[GRASS-user] Repeated r.watershed runs

[Ken Mankoff]

Hi Tom,

You sound like my friend reviewer #2. :)

But seriously, I thank you for thinking critically about what I'm trying to do. However, the inputs I have come from a couples land/ice/snow/atmosphere regional climate model with rain, snowfall, eval, condensations, storage in snow and land, and melting of snow and ice. I think it would be incorrect to take this product and do anything other than route instantaneously and completely at this point. 

  -k. 

Please excuse brevity. Sent from pocket computer with tiny non-haptic feedback keyboard. 

> On 31 Aug 2017, at 22:49, Thomas Adams <tea3rd at gmail.com> wrote:
> 
> Ken,
> 
> That would be a gross misapplication of r.watershed and, I'm afraid, is just bad science; I just don't know how to be more clear on that...
> 
> Tom
> 
>> On Thu, Aug 31, 2017 at 3:52 PM, Ken Mankoff <mankoff at gmail.com> wrote:
>> Hi Tom,
>> 
>> What if we are assuming instantaneous flow and no storage and all of the input flow variable leaves the system. Then is r.watershed usable to calculate accumulation?
>> 
>>   -k. 
>> 
>> Please excuse brevity. Sent from pocket computer with tiny non-haptic feedback keyboard. 
>> 
>>> On 31 Aug 2017, at 21:17, Thomas Adams <tea3rd at gmail.com> wrote:
>>> 
>>> Ken,
>>> 
>>> You wrote:
>>> 
>>> 'If I give r.watershed a precip map as input (flow parameter), doesn't it route that water down the DEM? And if I know the x,y coordinate of a point in a stream, doesn't the accumulation at that point represent all of the up-basin precip that is routed past that point?'
>>> 
>>> ANSWER: NO!!!! r.watershed is NOT a hydrologic model and does NOT take precipitation as an input raster.
>>> 
>>> 'Daily water runoff from the Greenland ice sheet for ~50 years'
>>> 
>>> ANSWER: NO!!!! same as above
>>> 
>>> You want something like r.topmodel or r.sim.water in GRASS, not that I'm necessarily suggesting these would meet your needs. There is also ITZI (https://www.itzi.org/) which may be appropriate since flow over glaciers (if you're focused on glaciers) in Greenland, should be largely impervious.
>>> 
>>> You would need r.watershed and r.water.outlet to aid you in the application of hydrologic models like topmodel, ITZI, and sim.water. But, there are many dozens more hydrologic models, such as VIC, PRMS, etc.
>>> 
>>> Tom
>>> 
>>> 
>>>> On Thu, Aug 31, 2017 at 3:01 PM, Ken Mankoff <mankoff at gmail.com> wrote:
>>>> Hi Tom,
>>>> 
>>>> > On 31 Aug 2017, at 20:30, Thomas Adams <tea3rd at gmail.com> wrote:
>>>> > You "want 14,000 values" of what??
>>>> 
>>>> One grid cell from each of 14,000 accumulation rasters.
>>>> 
>>>> > 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.
>>>> 
>>>> If I give r.watershed a precip map as input (flow parameter), doesn't it route that water down the DEM? And if I know the x,y coordinate of a point in a stream, doesn't the accumulation at that point represent all of the up-basin precip that is routed past that point?
>>>> 
>>>> > And you say you have "14,000 flow rasters to be used as input" -- what exactly are these 'flow rasters';
>>>> 
>>>> Daily water runoff from the Greenland ice sheet for ~50 years.
>>>> 
>>>> > what is your goal? I may not understand...
>>>> 
>>>> To find the daily stream flow at a point, based on the runoff that feeds into that point, or any upstream runoff that eventually makes it to that point.
>>>> 
>>>> Clearly there is a miscommunication issue here. I apologize if I am not being clear or using incorrect terminology.
>>>> 
>>>>   -k.
> 
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