[GRASS-user] Repeated r.watershed runs

[Ken Mankoff]

Hi Micha,

We are getting closer to the issue. Unfortunately I'm also becoming more certain the limitation is real and in GRASS and not my mental model.  If this email does not clarify it, I will draw a picture which may help. 

> On 01 Sep 2017, at 00:08, Micha Silver <tsvibar at gmail.com> wrote:
> 
> You won't have many upstream cells for those cells along the basin boundary, only the few that drain exactly along the watershed divide.

I think you are picturing the NC data set and mountains. Greenland is flatter. Why can't a divide in an extreme case be near or across a lake? Or alternatively, why can't a major stream flow along the boundary outside of a divide? In these cases the boundary cells contribute 49%, but have large (massive?) upstream catchments themselves, all of which is excluded if a mask is generated from r.water.outlet. If I were only losing the boundary cells (max of n of them, for a boundary n cells long), I would not worry. This seems like the case in mountainous regions, but perhaps not on the flatter Greenland ice sheet. 


> The only way that r.watershed can return different results is if you input a different elevation grid. 

R.watershed w/o flow -> r.water.outlet produces a "minimum" basin where partial contributor cells (and the upstream catchments of those cells) are not included. 

R.watershed WITH flow produces runoff at point x,y with the contribution from other catchments that partially contribute to this catchment. 

Correct?

Therefor, running r.watershed 14,000 times gives the correct (including partial contributions) runoff each day. 

> The only situation that I can envision where you would rerun r.watershed is when massive earthwork was done, and you have a new/revised elevation dataset

Or I want access to the cells that contribute partially to my drainage basin. This matters when those boundary cells may tap into large lakes or catchments themselves. This scenario might only exist on the unique topography of Antarctica and Greenland. 

  -k. 
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