Groundwater storage in and storage out

[John M]

I am looking for literature that clearly explain groundwater storage in and storage out in a groundwater system tend to be intuitively the opposite. Anyone who can help explain this I am expecting at least one answer from Richard Winston.

Thank you

John Maw

[Richard Winston]

Flow into or out of storage can balance other components of groundwater flow.

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[An Ho Antonio Taylor]

This is the answer from Bill Dolinar from GMS development Team:

If you were to write a balance equation you could write:

IN - OUT = ACCUMULATION

An excess of IN causes ACCUMULATION. The IN and OUT parts come from change due to boundary conditions and the ACCUMULATION comes from change in STORAGE. Let's separate ACCUMULATION into two parts:

IN - OUT = STORE_INCREASE - STORE_DECREASE

If there is no in and out in a cell, any increase in storage comes from STORE_INCREASE and decrease from STORE_DECREASE. Now let's move parts of accumulation to each side:

IN + STORE_DECREASE = OUT + STORE_INCREASE

This matches up to the values given in the balance. STORAGE_DECREASE values are given as a positive value and STORAGE_INCREASE are given as a positive values. This would mean that an increase of storage on the IN side goes out of the cell and the OUT side goes in the cell.

  Time series plots of the temporal changes in the rate of aquifer storage would help to visualize that STORAGE IN is the rate of mass added to the system through sources (precipitation, regional groundwater recharge) and STORAGE OUT is the rate of reduction in mass storage.

Also, this contains interesting comments about storage terms and Balance interpretation:

http://inside.mines.edu/~epoeter/583CSM/13_CommentsTransientModelingReports.pdf

Best Regards

An Ho

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[John M]

Thank you all: I was looking for reference where these things are well explained. My question was Storage in (in from where????) Storage Out (from where). I always imagined this as flows between Aquifer and Model. (into storage : I assume its from Model into storage(aquifer)  Storage out means water from Aquifer to the model. Is this correct???

I do know Flow into or out of storage can balance other components of groundwater flow. my question was who can explain to me clearly what is happening.

Thank you

John

[Melvin Raymund Dangin]

Hello John,

I'm actually struggling to understand this as well. Have you figured this out? I also can't seem to imagine storage coming in and out of a model.

Thank you.

Melvin

[Ashutosh]

Hi,

If you look at the transient groundwater equation specific storage and specific yield terms exist. This part of the equation is what you see in storage in out terms.

Ashutosh Singh

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On 22 Jun 2023, at 14:58, 'Melvin Raymund Dangin' via MODFLOW Users Group <mod...@googlegroups.com> wrote:

Hello John,

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[Diganta Sarma]

In the overall MODFLOW water budget calculation, when the aquifer gains storage it is an outflow from the flow system and it is a negative term by convention. The opposite, when water is released from storage, it is an inflow to the flow system and is a positive term. See section on Volumetric Budget in the MODFLOW 2005 manual.

Diganta

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[Jakab Andras - Gmail]

Although I'm not Richard, let me share my thoughts about this matter. Storage shall simply be considered as a sink/source term for a transient model. If for instance, a cell has a negative volumetric balance, then it adds/supplies some volume of water to the model, which in this case is considered as Storage In. The reverse is Storage Out. Volumetric and mass balances are always expressed with respect to the model. I know this may be confusing.

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[Melvin Raymund Dangin]

Thank you everyone for explaining. I now have a better understanding of the concept. My water budget also has constant head and head dep bounds terms because my model has CHD and GHB. Do the IN and OUT values mean that water is added or removed to maintain the constant/specified heads? Thank you, I appreciate how supportive this group is. 

[Jakab Andras - Gmail]

To reiterate what I wrote in my previous message, CHD or GHB In means that volume is added to the model through the respective boundary condition. And yes, in the case of the CHD bc. it adds or removes an amount that is required to maintain the specified head.

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[Melvin Raymund Dangin]

Dear Mr. Jakab,

Apologies for the late reply. Thank you for confirming my query. I'd just like to ask one more question on boundary conditions, specifically for DRN and RIV.

In my model, I'm using the RIV package to define river boundary conditions, although I'm aware that some people prefer to use DRN especially when the river is effluent. What confuses me is when both packages are used on the same model for river boundaries. Wouldn't that be redundant in some sense? I hope you could clarify my notion on this setup.

Thank you also to anyone who would want to answer this question.

Sincerely,

Melvin

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[Jakab Andras - Gmail]

Do you mean using both RIV and DRN for the same surface water feature? That is definitely not recommended. Depending on the case, use either RIV or DRN (or something else). You may of course use both RIV and DRN in the same model, but each for a different boundary feature.

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[Melvin Raymund Dangin]

Thank you for the clarification, Mr. Jakab. I actually tried applying both RIV and DRN boundary conditions for the same river network in my model just to test it out. The mass balance showed the same OUT values (±5m³) for both, which confirmed my idea of it being redundant. 

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