## Tutorial: Calculate time of concentration (Kirpich)

### 4. Find the first and last node of the river

In this step we're going to find the first and last node of the river. For the Kirpich equation we later need the distance between these points and their elevation difference.

1. Open the *Processing Toolbox*: from the main menu choose **Processing | Toolbox**.

2. click on the tool **Vector Geometry | Extract specific vertices**

This tool can be used to extract specific vertices.The first vertex corresponds to an index of 0, the second vertex has an index of 1, etc. Negative indices can be used to find vertices at the end of the geometry, e.g., an index of -1 corresponds to the last vertex, -2 corresponds to the second last vertex, etc.

3. In the

*Extract Specific Vertices*dialogue choose*river_dissolved*as*Input layer*, for*Vertex indices*type`0, -1`

and save it to the *Kirpich_data*GeoPackage with the name*firstlastnode*. Click*Run*.4. Click

*Close*after processing.5. Click right on the

*firstlastnode*layer and choose**Open Attribute Table**.You can see in the field

*vertex_pos*the index of the vertex and in the*distance*field the distance from the first node in meters. So here the distance of the last node is 3745 m. We need that value in the equation.We also need the elevation difference between the first and last node. In the next step we're going to sample the elevation on the two nodes.