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.
Now we're ready to calcute TWI.