r.water.fea problem and solution

[John Talcott]

Not long ago, I posted the following problem I had with
the contributed GRASS hydrology program r.water.fea:

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The following is what happens when I am running r.water.fea,
selecting watershed basins:

Please adjust location of your monitor.
[Hit RETURN to continue]

Drawing maps .....................

Buttons
 Left: select basin
 Right: quit
Basin <6> selected
Basin <10> selected
Basin <12> selected
Basin <8> selected

These are the basins that you have selected.


These are the corresponding streams.

Your stream network should be continuous.
Do you wish to delete any basins ?
(y/n) n
Building Channel Topology ......                WARNING: [fea.stream.xxx in
cwcm
] - read request for row 298 is outside
         region
WARNING: [fea.stream.xxx in cwcm] - read request for row 298 is outside
         region


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Dr. Baxter Vieux - one of the r.water.fea authors - was able to provide
a solution to the problem. The problem arose because my region contains
a coastline. I assume that users of r.water.fea will have similar regions
and if your region does contain a coastline, you may have exactly the same
problem I did. The problem occurred only when I had selected basins that
contacted the coastline.

It is mentioned in the notes that you should not
select any basins that contain negative drainage values. In addition to
the edge of the region where you expect to encounter negative drainage
values, the basins that drained into the ocean along the coast also
contained negative values. I had used a land mask in conjunction with
r.watershed to compute the basin, stream, drainage, and accumulation maps.
I wanted to include the coastal basins so the workaround was to create a
mask
that included only the positive drainage values and multiply each map
created by r.watershed by the mask. Not only did this solve the problem,
but now I can select any basin for processing without wondering whether
they contain negative drainage values - even the basins that contact the
edge of my region! Thanks to Dr. Vieux and Sree, one of his students for
their help!

-- 
John C Talcott talcott at trg.saic.com
Science Applications Intl. Corp. (SAIC)
10260 Campus Point Drive
San Diego CA 92121
(619) 546-6447