Dear Grass users and developers,
The run-time messages of command r.watershed is pasted below.
This I ran as per suggestions on a threshold area of 100 sq km. Considerable number of basins are in the limit 1 to 100 sq km. The results represent are close to natural distribution of watersheds. However, from the results, it appears to me that the threshold limit should be considered only as a guideline and not to find basins of size larger than the threshold limit.
(Thu Nov 24 17:07:28 2011)
r.watershed elevation=ybdem@work threshold=12345 accumulation=ybdem100sqkm_acc drainage=ybdem100sqkm_fdir basin=ybdem100sqkm_basin stream=ybdem100sqkm_stream
SECTION 1a (of 5): Initiating Memory.
SECTION 1b (of 5): Determining Offmap Flow.
SECTION 2: A* Search.
SECTION 3: Accumulating Surface Flow with MFD.
SECTION 4: Watershed determination.
SECTION 5: Closing Maps.
(Thu Nov 24 17:09:38 2011) Command finished (130 sec)
In this part, I tried to ran r.watershed with SFD flag.
(Thu Nov 24 17:21:40 2011)
r.watershed -s elevation=ybdem@work threshold=12345 accumulation=ybdem100sqkm_acc_sfd drainage=ybdem100sqkm_fdir_sfd basin=ybdem100sqkm_basin_sfd stream=ybdem100sqkm_stream_sfd
SECTION 1a (of 5): Initiating Memory.
SECTION 1b (of 5): Determining Offmap Flow.
SECTION 2: A* Search.
SECTION 3: Accumulating Surface Flow with SFD.
SECTION 4: Watershed determination.
Subprocess failed with exit code 9
category information for [ybdem100sqkm_acc_sfd] in [work] missing or invalid
category information for [ybdem100sqkm_fdir_sfd] in [work] missing or invalid
category information for [ybdem100sqkm_basin_sfd] in [work] missing or invalid
category information for [ybdem100sqkm_stream_sfd] in [work] missing or invalid
(Fri Nov 25 09:46:01 2011) Command finished (59061 sec)
One can see that I had manually terminated the command from “system monitor”. It didn’t complete.
I tried again.
(Fri Nov 25 09:46:35 2011)
r.watershed -s --overwrite elevation=ybdem@work threshold=12345 accumulation=ybdem100sqkm_acc_sfd drainage=ybdem100sqkm_fdir_sfd basin=ybdem100sqkm_basin_sfd stream=ybdem100sqkm_stream_sfd
SECTION 1a (of 5): Initiating Memory.
SECTION 1b (of 5): Determining Offmap Flow.
SECTION 2: A* Search.
SECTION 3: Accumulating Surface Flow with SFD.
SECTION 4: Watershed determination.
Please note that the data are left in inconsistent state and may be corrupted
(Fri Nov 25 10:23:45 2011) Command aborted (2230 sec)
It wasn’t working. I found in “system monitor” that the program r.watershed.ram is using only 2gb ram.
The issue may be of interest to GRASS developers.
So, I tried again with “memory” mode and it worked.
(Fri Nov 25 10:24:33 2011)
r.watershed -s -m --overwrite elevation=ybdem@work threshold=12345 accumulation=ybdem100sqkm_acc_sfd drainage=ybdem100sqkm_fdir_sfd basin=ybdem100sqkm_basin_sfd stream=ybdem100sqkm_stream_sfd memory=12000
SECTION 1 beginning: Initiating Variables. 5 sections total.
SECTION 1a: Mark masked and NULL cells
SECTION 1b: Determining Offmap Flow.
SECTION 2: A* Search.
SECTION 3: Accumulating Surface Flow with SFD.
SECTION 4: Watershed determination.
SECTION 5: Closing Maps.
Closing stream segments map
Closing basins map
Closing accumulation map
Closing flow direction map
(Fri Nov 25 10:28:46 2011) Command finished (252 sec)
It was suggested that from the accumulation values equal to the number of cells of area required (i.e. 12345 for 100 sq km), one can find tentative points having watershed equal to the area desired.
In the accumulation values , I found that there is no value near 12345. And there are lot of negative values.
