How to delineate watersheds in QGIS?

Question

I am using qgis 2.14.17 with ubuntu16. Having as input data a DEM of the region and a river network. In the manual of qgis they suggest to use a toll of qgis: Catchment area (Parallel); I dont have this in my toolbox, anybody made the same experience and could tell me the reason for it and how to get it. It seems to offer the best solution for me. Tools which are available are from GRASS7 and TauDEM, the latter need much more data it seems. And I am not sure if I am able to create these just out of a DEM? The GRASS7 has one tool r.water.outlet: but here one has to put the outlet coordinates, which clearly I could get out of the river network, but the idea is that they are generated by delineating the watersheds.

Somebody has an idea how to solve it?

I managed now to get SAGA run in the toolbox, nevertheless would be great if somebody could help me with the qgis tool, since it should appear?!

Answer 1

Assuming your QGIS 2.14.17 is equipped with SAGA 2.3.1 or 2.3.2, let me answer only the first question "where to find Catchment area (Parallel) in the QGIS Processing Toolbox?"

Try and find SAGA | Terrain Analysis - Hydrology | Catchment area

This tool uses parallel processing, so you would find it was called Catchment area (Parallel) in some older documents.

SAGA-wise, the tool has been renamed to Flow Accumulation (Top-Down) (from SAGA 2.1.3, I think). In the QGIS Processing Toolbox, it still keeps a plain name Catchment area, but this new name appears in the log while you are running the tool.

library path: C:\PROGRA~1\QGIS2~1.14\apps\saga-ltr\modules\
library name: ta_hydrology
library : Hydrology
tool : Flow Accumulation (Top-Down)
author : O.Conrad (c) 2001-2016, T.Grabs portions (c) 2010

In the meantime I am almost certain you will encounter an error Unknown option 'CAREA' using QGIS 2.14.17 (or even latest 2.14.19). If it occurs please try QGIS 2.18.13. (See this post Error in catchment area calculation by André Lourenço).

If successful, you will get something like this:

Usual workflow is to get this Catchment area first, next step is Channel network, and then Watershed basins.

If your River network is the same as Channel network (raster), you do not have to do this task. (I guess your river network might be vector...)

Answer 2

The GRASS option from the toolbox

1. use the r.watershed with the DEM as input to obtain as output a drainage direction raster map and the accumulation raster map (Number of cells that drain through each cell).
2. use the r.water.outlet with the drainage direction as input and a chosen point of the river taken from the acculation map

Answer 3

Circa 2023, you can use the Global Watersheds API to find the watershed upstream of a point based on its latitude and longitude coordinates. Instructions are here:

https://mghydro.com/watersheds/help.html#api

Briefly, you just need to create a URL with the coordinates and the options you want. For example:

https://mghydro.com/app/watershed_api?lat=43.253&lng=-77.609&precision=high

You can test the URL in a web browser, and copy and paste the results to preview them on https://geojson.io.

To load the watershed on your map in QGIS:

• Choose Layer > Add Layer > Add Vector Layer.

• Under Source Type, choose Protocol: HTTP(S), Cloud, etc. You can leave Encoding as Automatic. (The API returns UTF-8.)

• Under Protocol, for Type, choose GeoJSON.

• In the field, URI, type or paste the URL, then click Add.

Authentification is not required for the Global Watersheds API, so you don't have to make any changes in the last section.

The result should look something like this:

You can then adjust your symbology, or export the layer to save it on your hard drive.

Note that the web app is based on 90m resolution data from MERIT-Hydro. If you are working in the US, you will get more accurate results with the USGS' National Hydrography Dataset (NHD). The app is better at finding large watersheds, and the results can be poor in certain areas. Make sure to always inspect the results to make sure they make sense.