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

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