Modflow-usg dry cells

[Martin1994]

Hello community.

I am currently trying to build a model in MODFLOW-USG similiar to a model in MODFLOW-NWT. My NWT model is fast and robust.

I have tried to build a similar model of the same area in USG in different ways, ending with the same problem. The model will not converge if any cells is dry. I discovered this by adjusting the recharge parameter. My USG model converged when the recharge was extremely high and no cells were dry.  

I tried to "convert model to MODFLOW-USG simulation" in GMS to check if the model would converge when there was no difference in grid structure. I get the same result. I need to increase the recharge to have no dry cells and a model that does not crashes.

Can anyone tell me if the SMS solver in MODFLOW-USG can be set up in the same way as the NWT solver to handle dry cells, and how?

Martin

 

[Sean Czarniecki]

So let me tell you something that might help you to figure out what is going on.  I had built a model using MODFLOW-USG....one of the reasons was because it managed dry cells without actually making them dry (model converged with no issue).  Well, after months of working with this model, a newer MODFLOW-USG executable came out (and was being used in GMS) that eliminated that part of the programming (I guess it was not helpful to some people - enough complaints got the programming changed).  Well, that certainly messed up my modeling.  It wouldn't converge anymore.  The solution was ultimately to set GMS to use the older executable.  Soooo.....long story short.....the older MODFLOW-USG executable may solve your original model.

[Martin1994]

Hi, thank you for your thoughts on this. It might be the problem.

It is still my belief that it is possible to have MODFLOW-USG manage dry cells. At least that is what i get from reading the documentation of the code, and som litterature. I will try working on with it.

[julian.weir]

I had the same problem, and the helpful guys at USGS helped me solve it.  I was able to replicate the NWT solution by setting all layer types to 'Convertible Upstream' (in the LPF Package options) and using the '(1) Newton with Delta-Bar-Delta' nonlinear solution method (under the SMS solver options).  When theses are selected, it reverts to the equivalent of MODFLOW-NWT, and I understand that the cell wetting options are not used.  Hope that helps.

[Martin1994]

On 7/14/2019 at 10:45 PM, julian.weir said:

I had the same problem, and the helpful guys at USGS helped me solve it.  I was able to replicate the NWT solution by setting all layer types to 'Convertible Upstream' (in the LPF Package options) and using the '(1) Newton with Delta-Bar-Delta' nonlinear solution method (under the SMS solver options).  When theses are selected, it reverts to the equivalent of MODFLOW-NWT, and I understand that the cell wetting options are not used.  Hope that helps.

Thanks, this solved my problem. Even though my result is slightly different from my similar model solved with modflow-nwt my model now converges with the dry cells. Thanks.

[RSP21]

Hi, 

I have the same problem. My MODFLOW-USG model is running with certain horizontal and vertical hydraulic conductivity values. However, if I changed the  horizontal and vertical K, the model wont converge as a results of dry cells. 

I am using '(1) Newton with Delta-Bar-Delta' with layer types of 'Convertible' in the LPF Package options (the model wont converge by selecting the 'Convertible Upstream' option).  

I also using the latest version of MODFLOW.  

Any advice is greatly appreciated.