 Hello, I'm Hans van der Kwas, senior lecturer at IHE Delft Institute for Water Education. In this video I'm going to show you how to calculate the Topographic Wetness Index. The Topographic Wetness Index is also known as the Compound Topographic Index, which is a steady state wetness index. It is commonly used to quantify topographic control on hydrological processes. The index is a function of both the slope and the upstream contributing area per unit width orthogonal to the flow direction. You can apply the Topographic Wetness Index to whole catchments and subcatchments but also to parcels. In this example I will apply it to a parcel. The first thing we need to do is to calculate the slope of this digital elevation model. In the main menu we go to Raster, Analysis, Slope. There we can choose to save and bear a torn formula instead of the horns one if you have more smooth areas. If you have steep areas you don't check the box, you can check this box. And we want slope in degrees. So we'll choose an output folder to save the file. And let's call it slope degrees. Then we can run the algorithm and you can see that it uses the GDALDAM function. Let's do a bit of styling. We can see already in the map canvas that we lose the sites that is inherent on these kind of calculations. So it's better to choose a larger area than the area you need. So if we apply some styling we can see already which areas are steeper and which are flatter. The next step is to correct this slope map and situate that we don't have zeros in the map. So therefore we go to Raster Calculator. This is needed because later we take the logarithm, the natural logarithm. So we start with the bracket. We add slope in degrees and we say when it's less than zero or equal to zero that we have to correct it. So we do times one and for the other values we keep the slope. And we call this one slope degrees modified. We run it and we can copy the style. There it is. And now we need to convert it to radians. So we go back to the Raster Calculator. To convert it to radians we use zero, zero, one, seven, four, five. And then it's slope radians.tiff. We can also style these layers. Nob and pseudo color, color ramp, inverted so the higher slopes become red. So the next step is to calculate the upslope contributing area. And we can do that with the flow accumulation tool from Saga. As an input we choose our DTM. We keep the default fills things temporarily. And we save it to an output file which we call upslope area. This will basically calculate for each pixel the contributing upslope area. So it's done so we can also style this one. And we can keep the red to blue ramp. But maybe we can use another way of dividing the classes to get a bit more contrast. Now the final step is to apply the Topographic Wetness Index Equation. It starts with a natural logarithm, a few brackets. We add the upslope area. And we need to correct it for the pixel size. In this case we have a five meter digital terrain model. So we multiply it by five. And then we need to divide it by the tangent of the slope in radians that we previously calculated. And we can save the output as twi.tiff. And when we run it, we have our Topographic Wetness Index. We can style it to make more sense out of the contrast. And we can keep the red to blue ramp as we see here. And this will give us the wetter and the drier areas in this parcel. There are some other tools in QDS to calculate the Topographic Wetness Index. But this is the basic equation that you can simply build up in the raster calculator. The other tools you can find under the saga tools.