[GRASS-user] r.watershed: output basin map

[Rich Shepard]

On Thu, 19 Apr 2012, Margherita Di Leo wrote:

> Please post information about the region, r.info of the map and the
> command line of r.watershed that you use. Also, do you use a mask? Pankaj
> Kr Sharma, please avoid to reply attaching the whole digest.

madi, Micha, Pankaj:

   The Level 6 HUC basins (a.k.a. HUC-12 or sub-watersheds) are defined by
the USGS to enclose areas of 10k-40k acres (15.625-62.5 square miles;
40.5-161.9 square kilometers). These are a bit too large, but vector
boundaries for 14- or 16-digit hydrologic units are not available for this
area (or for my other project area which is approximately 125 square miles,
323.75 square kilometers) so I tried different threshold values for
r.watershed and 150000 seems to provide additional resolution without
generating a separate drainage for each individual stream segment. Yes, it's
arbitrary. But for the purposes of these two projects it is a practical
balance between too coarse and too fine.

   The region was set by the base, 10m DEM:

projection: 99 (Lambert Conformal Conic; units in International Feet)
zone:       0
datum:      nad83
ellipsoid:  grs80
north:      735780
south:      655650
west:       2042100
east:       2138580
nsres:      30
ewres:      30
rows:       2671
cols:       3216
cells:      8589936

   The r.watershed command line (all on one line but wrapped in the MUA):

r.watershed -f elev=dem10m.avg acc=dem.acc drain=dem.drain bas=dem.basin
stream=dem_str.seg thresh=150000 --o

   And r.info for dem.basin:

    Type of Map:  raster               Number of Categories: 94
    Data Type:    CELL
    Rows:         2671
    Columns:      3216
    Total Cells:  8589936
         Projection: Lambert Conformal Conic
             N:     735780    S:     655650   Res:    30
             E:    2138580    W:    2042100   Res:    30
    Range of data:    min = 2  max = 94
    Data Source:
     dem10m.avg
    Data Description:
     generated by r.watershed
    Comments:
     Processing mode: MFD
     Memory mode: All in RAM
     r.watershed -f elevation="dem10m.avg" accumulation="dem.acc" drainag\
     e="dem.drain" basin="dem.basin" stream="dem_str.seg" threshold=15000\
     0 convergence=5 memory=300

   If the white areas with vector streams (the blue lines) represent basins,
then that's OK. What puzzles me is why a couple of basins would be white
while the others are different colors and the small white areas without
mapped streams are appently external to the basins of interest.

   As far as I know, there's no attribute table for raster maps that hold
descriptions of the generated basins, only information on the contents of
each cell.

Rich