[GRASS-user] Repeated r.watershed runs

[Moritz Lennert]

Le 1 septembre 2017 09:33:52 GMT+02:00, Moritz Lennert <mlennert at club.worldonline.be> a écrit :
>On 01/09/17 06:41, Ken Mankoff wrote:
>> 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 
>>> <mailto: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?
>
>I think so. If you know how much overland flow you have in each cell (=
>
>flow parameter), then r.watershed should calculate the accumulation of 
>that flow along the way based on the extremely simplified assumption 
>that all that flow remains overland across such a large area.

P.S. One important issue with your approach is that r.watershed is static in time. There is no notion whatsoever of how long it takes for flow to move from one cell to the next... 

Moritz