[GRASS-user] Repeated r.watershed runs

[Thomas Adams]

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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