Monday, April 07, 2008

MODFLOW & Vertical Leakance Coefficient

I seem to get a lot of questions about vertical leakance vs. vertical hydraulic conductivity in MODFLOW. In the original versions of MODFLOW (88 and 96), the Block Centered Flow (BCF) Package required you to enter a value for vertical leakance or VCONT. That value was based on the half-thickness of layers and the vertical K of the layers. All preprocessors that support MODFLOW88/96 compute this for the user, although there are slight differences in how they compute VCONT. Like most preprocessors, Groundwater Vistas computes VCONT based on the saturated thickness of the layer. So as the starting heads change during the evolution of a model, the VCONT may change as well for unconfined layers. However, during any particular simulation, VCONT is constant, even if the saturated thickness of the layer changes.

In MODFLOW2000, the BCF Package is still available with the same assumptions. A new package was introduced that can be used in place of BCF. The new package is the Layer Property Flow Package (LPF). Instead of a vertical leakance, LPF takes as input the vertical hydraulic conductivity directly and computes a vertical conductance internally. The main difference is that it updates vertical leakance at each solver iteration for partically saturated convertible layers. This was a big change and caused many models developed in MODFLOW96 to give different results in MODFLOW2000.

MODFLOW2005 is essentially the same as MODFLOW2000 with one exception. A new option was added that makes LPF more BCF-like. You can choose to have the vertical leakance computed once at the beginning of the simulation and then it remains constant as in BCF.

Which is correct? I think that theoretically the LPF approach is the better one. It is hard to say, though, that all of our older MODFLOW96 models are incorrect. Perhaps with all of the other uncertainties in a model it really does not matter.


Post a Comment

<< Home