8  Concept of hydrology analysis in GIS

This exercise introduces you to the concept of hydrology analysis in QGIS. We will use the following data for the exercise. This is very small raster DEM data with 6 rows and 5 columns.

8.1 How the data was created

The data was created using a text editor with following values and saved as ESRI ascii file which can be opened in QGIS. The data doesn’t have any coordinate reference system (CRS) as the ESRI ascii format does not support it. In the next step we will convert it into GeoTIFF (format).

ncols 5
nrows 6
xllcorner 378923
yllcorner 4072345
cellsize 100
nodata_value -9999
480 500 490 480 485
505 495 490 475 480
500 485 455 470 475
495 490 480 465 470
490 485 475 455 450
500 490 480 460 445

8.2 Converting ESRI ascii format to GeoTIFF

  • Open OSGeo4wShell and run the following command.
gdal_translate dem.asc rast01.tif -a_srs EPSG:32645

8.3 Convert the DEM to point data

It is only for display. Use processing tools $r.to.vect. In the dialog box, set feature type to point. Point data will be created. If you display the label it will look as shown.

DEM values

8.4 Concept of sink

When a cell or a group of cell is at lower elevation than their surrounding cell these cells are called sink cells. In the above figure we can see that the cell-C3 (Named as in Excel) has lower elevation compared to all its surrounding cells.

In the hydrology analysis, the first step is to fill the sink so that the water flows out of the cell.

Let us try two different methods.

  1. Run r.fill.dir from Processing Toolbox. This Grass-GIS implementation of sink fill.
  2. Run SAGA \(\Rightarrow\) Terrain Analysis - Preprocessing \(\Rightarrow\) Fill Sink (Wang & Liu) (Note: you can find this tool just by searching the term fill sink in the processing toolbox)
    • Save all the files as shown below Fill Sink (Wang & Liu)

The result of above two methods is shown below

Sink Fill Comparison We can see that grass tool replaces the elevation of sink cell to the lowest elevation of the surrounding tool. Saga tool further adds the elevation based on the slope parameter provided.

8.5 Flow direction

The saga tool produces Flow direction raster which we can show as arrows. The steps are explained below.

  1. Install Crayfish plugin.

    • Plugins \(\Rightarrow\) Manage and Install plugins
    • Search Crayfish and install

  2. Open Crayfish \(\Rightarrow\) Conversions \(\Rightarrow\) SAGA Flow to Grib tool from Processing Toolbox

  3. Fill the parameters as below Flow to Grib

  4. Add the newly created mesh file (fillgrid.grb) to the project using Add Mesh Layer tool.

    Flow Mesh -> Add

  5. Now double click the flowgrid mesh to show the symbol. Flow mesh symbology

  6. Finally the flow direction will be shown as below.

    Flow Direction

  7. Flow Accumulation

    • Find r.flow tool in Processing Toolbox

    • Fill the parameters as shown below

      Flow Accumulation Tool

  8. Now we can see the flow accumulation values by converting flow-accumulation raster to vector.

    Flow Accumulation Result