specified head boundary condition for testing scenarios

[Azizallah Izady]

Dear MODFLOW users,

I have calibrated (7 years) and validated (3 years) my groundwater flow model. I have used “specified head” condition for boundaries. I want to test different scenarios for future prediction using validated model. For testing scenarios, I can change (reduce or increase) groundwater extraction or recharge of simulation period for future. In fact, I can change time series of groundwater extraction or recharge of simulation period and consider it for future. Now, for specified head boundaries, how can I consider boundaries for scenario testing? Actually, we cannot consider time series of boundaries in the simulation period for testing scenarios. Because, head of boundaries depends on groundwater extraction or recharge variations and they will certainly change with respect to the variation of groundwater extraction or recharge in the future.

I really need your help. Any help/insights would be appreciated. Thank you.

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Best,

------------------------------------

Azizallah Izady
Dept. of Water Engineering
Ferdowsi University of Mashhad
Mashhad, Iran.
Cell Phone: +98-915-187-1219
E-mail: az.i...@gmail.com

[Azizallah Izady]

Dear Jon,

Thank you for your comprehensive explanations and hints. I do appreciate.

There are two type boundary conditions in my study area. The south east flow path represents the general direction of flow from another plain to my study area. The east boundary of study area is bounded by the “Binalood” Mountainous with small-permeability sedimentary rocks, Marne and Neogene stones materials. However, intervening tributary drainage basins provide a source of groundwater inflow to the study area.

Therefore, another question will be raised regarding your recommendation. For specified flow boundaries, how can I consider boundaries for scenario testing?  In fact, specified flow boundaries will be certainly changed with regard to considered scenarios like specified head boundaries. Hence, how we can predict specified flow boundaries for future.

Thank you in advance.

On Sat, May 4, 2013 at 3:41 AM, Jon Traum <jtr...@gmail.com> wrote:

Hi Azizallah,

What is the physical nature of the boundary conditions?

For example, if it is something like a lake, then it would be pretty reasonable to assume that the specified head will continue to be the same into the future.  If it is something else, like your model is just local model within a larger groundwater basin, then you might want to reconsider using a specified head boundary.

One idea would be to replace the “specified head” boundary condition from the historical model with a specified flow boundary condition (FHB package) for the future model.  You could estimate the specified flow rate by extracting the boundary flow rates out of your historical model’s groundwater budget.

This assumption is generally pretty reasonable if you don’t have anything else to go by.  Because for example, if you are pumping more under a future conditions scenario, it is reasonable to assume that your neighbors (outside of the model area) will also be pumping more, so the subsurface flux between you and your neighbor will remain unchanged from historical conditions.

Hope this helps and makes sense.  And remember this is just an idea; like I said above, the physical nature of the boundary condition really dictates how it should be simulated in the model.

Jon

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[Azizallah Izady]

Dear Jon,
Thank you again for your precious comments.
I think it is good idea that MODFLOW users could write their
invaluable experiences about this issue when they faced such problem.
I do believe that their experiences will help other users to solve
their problems.

Best,

Aziz

On 5/5/13, Jon Traum <jtr...@gmail.com> wrote:
> Hi Azizallah,
>

> For the mountain boundary, it is a reasonable assumption that the tributary
>
> drainage will probably be the same in the future as it was in historical
> conditions. Thus, it would be reasonable to assume the same specified
> flows for the scenarios that you used for the calibration model. This
> would be a fairly simple approach, and you could get way more complex if
> you wanted (such as incorporating future climate change).
>
> For the subsurface flow from another basin to your study area, there are a
> few ideas for estimating the future boundary conditions.
> First, like I said above, one assumption is that the subsurface flow will
> remain the same in the future as under historical conditions. The
> justification for this assumption is that your neighbors will do behave the
>
> same as you do, and thus their activities won’t alter the regional
> groundwater flow gradient.
> If you do want the flow to change for each scenario, another idea would be
> to use a general head boundary condition (GHB package). To predict the
> head at the boundary in the future, you could look at the trends in
> historical boundary heads and then extrapolate the heads for the future.
>
> Hope these ideas help. Determining future boundary conditions is one of
> the trickier parts of groundwater modeling, and usually a large source of
> uncertainty. Good luck!

[An Ho Antonio Taylor]

Why don't try injection well?, you can use a tributary area for recharge, like a flux and inject direct over a screen, you can calibrate this wells within pest, there is will be non uniqueness and a lot uncertainities, but for the first guess, i think it much more flexible, i thought.

2013/5/4 Jon Traum <jtr...@gmail.com>

Hi Azizallah,

For the mountain boundary, it is a reasonable assumption that the tributary drainage will probably be the same in the future as it was in historical conditions.  Thus, it would be reasonable to assume the same specified flows for the scenarios that you used for the calibration model.  This would be a fairly simple approach, and you could get way more complex if you wanted (such as incorporating future climate change).

For the subsurface flow from another basin to your study area, there are a few ideas for estimating the future boundary conditions.
First, like I said above, one assumption is that the subsurface flow will remain the same in the future as under historical conditions.  The justification for this assumption is that your neighbors will do behave the same as you do, and thus their activities won’t alter the regional groundwater flow gradient.
If you do want the flow to change for each scenario, another idea would be to use a general head boundary condition (GHB package).  To predict the head at the boundary in the future, you could look at the trends in historical boundary heads and then extrapolate the heads for the future.

Hope these ideas help.  Determining future boundary conditions is one of the trickier parts of groundwater modeling, and usually a large source of uncertainty.  Good luck!

[Azizallah Izady]

Dear Antoni,
Would you please explain more about tributary area for recharge? What
about the relation between your suggestion and considering specified
flow boundary for future scenario testing. I do appreciate your
insights.

Best,

Aziz

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[An Ho Antonio Taylor]

I recall the superposition principle, The principle of superposition states that problem solutions can be added together to obtain composite solutions. note: This principle applies to linear systems.  The tributary area depends your geological settings and the precipitation withing the area to calculate a flux: http://funnel.sfsu.edu/students/student/courses/G475_775/Tutorial/WinPest/VMOD_WinPEST_Tutorial.pdf

They use a specified flux located at the Western boundary of the model have a positive 

pumping rate - these are used to simulate a specified flow across the boundary and into the model 

domain.

2013/5/9 Azizallah Izady <az.i...@gmail.com>

[Azizallah Izady]

Dear Antonio,

Thank you for your insights. 

Best,

Aziz

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