The modulehas been designed to perform the delineation and the morphometric characterization of a given basin, on the basis of an elevation raster map and the coordinates of the outlet. Here a tutorial based on NC sample dataset is presented.
As a first step, we set the computational region to match the elevation raster map:
g.region rast=elevation@PERMANENT -ap projection: 99 (Lambert Conformal Conic) zone: 0 datum: nad83 ellipsoid: a=6378137 es=0.006694380022900787 north: 228500 south: 215000 west: 630000 east: 645000 nsres: 10 ewres: 10 rows: 1350 cols: 1500 cells: 2025000
For the basin's delineation, a pair of coordinates is required. Usually coordinates belonging to the natural river network don't exactly match with the calculated stream network. What we should do is calculating the stream network first, and then find the coordinates on the calculated stream network closest to the coordinates belonging to the natural stream network.
# Calculate drainage map r.watershed -a elevation=elevation@PERMANENT accumulation=accum # Extract the stream network r.stream.extract elevation=elevation@PERMANENT accumulation=accum@testing threshold=20 stream_rast=stream_network
We no longer need the accumulation map:
Now that we have the calculated stream network, we should choose a pair of coordinates for the outlet belonging to it. Let's choose:
We no longer need the stream network map:
Usage of r.basin
We can run r.basin:
r.basin map=elevation@PERMANENT prefix=out easting=636654.791181 northing=218824.126649 threshold=20
Prefix parameter is a string given by the user in order to distinguish all the maps produced by every run of the program, i.e. every set of outlet's coordinates. Prefix must start with a letter. Threshold is the same parameter given in r.watershed and r.stream.extract. Physically, it means the number of cells that determine "where do the river begins". (This is an open issue for the hydrological science and a wide literature has been produced on the topic).