RiverBasinObject: Sub-catchment delineation
A simple HOW-TO using GRASS.
- Start GRASS (the Geographic Resources Analysis Support System)
on your client computer;
GRASS is a public domain raster based GIS, available for free download from
a number of sources, including:
An excellent preparatory step is to READ the GRASS Manuals (see 5. below)
and be at least vaguely familiar with the basic terms and concepts
of GRASS (and GIS in general).
- Create a new mapset, using the parameters defined in the current WaterWare applications
For he Murrumbidgee river example:
UTM, zone 55, north=6280000, s=5980000, e=740000, w=440000, resolution=90)
- Create a flow_direction file from a high-resolution DEM
(e.g., any local/national source or from the public domain such as
the ASTER 30m resolution data file; this can be obtain from:
NASA/USGS MODISAster30 m DEM ImageScenes,
Highest resolution DEM data in the public domain.
The flow direction file (or drainage raster map) is generated with r.watershed command in GRASS.
Shortcut for the Murrumbidgee River application:
download flow_direction file: 22.214.171.124/dr5.tif
- import dr5.tif with r.in.gdal
- Delineate new catchments using r.water.outlet
(see the on-line GRASS 6 manual for detailed instructions on GRASS usage:
The result is the area it found, indicated as XXXXXXXXX ?.
Success or failure is up to the interpretation of the (expert) user:
is the area shown a likely, a large enough basin
area in the expectd (correct ?) location and extent or not.
A solution over the DEM should be easy to interpret,
with the boundaries expected on (visible) ridges.
If the function fails to produce a "plausible" solution:
try again with a different start point up-river ?
Reasons for failure may be that there was a sink, edge of
opposite direction - that should be visible from
dem and flow direction - overlays the user can display,
or one can use the map calculator to highlight those
areas. Understanding the reason may be a good basis
for the selection of a new seed point to start from.
- Final steps:
This shape file can now be imported as the geometry
assocaiated with a sub-catchment object in WaterWare,
using the standard import dialog available from the
geometry button at the OBJECT page.
- Raster to vector conversion of the resulting basin area;
- Export the new basin boundary as a shape file