Tutorial Stream and Catchment Delineation using PCRaster in QGIS

The next step is to calculate the channel network, based on the Strahler threshold determined in the previous step.

First we're going to create a layer with the Strahler orders for the rivers by selecting the cells with order equal to or higher than the threshold determined in the previous step. Here we'll use order 8 as threshold.

With the spatial tool you can make rasters with for all cells the input nonspatial value and the chosen data type.

1. Create an ordinal layer with value of the threshold, i.e. 8. In the Processing Toolbox go to PCRaster | Data management | spatial

2. In the Spatial dialogue type 8 for Input nonspatial. Choose Ordinal for Output data type and strahler as the Mask layer. Call the Output raster layer ordinal8.map.

3. Click Run and Close the dialogue after completion.

Now we're going to create a boolean layer with 1  (True) for all strahler cells that are >= 8:

4. In the Processing Toolbox go to PCRaster | Conditional and boolean operators | comparison operators.

5. In the Comparison Operators dialogue choose strahler as Input raster, >= as Comparison operator and ordinal8 as the second Input raster.

6. Click Run and Close the dialogue after processing.

Now we have a boolean layer with the channels that we can use now to assign the river strahler orders.

7. In the Processong Toolbox go to PCRaster | Conditional and boolean operators | ifthen

8. In the If Then dialogue choose channels as the Input Boolean Condition Raster and strahler as the Input True Raster. Call the Output raster channelsstrahler.map.

9. Click Run and Close the dialogue after processing.

Now we have a raster with the Strahler orders for the channels. This can be visualised better as a vector, whick will be explained in the next section.