[GRASS-user] Repeated r.watershed runs

[Ken Mankoff]

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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