Tutorial: Calculate time of concentration of a river (Kirpich equation)

2. Select the longest path of a river

The first step is to select the longest path of the river for which you want to calculate the time of concentration. Unfortunately, there's not a simple tool that does that. Here we'll use a manual procedure.

1. Start QGIS

2. Open the Kirpich project from the GeoPackage provided with this tutorial. In the main menu choose Project | Open from | GeoPackage...

3. In the Load project from GeoPackage dialogue browse to Kirpich_data.gpkg and choose Kirpich as project.

The project has the catchment polygon (catchpoly), channels, DEM (SRTM 1-Arc Second) and is styled using blending with a hillshade.

4. Click on channels

5. Click the Select Features by area or single click icon

6. Now select the channel segments of the longest path from upstream to downstream. Keep the <Shift> button pressed to add the segments. Zoom in if necessary. You can use the scroll button of your mouse to zoom in/out and you can use the spacebar in combination with moving the mouse to pan. If you accidently have selected a wrong segment, click on it again with to deselect it.

After selecting the longest path the selected segments should be yellow as in the picture above.

Now we're going to export this longest path to a new layer.

7. Click right on channels in the Layers panel and choose Export | Save selected feature as...

8. In the dialogue save it to the Kirpich_data GeoPackage, call the layer river and click OK.

9. Copy the style from channels to river: click right on channels and choose Styles | Copy Style | All Style Categories... and click right on Kirpich_data river and choose Styles | Paste Style | All Style Categories.

10. Remove the channels layer.

Now we have the longest path of the river.

In the next step we're going to dissolve the segments of the longest path to one feature.