Model with all layers treated as confined

[Philip Margarit]

Hi All,

I am currently working with a MODFLOW 6 model to do some capture fraction mapping. The model was built by another organization with each layer including the surficial aquifer represented as a confined layer rather than convertible. Likewise, I noticed some of the head-dependent boundary conditions have their heads and bottoms (drn and RIV) below the cell bottom of the first layer, however the model was able to run when the layers were all confined.

I wanted to ask whether anyone has experience working with models that represent all layers as confined rather than convertible, and whether the resulting constant-transmissivity assumption has significantly affected stress-response predictions. My concern is that this assumption could potentially bias capture-fraction estimates, particularly transient capture fractions. If a water-table aquifer is represented as confined, transmissivity remains fixed rather than decreasing as saturated thickness declines. That could overstate hydraulic diffusivity and allow the response to pumping to propagate more efficiently than it would in a convertible/unconfined formulation which may overestimate capture fractions or change their timings. 

Has anyone ran into this in their work? Thanks all! 

[Christian Langevin]

There is a paper published in Groundwater on this topic.  Would be worth a look if you haven't seen it.

Simulation of Water-Table Aquifers Using Specified Saturated Thickness

by , , , , 

First published: 06 February 2014

 

https://doi.org/10.1111/gwat.12164Digital Object Identifier (DOI)