Tutorial: Calculate the Topographic Wetness Index from a DTM

4. Calculate the Contributing Upslope Area

In this section we're going to calculate the contributing upslope area for each pixel in the DTM.

1. Go to the Processign Toolbox and choose SAGA | Simulation | Flow accumulation (qm of esp).

2. In the dialogue choose dtm_voidfilled as DEM, keep the default Preprocessing to fill sinks temporarily and save the Contributing Area to contributing_upslope_area.sdat.

3. Click Run. Close the dialogue after processing.

The result is for each pixel the amount of pixels that are upstream.

Contributing Upslope Area

Now we're ready to calcute TWI.