This exercise will demonstrate the process involved in watershed delineation using open source data and software. A preliminary analysis for any hydro-geological study will require us to understand the basic hydrological pattern and flow direction. An open platform to depict these parameters will be discussed in the paper to enable Indian citizen-scientists to understand their watershed and take participatory measures to ensure sustainable development. Methodology
1. Download the Digital Elevation Model (DEM) from the open source ISRO web platform
Download the Cartosat-1 satellite sensor product or tile for the required location.
2. Preparation of DEM : This section deals with the analysis of DEM files (tiff image). It provides instructions on how to create a hillshade map, shaded colour and relief map.
Step 1: QGIS Plugins -> GRASS
Enable gdal tools in QGIS from plugins/Manage Plugins
Step 2. Open GRASS GUI –> Create Directory
Define Location Wizard – WGS 84 (ES: 4326)
Start GRASS [ Raster -> Projections -> Warp]
Step 3: Load the raster (DEM) and the vector shapefile (Create shp of lake/river using google earth (kml file)) in QGIS.
3.1 Use Clipper tool (Raster/Extraction/Clipper) to select DEM into smaller area of interest according to the shapefile. Use the option of extent.
3.2 If Multiple DEM files are needed use Raster/Miscellaneous/Merge option to combine the raster.
3.3 Convert the projection of DEM from WGS 1984 to WGS 1984/UTM 43N using Raster =>Projections => Warp option. The raster is converted from geographic coordinate
system to projection coordinate system. In simple terms, it would help us in analysis of vector files by giving the values in meters instead of degrees.
ANALYSIS OF DEM
Open the Grass Location by clicking ‘ ’. Open the Grass tools by clicking ‘ ’.
Load the DEM into grass. Search and select r.watershed.
Select the DEM from the list and give the file names.(See figure 1.2 on page 1.2) Use threshold of 10000 which would create watersheds with minimum area of 1 sq.km. Preferred names are fdrain for the first map that gives the drainage flow. Use fdir for second map which gives the drainage direction. Use stream for third map which gives the stream segments. Use catch for the fourth map which gives the raster map.
Convert the stream raster to vector by using r.to.vect.line. Similarly convert the catch raster using r.to.vect.area module. The output will be similar to figure 1.3
Identify and select the polygons you need in the basin and delete the remaing polygons from the attribute table.
Hint: If you use higher threshold 1000000 then it would create bigger watersheds.
By following the step 5 basin can be created easier. Then use clip tool from Vector/Geoprocessing to crop the smaller watershed vector.