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Saubhagya Singh

Flow budget in an unconfined aquifer (MODFLOW)

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Dear community members,

I am trying to numerically evaluate the discharge in an unconfined coastal aquifer with a constant head as the inland boundary. After performing SEAWAT simulation, the flow rate value in the flow-budget for any cross-section close to the inland boundary reflects only the flow below the sea-level. I also confirmed this by plotting velocity-vector plot, in which there were no velocities plotted in the region above the sea-level and below the water table. I have attached the picture for clarity. Is there a way I can get the total value of the freshwater discharge in the aquifer? I believe, a simple single-density flow model would also have a similar issue with the flow-budget in the unconfined case.

 

Unconfined_flow_budget.JPG

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Here is an example of a single-density flow width domain length as 53 cm, hydraulic conductivity as 20 cm/min and boundary heads as 21 cm and 22 cm. Theoretical calculation based on Darcy's law yields 8.11 cm^2/min, however, the flow budget in the model described below shows 7.9186 cm^2/min as the flow near the inland boundary which is exactly the value we would obtain if we consider a confined aquifer of thickness 21 cm and other parameters same as above. This suggests that flow budget does not account for the flow above the elevation equal to the lower boundary head. Also, there are not velocity vector generated in that region. Any guidance on how to proceed to get the accurate flow budget for the unconfined aquifers will be helpful. 

 

SIngle_density_flow.thumb.JPG.3ce6b308de830a23d3111530849b1f04.JPG

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Hi, what about the dry cells in the above picture? Couldn't it be the explanation for what you observe? This could cause reduction in aquifer thickness and reduction of volumetric flow rate therefore. Is rewetting on?

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Hi Michael, 

Thank you for your answer. Yes, I also think that. I did not have rewetting on. However, I tried various combinations of rewetting parameters but nothing helped. Finally, I had initial heads set to higher boundary-head, which then did not require rewetting and it worked (Figure attached). However, do you have any suggestion how can I have rewetting enabled? I used the wetting parameter as low as 0.01 still rewetting did not happen.   SIngle_density_flow_higher_initial_heads.thumb.JPG.7a9febd3df35580d2c20c24f11bd2de8.JPG

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This result looks good to me. There seems to be no more flooded cells. What about the budget?

The wetting capability is turned on in the LPF or BCF package. I usually use the default values first unless there is a problem with convergence.

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The figure in my previous comment shows the result when no rewetting was required as initial heads were equal to the higher boundary-head.  But in the case where I have initial heads equal to lower boundary-head, the cells near the higher-head boundary never get wetted. I will try some more combinations of wetting parameters. 

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Yes, but if the model is steady, than the best approach is to set the initial heads high, because during the consecutive aproximation of the solution the chance of making cell inactive as a result of undershoot is lower. It can happen anyway, but in your case it looks fine.

Btw. I think the cells never get rewetted because they are initialy dry.

Edited by Michal

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Ah! Now "Rewetted" makes sense to me. Don't know how I missed that haha! Thank you so much!

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