# 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:

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