Watersheds delineation with SAGA and QGIS

A simple workflow to create watersheds and streams networks from SRTM data.

Final product shows delineated basins in different colours as well as drainage network of area of interest.

What is needed
in SAGA
in QGIS
Final products
References

What is needed

Software used:
- SAGA GIS 2.3.2 LTS
- QGIS 3.14 LTS
To start with, SRTM30 data is needed. Source tiles from USGS EarthExplorer^* web site. Use geoTIFF tiles. Coordinates system of downloaded files is WGS84. This to be converted to a projected coordinates system. Do it in QGIS: drag and drop the SRTM file into QGIS, right-click on the name in the Legend and Save as.... Then define the coordinates system.

1 - In SAGA

Following steps will be done in SAGA:

A - Import projected SRTM
B - Fill sinks in SRTM and create watershed basins
C - Compute flow accumulation
D - Compute channels network

A - Import SRTM

Open SAGA
Click "Open" icon and select projected SRTM file
A new project is created. Its frame is equal to the dimension of the DEM file

Results

01. DEM

B - Fill sinks in SRTM

First step is to process SRTM data in order to correct for irregularities in slope values. Raw SRTM data does not flow smoothly along real slopes, instead elevation values along a real stream profile create dumps and holes along that profile. To correctly compute stream network it is thus necessary to fill all sinks along streams path.

By-product from this step in a watershed raster grid. This is one product of interest.

Geoprocessing -> Terrain Analysis -> Preprocessing -> Fill Sinks (Wang Liu):
Setup Grid system, DEM and Watershed Basins:

Results

02. DEM [no sinks]
03. Flow Directions
04. Watershed Basins

C - Compute flow accumulation

This step is a preliminary computation to get to streams networks.

Geoprocessing -> Terrain Analysis -> Hydrology -> Flow Accumulation -> Flow Accumulation (Top-Down):
Setup Grid system, Elevation and Method:

Results

05. Flow Accumulation

Open Flow Accummulation map, move mouse over map and note that in dark blue areas (streams) accumulation values are generally very high (> 1 000 000). This fact will be used in next step.

D - Compute Channels Network

In this step a river network is created from flow accummulation data. This is the second product of interest.

Geoprocessing -> Terrain Analysis -> Channels -> Channel Network:
Setup Grid system, Elevation and Initiation Grid and Initiation Treshold (IT, in function on accummulation values seen above):

Results

06. Channel Network - raster
07. Channel Direction - raster
01. Channel Network - vector (shapefile)

Channel Network - vector. Dark blue: IT = 3 000 000; Fushia: IT = 1 000 000

Initiation Treshold (IT) value can be changed to obtain more or less detailed channel network.

Save work

Save everything to disk. SAGA creates lots of files. The ones used by QGIS are .sdat files.

2 - in QGIS

Following steps will be done in QGIS:

Import stream network
Import watersheds file
Vectorise basins
Organise basins into a single shapefile
Style basin shapefile

A - Import stream network

SAGA has created a shapefile for the stream network. Locate it (*.shp - Channel Network_1.shp) in File Explorer and drag it to QGIS canvas.

Style it to represents rivers/streams.

B - Import watersheds file

SAGA also created a watersheds file (by default Watershed Basins.*). Locate Watershed Basins.sdat and drag it to QGIS canvas.

Watersheds are shown in different shades of grey:

C - Vectorise basins

Watersheds raster file is to be converted to shapefile using Raster -> Conversion -> Polygonize (Raster to Vector)...:

Results

OUTPUT vector. Can be saved to shapefile or otherwise later.

D - Organise basins into a single shapefile

At edges of OUTPUT there are lots of badly defined watersheds, simply because there is not enough data there! As area of interest lies in map centre, only relevant basins will be kept.

With selection tool , select basins of interest (9 here), and then inverse selection.