Overview
Overview of watershed delination using QGIS. The Manual
approach uses a series of functions available in SAGA GIS. The Automated
approach uses QSWAT+ which is recentely implmented in QGIS 3. Other approaches are introduced in youtube videos by Hans van der Kwast.
Manual
Create a merged DEM layer
(1) Download Adjusted DEM from MERIT Hydro. Select area you need.
(2) Raster
> Miscellaneous
> Build Virtual Raster
(3) Select layers to be merged, uncheck the checkbox Place each input file into a separate band
, and Run
(4) Merged DEM
Raster channel network
(1) I create a binary raster data with 1
indicating stream networks (0
otherwise) to burn streams into DEM. Download Upstream Drainage Area from MERIT Hydro
(2) Raster
> Raster Calculator
(3) Input a formula to extract pixels with > XXX drainage area ("epsg4326_MWupa@1" >= 1
). In this example, I set a value of 1 as pixel values were entered in unit ‘km2’. Unit varies by file, so make sure before deciding the value.
(4) Binary channel data - pixels with > 1 km2 drainage area have 1, otherwise 0.
Burn streams into DEM
(1) Raster
> Raster Calculator
. Subtract a value from the original DEM using the Binary channel layer ("epsg4326_MWdem@1" - "epsg4326_MWupa_channel@1"*20
). In this case, subtracting 20 m from the original DEM. Important: every single pixel must match between layers.
(2) Burned DEM - pixels where streams flow have low elevations.
Reprojection
(1) Raster
> Projection
> Warp
(2) Reprojection setup
-
Input Layer
: burned DEM -
Source CRS
: current layer CRS -
Target CRS
: target CRS -
Resampling method to use
:bilinear
for DEM -
Nodata value for output bands
: -9999 -
Output file resolution in target georeferenced units
: 90 m -
Additional command-line parameters
:-tap
Fill depression area
(1) Processing
> Toolbox
> SAGA
>Terrain Analysis - Hydrology
> Fill sinks (wang & liu)
(2) Fill sinks setup
-
DEM
: Burned DEM -
Minimum Slope
: 0.001 (default 0.01, but does not work for Midwest)
Create a channel network
(1) Processing
> Toolbox
> SAGA
> Terrain Analysis - Hydrology
> Catchment area
-
Elevation
: Filled DEM -
Method
: Deterministic 8
(2) Processing
> Toolbox
> SAGA
> Terrain Analysis - Channels
> Channel network
-
Elevation
: DEM used for catchment area calculation -
Initiation Grid
: Catchment area grid produced by 'Catchment area' -
Initiation Type
:Greater than
-
Initiation Threshold
: Minimum catchment area to be a channel
(3) Channel network produced
Automated (QSWAT+)
Download SWAT+
(1) Go to SWAT+ website and install the software onto your computer. Install guide is available on the website linked.
(2) Plugins
> Manage and Install Plugins
> Search SWAT
> Check QSWATPlus3_64
to activate QSWAT+
Create streams
(1) Create a working directory where all the output files will be stored
(2) Create a QGIS project in the working folder (otherwise QSWAT+ will ask you to create)
(3) Click S+
icon in the menu bar
(4) Click Delineate Watershed
in the QSWAT+ inbox
(5) Create streams (click Create streams
once you are ready)
-
Select DEM
: Choose raw or burned DEM (or filled DEM, but the following process involves filling depression areas). -
Burn in existing stream network
: [Optional] check and choose a vector stream file for burn-in if you prefer -
Channel threshold
andStream threshold
: Define minimum catchment area to be a channel or stream
(6) Create watersheds (click Create watersheds
once you are ready). In the following example, I drew two outlet points indicated by blue triangles
-
Use an inlets/outlets shapefile
: [Optional] check and choose a vector file that specifies inlets/outlets or draw inlets/outlets (Draw inlets/outlets
). If you use a predefined point vector, the file must contain the following fields in its attribute table -ID
,RES
,INLET
,PTSOURCE
. The fieldID
is a series number that uniquely identifies each point, while others are an indicator variable that speficies whether the point is either a reservoir (RES
), inlet (INLET
) or PTSOURCE (PTSOURCE
). If the point is either of these, the corresponding column should be indicated as1
(otherwise0
). If all the columns butID
are0
, then the point is recognized as an outlet. -
Snap threshold (meters)
: If you draw inlets/outlets, those must be exactly on a stream line. QSWAT+ snap points to the nearest stream line with a threshold value of 300 m by default.
(7) Stream and watershed layers created for each outlet
(8) To merge subbasins, go to Merge subbasins
tab at the bottom > Select subbasins
. QAWAT+ will let you select subbasins that you want to merge. Note that the QSWAT+ will not merge subbasins that do not share an outlet.
(9) Merged watershed polygons
(10) Output files are stored in the working directory: Go to your working directory > Watershed
> Rasters
or Shapes