[GRASS-user] help with r.gwflow

[Sören Gebbert]

Hi Vishal,

2012/7/18 Vishal Mehta <vishalm1975 at gmail.com>:
> hi Soren,
>
> my constan head boundary conditions on the edges are causing water tables to
> build up everywhere else. so i estimated a constant flux 0f 0.0019 m3/s that
> i want to apply for edge cells. this is what i am trying in order to impose
> a constant flux on the edges
>
> r.mapcalc "sink.init=if(row()==1 || row()==444 ||col()==1
> ||col==477,-0.0019,null())
>
> #sink.init is the constant flux on edges
> # my r.gwflow script runs at monthly time step- ihave a loop $month-here's
> the r.gwflow snippet
>
> r.gwflow --o -s solver=cg top=top bottom=bottom status=bc2 hc_x=k.1 hc_y=k.1
> s=s.1 type=unconfined dt=2592000 error=0.05 phead=sim.$((prevmonth))
> r=gwnaturalms.$((month)) q=sink.init output=sim.$((month))
>
> my boundary condition map (bc2) has contant head in strea pixels. all else
> are calculated.
>
> do you think the above implementation will correctly impose the constant
> flux on the edges? it seems though that since this is not a boundary
> condition, how can it ensure a constant flux at edges?

Your script looks reasonable, except that the error term is much to large.
You may need to use a smaller number like 10⁻7,
otherwise your results my be wrong.

I have attached a small r.gwflow example to show you how to estimate
the constant flux at
a western boundary using the budget computation. Since the flow will
be specified
as a source term, it will be constant the whole computational time.

Maybe you can apply this method to estimate the boundary flux in your area?

JFYI i am using r.gwflow of GRASS 7.

Best regards
Soeren

>
> Thanks for any help you can provide,
> Vishal
>
> On Mon, Jul 16, 2012 at 1:46 PM, Vishal Mehta <vishalm1975 at gmail.com> wrote:
>>
>> Thanks Soren,
>> From you response, can you please tell me how to do the following two
>> tasks (which i dont find in the online manual for r.gwflow): the remainder
>> of your comments i have figured out.
>>
>> - How can i compute the flux in [m^3/s] for each cell with r.mapcalc?
>> and
>>
>> - i have set the bc of the edges and stream cells at constant head for
>> now- how can i get the budget raster maps you mention?
>>
>> thanks again,
>> Vishal
>>
>>
>> On Sat, Jul 14, 2012 at 12:05 PM, Sören Gebbert
>> <soerengebbert at googlemail.com> wrote:
>>>
>>> Hi,
>>> sorry for the delay.
>>>
>>> 2012/7/11 Vishal Mehta <vishalm1975 at gmail.com>:
>>> > Thanks Soren,
>>> > That explains some of the results i'm getting, with water piling up
>>> > above
>>> > the surface in the edges in low-lying areas.
>>> >
>>> > Can you please tell me how i can change that to constant flux or
>>> > constant
>>> > head? If constant flux, should that be in m/s units?
>>>
>>> Constant flux can currently only be defined using sources/sinks with
>>> unit [m^3/s], that is option q.
>>> But i can add two new options (fn, fe) that defines the flux in
>>> northern or eastern direction using the unit [m/s]
>>> that will be multiplied internally with the northern or eastern face
>>> area of the cell?
>>>
>>> Otherwise you need to compute the flux in [m^3/s] for each cell with
>>> r.mapcalc.
>>>
>>> >
>>> > My problem though is that i dont know what a constant flux or head at
>>> > the
>>> > edges should be set to. For now the only bc i have put in there
>>> > deliberately
>>> > (beyond the default you mention) is that i have set constant head in
>>> > stream
>>> > pixels. I'll have to let flow through at the edges but i have no idea
>>> > what
>>>
>>> You can use the river boundary condition to specify the flux in stream
>>> pixel.
>>>
>>> > that flow should be. Are there some ways of setting the edge conditions
>>> > such
>>> > that the gw evolution in the central areas of interest are not highly
>>> > influenced?
>>>
>>> You can set the boundary of interest to constant head pixel and
>>> compute the flow throw the boundary pixel using the budget option.
>>> The resulting budget raster map shows the flow from active cell into
>>> sources, sinks and constant heads in [m^3/s].
>>>
>>> Best regards
>>> Soeren
>>
>>
>>
>>
>> --
>> Vishal K. Mehta, PhD
>> Scientist
>> Stockholm Environment Institute - US
>> 133 D St Suite F
>> Davis CA 95616
>> www.sei-us.org
>
>
>
>
> --
> Vishal K. Mehta, PhD
> Scientist
> Stockholm Environment Institute - US
> 133 D St Suite F
> Davis CA 95616
> www.sei-us.org
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