS. The input data is a DTM.

After this tutorial you'll be able to:

  • Interpolate voids in a DTM
  • Calculate slope in degrees
  • Convert slope from degrees to radians
  • Calculate TWI