Tutorial: Calculate time of concentration (Kirpich)
|Site:||IHE DELFT OPEN COURSEWARE|
|Course:||GIS OpenCourseWare for Hydrological Applications|
|Book:||Tutorial: Calculate time of concentration (Kirpich)|
|Printed by:||Guest user|
|Date:||Tuesday, 19 October 2021, 11:38 AM|
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)
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:
- Select longest path of a river
- Dissolve the line segments
- Find first and last node on river
- Merge first and last node into one point vector layer
- Sample elevation from the DEM at these points
- 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.