Unsaturated flow model using the UZF - dry cells

[Nahuel Villarroel]

Hello everyone, does anyone know how to set up a four-layer unsaturated flow model using the UZF package? I have the water table in the last layer, so I initialize the first three layers as dry (with the water table below the bottom of each layer) in the BAS package. However, when I start with dry layers, the model doesn't run and gives the following error:

forrtl: error (72): floating overflow.

  Thanks

[Oscar Fabian Sanchez Ortiz]

Maybe not a real solution, but I would try using mf6 nwt to avoid dry cells. it seems your problem is not linear, and the Newton-Raphson approximation may be better. Anyway, that looks like a bug instead of a problem with the implementation. another option is using Modflow 6, which has ongoing support and also has an unsaturated package

Cheers

[Nahuel Villarroel]

Thanks, Oscar. It seems like a good idea to try it with MF6, although it requires more time. For now, I will start the model without dry cells ( This configuration allows the model to run), and dry them out by having some time periods without water input. Then, I will begin infiltration in dry cells to represent flow in unsaturated zones. I will keep updating on the progress—if there are any ideas to solve the problem, please leave them in the comments.

 Thanks

[Oscar Fabian Sanchez Ortiz]

Sorry, I meant mf nwt (mf2005 with newton raphson), or as I said later, modflow 6, which also has UZF and newton raphson(optional), with a more active development and support from the USGS

[Dawood Muhammad]

Do you expect the water table to rise to the upper three layers? If not, there is no need to include those dry layers in the model.

Sent from my iPhone

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[Oscar Fabian Sanchez Ortiz]

I'm assuming that you are using mf2005 because you're using the bas package and you got dry cells, but if you use mfnwt, you get the newton raphson approach(no rewetting), and you would use almost the same files(of mf2005), you only need to update the solver(and the executable), but it's the same software with the same structure.

I think any GUI would let you change it easily, and even in Flopy, it would be very easy to do.

Cheers

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This group was created in 2004 by Mr. C. P. Kumar, Former Scientist 'G', National Institute of Hydrology, Roorkee. Please visit his webpage at https://www.angelfire.com/nh/cpkumar/
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[Nahuel Villarroel]

 I am using MFNWT and the following packages: ['DIS', 'BAS6', 'NWT', 'UPW', 'OC', 'GHB', 'UZF']. The entire model has been implemented with FloPy. Now, I am considering removing the GHB package since I have implemented constant heads at the boundaries using the BAS package, the IBound options with negative values, and the STR option for initial levels in all layers.  

[Nahuel Villarroel]

Dear Dawood, i do not expect the water table to rise; the lower layers are included to track the flow from the infiltration zone to the deeper layers. Theoretically, the bottom of the last layer represents the water table, meaning the entire domain would be the unsaturated zone. However, if the cells are dry, the model will not run. The discretization into layers in the Y direction is based on the geological formations present in the area.  

Thaks,

[Jakab Andras - Gmail]

You should have at least the bottom layer set up with initial and boundary conditions that render it non-dry. The overlying layers representing the unsaturated zone can be set as inactive cells for MODFLOW-NWT and will only serve to provide geometrical requirements for the UZF1 package. You may then set the vertical hydraulic conductivity of these unsaturated zone cells using two options.

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