Tutorial: Calculate the Topographic Wetness Index from a DTM
Completion requirements
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:
TWI =
Where:
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