Now we need to polygonize the raster layer with the catchment.
1. In the Algorithms panel choose Polygonize (raster to vector).
2. In the dialogue change the Description to Polygonize catchment. Choose the Upslope area as Input layer and leave the rest as default.
3. Click OK.
Now we need to select and save the attribute with value 100, which is the catchment.
4. In the Algorithms panel choose the Extract by attribute algorithm.
5. In the Extract by attribute dialogue replace the Description with Save catchment polygon only. As Input layer choose the output of the previous algorithm. Type for Selection attribute DN (that's the field where with the values 0 and 100), keep the Operator as = and type 100 for the Value, so we'll select the feature with DN = 100, which is the catchment boundary. Save the Extracted polygon as Catchment Boundary.
6. Click OK.
The model should now look like the figure below.
7. Save the model by clicking the button and close the Graphical Modeler window.
Next, we'll run the model.