Dear grass users,
I am trying to find sub-basins and their outlets exactly for each 300 sq. km. in a bigger basin.
I think accumulation value indicates the no. of cells draining at a cell. And will thus be a good starting point for such calculation.
The other option is to start backwards from the basin outlet and keep on investigating upwards in a stream network.
Can someone provide some hint / help in this regard.
Thanks.
Dear grass users,
I am trying to find sub-basins and their outlets exactly for each 300 sq. km. in a bigger basin.
I think accumulation value indicates the no. of cells draining at a cell. And will thus be a good starting point for such calculation.
The threshold value is what you’re looking for. You need to calculate how many raster cells it will take to make 300 sq. km - depending on the raster resolution. For example, if the resolution is 90 m. (i.e. SRTM elevation data) then each cell = 8100 sq.m. then you will need a threshold of 370,000 to create basins of about 300 sq.km.
On 20/11/2011 12:50, Pankaj Kr Sharma wrote:
Dear Micha,
by the method of threshold value, I have tried and output varies from 1 sqkm to 6000 sqkm.
Not what I want.
Give us some more information:
g.region -p
r.info
the exact command you used to run r.watersheds
On Sun, Nov 20, 2011 at 2:44 PM, Micha Silver <micha@arava.co.il> wrote:
On 20/11/2011 08:58, Pankaj Kr Sharma wrote:
Dear grass users,
I am trying to find sub-basins and their outlets exactly for each 300 sq. km. in a bigger basin.
I think accumulation value indicates the no. of cells draining at a cell. And will thus be a good starting point for such calculation.The threshold value is what you’re looking for. You need to calculate how many raster cells it will take to make 300 sq. km - depending on the raster resolution. For example, if the resolution is 90 m. (i.e. SRTM elevation data) then each cell = 8100 sq.m. then you will need a threshold of 370,000 to create basins of about 300 sq.km.
The other option is to start backwards from the basin outlet and keep on investigating upwards in a stream network.
Can someone provide some hint / help in this regard.
Thanks.This mail was received via Mail-SeCure System.
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Pankaj Kr Sharma wrote:
Dear grass users,
I am trying to find sub-basins and their outlets exactly for each 300 sq.
km. in a bigger basin.
I think accumulation value indicates the no. of cells draining at a cell.
And will thus be a good starting point for such calculation.
The other option is to start backwards from the basin outlet and keep on
investigating upwards in a stream network.
Can someone provide some hint / help in this regard.
Thanks.
You could run r.watershed in SFD mode, then select all flow
accumulation cells whose values are a multiple of 300 sq km converted
to number of cells. These cells can then be used as input for
r.stream.basins to determine the corresponding basins.
HTH,
Markus M
Hello Pankaj
(Posting back to the list also)
Hi, Delay is sincerely regretted. You can find the outputs below. The result of r.report is attached separately. (Thu Nov 24 17:07:28 2011) r.watershed elevation=ybdem@work threshold=12345
I see from your g.region output that the region resolution is 3 arcsecs. That’s about 90m. So each cell is 8100 sq m. By choosing a threshold of 12345, as above, you’re asking for catchments with a minimum size of 0.0081 X 12345 ~= 100 sq km.
If you wanted a minimum of 300 sq km, then your threshold should be about 37,000.
( I hope I’m not repeating things you already know 
)
Looking thru the r.report file, many of the categories are > 100 sq km, matching your threshold value of 12345. There are also many categories with a small total cell area near 0.0. These might be areas near the edge of the region which flow out of the region, thus are not part of any large basin.
Another thing that surprised me, In your attached r.report file, I saw that over 600,000 sq km were left with nodata? Why would that be? Perhaps the DEM doesn’t cover the whole region?
r.info ybdem@work ?
Regards,
Micha