The results of r.report are pasted below:
±----------------------------------------------------------------------------+
| RASTER MAP CATEGORY REPORT |
| LOCATION: only_yamuna Fri Nov 25 11:00:55 2011 |
|---|
| north: 32:00:02.953919N east: 82:00:02.881919E |
| REGION south: 21:59:59.96832N west: 72:59:59.89488E |
| res: 0:00:02.999999 res: 0:00:02.999999 |
| ----------------------------------------------------------------------------- |
| MASK:none |
| ----------------------------------------------------------------------------- |
| MAP: Watershed accumulation: overland flow that traverses each cell (ybdem100 |
| ----------------------------------------------------------------------------- |
| Category Information |
| # |
| ----------------------------------------------------------------------------- |
| -43691083–43505559.796078 |
| -43505559.796078–43320036.592157 |
| -43320036.592157–43134513.388235 |
| -43134513.388235–42948990.184314 |
| -42948990.184314–42763466.980392 |
| -42577943.776471–42392420.572549 |
| -42392420.572549–42206897.368627 |
| -42206897.368627–42021374.164706 |
| -38496433.290196–38310910.086275 |
| -38310910.086275–38125386.882353 |
| -38125386.882353–37939863.678431 |
| -32374167.560784–32188644.356863 |
| -31632074.745098–31446551.541176 |
| -31261028.337255–31075505.133333 |
| -27550564.258824–27365041.054902 |
| -18274404.062745–18088880.858824 |
| -18088880.858824–17903357.654902 |
| -17903357.654902–17717834.45098 |
| -17346788.043137–17161264.839216 |
| -17161264.839216–16975741.635294 |
| -16975741.635294–16790218.431373 |
| -9925859.886275–9740336.682353 |
| -9369290.27451–9183767.070588 |
| -9183767.070588–8998243.866667 |
| -7885104.643137–7699581.439216 |
| -7699581.439216–7514058.235294 |
| -6957488.623529–6771965.419608 |
| -6029872.603922–5844349.4 |
| -5844349.4–5658826.196078 |
| -5658826.196078–5473302.992157 |
| -5473302.992157–5287779.788235 |
| -5287779.788235–5102256.584314 |
| -5102256.584314–4916733.380392 |
| -4916733.380392–4731210.176471 |
| -4731210.176471–4545686.972549 |
| -4545686.972549–4360163.768627 |
| -4360163.768627–4174640.564706 |
| -3803594.156863–3618070.952941 |
| -3618070.952941–3432547.74902 |
| -3432547.74902–3247024.545098 |
| -3247024.545098–3061501.341176 |
| -3061501.341176–2875978.137255 |
| -2875978.137255–2690454.933333 |
| -2690454.933333–2504931.729412 |
| -2504931.729412–2319408.52549 |
| -2319408.52549–2133885.321569 |
| -2133885.321569–1948362.117647 |
| -1948362.117647–1762838.913725 |
| -1762838.913725–1577315.709804 |
| -1577315.709804–1391792.505882 |
| -1391792.505882–1206269.301961 |
| -1206269.301961–1020746.098039 |
| -1020746.098039–835222.894118 |
| -835222.894118–649699.690196 |
| -649699.690196–464176.486275 |
| -464176.486275–278653.282353 |
| -278653.282353–93130.078431 |
| -93130.078431-92393.12549 |
| 92393.12549-277916.329412 |
| 277916.329412-463439.533333 |
| 463439.533333-648962.737255 |
| 648962.737255-834485.941176 |
| 834485.941176-1020009.145098 |
| 1020009.145098-1205532.34902 |
| 1205532.34902-1391055.552941 |
| 1391055.552941-1576578.756863 |
| 1576578.756863-1762101.960784 |
| 1947625.164706-2133148.368627 |
| 2133148.368627-2318671.572549 |
| 2689717.980392-2875241.184314 |
| 3246287.592157-3431810.796078 |
| 3431810.796078-3617334 |
| * |
| ±----------------------------------------------------------------------------+ |
I have changed the colour of negative values to red and positive values to green.
I have read that negative values mean that area getting flow outside the current region.
That’s simply not possible in my case. I have created this basin from r.water.outlet. I used that as mask to cut the DEM of only that area.
Can anyone please shed some more light on it. Or share their experiences with us in this regard.
Basically, I have two queries now. 1. How to get positive accumulation values. 2. How to find the accumulation value representing 100 sq km basin area.
My idea to find 100 sq km basin area by walking along the rivers till we reach the desired point is still under development.