[GRASS-user] Repeated r.watershed runs

[Thomas Adams]

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.
>>
>> _______________________________________________
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>>
>
>
>
>
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