Blog post

Updated Watershed tool provides even more detailed catchment characteristics

Jul 7, 2022

by Helena Åström, helena@scalgo.com


We are constantly inspired by our users and their ideas on how to improve different tools and analyses in SCALGO Live. Today, we are proud to announce that some of the most frequently suggested improvements of the Watershed tool has been released. We have added a brand new set of catchment characteristics that enable you to gain an even deeper understanding of your watershed. We have thought long and hard about how to present this new information in a way that is fast, interactive and visual – we hope you like it.

The new information can be used for estimating hydrological response time when dimensioning culverts, as input to hydrological modelling, as a starting point when screening for measures to slow down water, and much more. Here’s a short introduction to the interesting new catchment characteristics and cool new visualisations.

Longest path in a catchment

Now SCALGO Live automatically computes the longest flow path in a catchment. Click at any point in the terrain to see the longest flow path to that point, including the flow path length and average slope.

Use this information to estimate the concentration time of the watershed, i.e. the time needed for runoff to travel from the most remote point to the outlet.

Figure 1. The longest flow path is shown as a red line through the watershed. In the watershed info box you see the length and slope of the longest path.

Length/area histogram - characterising the shape of your catchment

To supplement the longest path, we also give you the Length/area histogram with a more in-depth characterisation of all flow paths. This histogram describes how the area of a watershed is distributed across various flow path lengths. Why is this useful? Because all watersheds are unique and have very different shapes (even if they have the same longest flow path length), and this ultimately affects runoff behaviour.

Use the Length/area histogram to answer questions such as:

  • How large an area of the watershed is closer than x meters to the outlet?
  • How long are other flow paths in the watershed compared to the longest flow path?

As an extra bonus to our Danish users, we’ve also visualized the land cover distribution in each histogram column which allows you to assess e.g. the infiltration effects of impervious areas. This feature will soon be available in other countries too.

Figure 2. The Length/area histogram describes the how the area of a watershed is distributed across various flow path lengths. Here, red is buildings, orange is other impervious, and green is pervious.

Note that if you click on one of the bars in the chart you can interactively identify the part of the catchment corresponding to the chosen flow path lengths.

Figure 3. The area that has a flow path to the query point with a length less than the upper limit of the selected bar in the histogram is highlighted on the map.

How to read the histograms

Below are a few examples of what different shapes of histograms can tell you about your watershed:

1) Most flow paths in the watershed are much shorter than the longest path. This indicates that a large part of the catchment area drains towards the outlet faster than the concentration time.

2) Many flow paths are similar in length to the longest flow path. This indicates that most of the catchment area is far away from the outlet and runoff from these flow paths will reach the outlet approximately at the same time.

Figure 4. 1) Most flow paths are much shorter than the longest flow path 2) There are many flow paths in the catchment that are similar in length to the longest flow path.

What do you think?

Try the new expanded watershed tool and let us know what you think! If you have any questions don’t hesitate to contact us at info@scalgo.com.

You can read more about the new update in the manual.