Tutorial: Calculate time of concentration (Kirpich)

1. Introduction

In this tutorial we're going to calculate the time of concentration of a river. We'll use the Kirpich equation, but you can apply the method to other equations in a similar way.

The Kirpich equation:

tc = k * (dx ** const1) * (S ** const2)

Where:

tc = time of concentration in minutes
k = 0.0195 to convert to SI units
dx = distance between the first and last node in meters
S = the elevation difference between the first and last node in m/m
const1 = 0.77
const2 = -0.385

After this tutorial you're able to:

1. Select longest path of a river
   
2. Dissolve the line segments
   
3. Find first and last node on river
   
4. Merge first and last node into one point vector layer
   
5. Sample elevation from the DEM at these points
   
6. Calculate the Kirpich equation by hand or using the PyQGIS script
   

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.