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Cell Rewetting

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I am trying to figure out this wetting and drying thing and it just is not making any sense. My understanding is that the wet/dry flag is the threshold against which the head in the cells adjacent to the dry one is compared. If the flag is positive and the head in the adjacent cells is greater than this value, then the your selected equation will be evaluated to find the renewed head for the previously dry cell.

Well, I have been trying out a few examples and the numbers are not adding up. For example, I put a +20 in the wet/Dry flag for the whole grid. I assume that means that the head in the adjacent cells would need to be 20 feet higher than the bottom of the dry cell before it would be rewetted. This is a number that is much greater than would ever occur in my model. In this example, the cell was rewetted when the head in the adjacent cells was 9 feet above the dry cell bottom.

I was using equation 2: h = Bot+wetfact(Thresh) where: Bot = 435; WetFact = 2.0; and Thresh = 20 (wet/dry flag).

Therefore, I should get h = 435 + 2*10 = 455. But, what I got was a head in the rewetted cell of 439.6, which is appropriate given the heads in the adjacent cells, but it does not follow the protocol as I understand it.

I see two things that dont seem to be working as I understand it:

1. The wet/dry flag as a threshold does not seem to be working

2. The equation using wetting factor and threshold does not seem to be working.

Does anybody have any ideas on what I am doing wrong? I have uploaded a pdf of my notes on wetting and drying. Feel free to comment, correct, or add-on to what I have written. I will incorporate all of the comments and then upload the final as a guide for us lost rewetted souls.


Edited by Hydrology
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I am by no means an expert on this, but will provide my opinion to at least give you an answer to work with.

1) It looks like you've got things basically correct with your analysis of the equation and variables, except it appears you put a "2*10" instead of a "2*20"

2) I can't say why the cell was rewetted when your threshold was 20 and the head in the adjacent cells was only 9 above the dry cell bottom. Perhaps your solution is fluctuating during some iterations and the threshold does get exceeded?

3) It is my understanding that the head calculated by the equations are what you would consider a "starting head" just to rewet the cell. MODFLOW would then calculate the resulting head for the cell. This is why the equation provides a different head than the resulting head which, as you noted, was appropriate given the heads in the adjacent cells.

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