This is a practical demostration of Watershed Analysis with GRASS. This follows a click-this and click-that approach, followed by questions and exercises.

1. Watershed analysis with GRASS
Session - IV
Workshop on “Introduction to Remote Sensing”, July 7 – 11, 2014, JNEC Aurangabad

2. Strategy – watershed analysis
● Take an elevation map provided in the data
– Study the elevation map by drawing various profiles
● Use the elevation maps to create basins and
catchment accumulation lines
● Thin the catchment lines
● Convert the thinned lines to a vector map

3. Installing your dataset
1. Download your data from
http://grass.osgeo.org/download/sample-data/
1. Download your data from
http://grass.osgeo.org/download/sample-data/
2. Store the compressed file in your GRASS
project folder
2. Store the compressed file in your GRASS
project folder
3. Uncompress the file to obtain the
spearfish60 folder
3. Uncompress the file to obtain the
spearfish60 folder

4. 4. Select spearfish604. Select spearfish60
5. Select PERMANENT5. Select PERMANENT
6. Click Start GRASS6. Click Start GRASS

5. Two blank windows are displayed in parallel. The left one is known as the layer
manager and the right one is known as the MAP DISPLAY window
Two blank windows are displayed in parallel. The left one is known as the layer
manager and the right one is known as the MAP DISPLAY window

6. 7. Click here to add raster layer7. Click here to add raster layer
8. Select layer elevation.dem@PERMANENT8. Select layer elevation.dem@PERMANENT
9. Click OK9. Click OK
10. Elevation data displayed10. Elevation data displayed

7. 11. Click here. Select Profile
Surface Map
11. Click here. Select Profile
Surface Map
12. Select raster map to
profile. Click Ok.
12. Select raster map to
profile. Click Ok.

8. 13. Click here to draw a
transect for profiling the
terrain on the map window
13. Click here to draw a
transect for profiling the
terrain on the map window
14. Draw the profile. Switch
back to the Profile Analysis
Tool
14. Draw the profile. Switch
back to the Profile Analysis
Tool
16. Click here for rendering
the profile
16. Click here for rendering
the profile
17. Profile displayed here.
Close window after studying
17. Profile displayed here.
Close window after studying

9. 18. Click Raster > Hydrologic
Modelling > Watershed analysis
19. Select
elevation.dem@PERMANENT
18. Click Raster > Hydrologic
Modelling > Watershed analysis
19. Select
elevation.dem@PERMANENT
21. Minimum size of exterior
watershed basin (1000)
21. Minimum size of exterior
watershed basin (1000)
20. Click Input_options20. Click Input_options
22. Click Output_options22. Click Output_options
23. Name of output map (number
of cells draining through each
cell)
23. Name of output map (number
of cells draining through each
cell)
24. Name of watershed “basin”24. Name of watershed “basin”
25. Click Run25. Click Run

10. 26. Switch on / off the layer
displays using the check-boxes
26. Switch on / off the layer
displays using the check-boxes 27. Basins map27. Basins map
28. Accumulation map28. Accumulation map

11. 29. Go to the command console and type
r.mapcalc 'log_accumulation=log(abs(accumulation)+1)'
Press Enter
29. Go to the command console and type
r.mapcalc 'log_accumulation=log(abs(accumulation)+1)'
Press Enter
30. Add the layer named
log_accumulation using the layer
manager
30. Add the layer named
log_accumulation using the layer
manager
31. Tick the check box for the
log_accumulation layer
31. Tick the check box for the
log_accumulation layer
32. log_accumulation layer
displayed
32. log_accumulation layer
displayed

12. 33. Go the command console and type the
following command:
r.mapcalc 'inf_rivers=if(log_accumulation>6)'
Press Enter
33. Go the command console and type the
following command:
r.mapcalc 'inf_rivers=if(log_accumulation>6)'
Press Enter
34. Add the inf_rivers layer
using the layer manager
34. Add the inf_rivers layer
using the layer manager
34. Add the inf_rivers layer
using the layer manager
34. Add the inf_rivers layer
using the layer manager

15. 33. Go the command console and type the
following command:
r.mapcalc 'inf_rivers=if(log_accumulation>6)'
Press Enter
33. Go the command console and type the
following command:
r.mapcalc 'inf_rivers=if(log_accumulation>6)'
Press Enter
34. Add the inf_rivers layer
using the layer manager
34. Add the inf_rivers layer
using the layer manager
35. Check on the display of
the inf_rivers layer
35. Check on the display of
the inf_rivers layer
36. inf_layers displayed.36. inf_layers displayed.

16. 37. Click Raster > Transform Features
> Thin from the layers manager menu
37. Click Raster > Transform Features
> Thin from the layers manager menu
38. Select inf_rivers as input raster
map and set the output map name to
rivers_thin
38. Select inf_rivers as input raster
map and set the output map name to
rivers_thin
39. Click Run39. Click Run
40. Click Raster > Map type
conversion > Raster to vector
40. Click Raster > Map type
conversion > Raster to vector
41. Select rivers_thin as input raster map
and set the output vector map name to
rivers
41. Select rivers_thin as input raster map
and set the output vector map name to
rivers
42. Click Run42. Click Run

17. 43. The vector map rivers is displayed in the
Map Display window
43. The vector map rivers is displayed in the
Map Display window

18. Exercises
● Draw more profiles using the elevation map
– Can you now predict the shape of the terrain by
looking at the elevation map?
● Use the elevation.10m layer, and repeat the same
exercise again
– In case you chose to use the same layer names for
outputs, remember to use the overwrite option.
– See if more details in the river network happen. Why is
it so?
● Use another dataset e.g. North Carolina available
from the GRASS website to do the same exercise.