[GRASS-user] Repeated r.watershed runs

[Thomas Adams]

Micha,

I was thinking the same... but the ultimate result wanted is still unclear;
hope you are well!

Tom

On Thu, Aug 31, 2017 at 2:51 PM Micha Silver <tsvibar at gmail.com> wrote:

> I'm also not clear what you are asking. But risking a guess:
> You could run r.water.outlet *1 time* to get the basin. Then use that
> raster as a MASK, so that the next process will address only the pixels
> within the basin. Now do a loop with r.univar on all 14,000 flow rasters,
> and you'll get 14,000 results with total, min, max, mean, etc of the basin
> pixels for each of the flow rasters.
>
> --
> Micha
>
>
> On 08/31/2017 09:30 PM, Thomas Adams wrote:
>
> Ken,
>
> You "want 14,000 values" of what?? Your original email stated you were
> "trying to determine flow past a drainage basin outlet" -- r.watershed does
> NOT do this, if indeed this is what you want. And you say you have "14,000
> flow rasters to be used as input" -- what exactly are these 'flow rasters';
> what is your goal? I may not understand...
>
> Tom
>
> On Thu, Aug 31, 2017 at 2:24 PM, Ken Mankoff <mankoff at gmail.com> wrote:
>
>> Hi Tom,
>>
>> I have 1 DEM and 14,000 flow rasters to be used as input. I want 14,000
>> values, one at a specific coordinate from each acc output.
>>
>> I can do this by running r.watershed 14,000 times. That is slow, unless
>> I'm missing something (e.g. It works with I.group variables or Time Series
>> data more efficiently).
>>
>> An alternative approach is possible if I knew the complete drainage basin
>> *and* the fractional value of each cell that contributed to the basin. In
>> this case I don't need to route. But basins from r.watershed or
>> r.water.outlet, I think, use SFD not MFD (no cell is ever in 2 basins, are
>> they?), and I don't know how to get the fractional contribution from each
>> cell.
>>
>>   -k.
>>
>> Please excuse brevity. Sent from pocket computer with tiny non-haptic
>> feedback keyboard.
>>
>> On 31 Aug 2017, at 19:59, Thomas Adams <tea3rd at gmail.com> wrote:
>>
>> Ken,
>>
>> I'm confused about what you are trying to do with r.watershed, because
>> the output from the module is:
>>
>> accumulation=name
>> Name for output accumulation raster map
>> Number of cells that drain through each cell
>> tci=name
>> Name for output topographic index ln(a / tan(b)) map
>> spi=name
>> Stream power index a * tan(b)
>> Name for output raster map
>> drainage=name
>> Name for output drainage direction raster map
>> basin=name
>> Name for output basins raster map
>> stream=name
>> Name for output stream segments raster map
>> half_basin=name
>> Name for output half basins raster map
>> Each half-basin is given a unique value
>> length_slope=name
>> Name for output slope length raster map
>> Slope length and steepness (LS) factor for USLE
>> slope_steepness=name
>> Name for output slope steepness raster map
>> Slope steepness (S) factor for USLE
>>
>> I think you want a hydrologic model, and r.watershed is NOT that. What
>> are you trying to obtain?
>>
>> Tom
>>
>>
>>
>>
>> On Thu, Aug 31, 2017 at 1:47 PM, Ken Mankoff <mankoff at gmail.com> wrote:
>>
>>> Hi List,
>>>
>>> I'm trying to determine flow past a drainage basin outlet. The
>>> complicating factor is that I need to do this each day for 40 years. If I
>>> do "r.watershed" ~14,000 times I'll get the results, but it will take 3
>>> days. It seems that r.watershed is likely calculating many things each time
>>> through the loop. Is there a more efficient way to this? A flag to
>>> r.watershed that isn't documented? Something with time-series?
>>>
>>> Alternatively, because I only need the flow at the outlet, I could
>>> calculate the basin, not route the flow, and instead sum the values in the
>>> basin. I assume this would take seconds or minutes rather than days. In
>>> this case I'm not sure of the best way to define the basin. I tried doing
>>> r.water.outlet upstream from the outlet, but I think this uses SFD, which
>>> means the basin may be significantly underestimated.
>>>
>>> I also tried inverting/flipping the DEM and then running r.watershed
>>> with convergence=1, and a flow equal to 0 everywhere except 1000 at the
>>> outlet (now the source due to the inversion) to see where it flooded
>>> upstream (now downstream due to the inversion). This didn't seem to work...
>>> because basins are filled and flow routes to the edge of the DEM, I could
>>> not pick out the
>>>
>>> Any advice how to either a) efficiently route 14,000 FLOW rasters over 1
>>> DEM or b) determine the full basin will be much appreciated.
>>>
>>> Thanks,
>>>
>>>     -k.
>>>
>>> _______________________________________________
>>> grass-user mailing list
>>> grass-user at lists.osgeo.org
>>> https://lists.osgeo.org/mailman/listinfo/grass-user
>>>
>>
>>
>>
>>
>
>
>
> _______________________________________________
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>
>
> --
> Micha Silver
> Ben Gurion Univ.
> Sde Boker, Remote Sensing Lab
> cell: +972-523-665918
>
> --
Sent from Gmail Mobile
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