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

Completion requirements

### 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:

- 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