MODFLOW-NWT Source Code Released

And there was much rejoicing... The MODFLOW-NWT source code is out now (see link below). We will have it hooked up in Groundwater Vistas Version 6.04 Build 1, probably by tomorrow. If you see 6.04 build 1 on our web site (www.groundwatermodels.com - click Clients/Downloads), then you know we have added MODFLOW-NWT.

So now you'll probably want to know how to use it. It's actually pretty easy to get it working because it's essentially MODFLOW2005 with a new solver. So here is what you need to do:

1. Select Model|MODFLOW|Packages and change the version to MODFlOW2005
2. You must use the LPF Package (BCF not supported). The easy way to make that happen is to turn on "Automatically Reset Package Units" in step 1.
3. Select Model|MODFLOW2005|Packages and turn on the checkbox that says "Use Upstream Weighting (UPW) and Newton Solver (NWT) instead of LPF Package"
4. Select Model|MODFLOW2005|Options and click on the "NWT General" tab. The NWT Solver has lots of options under this tab and the one labeled "NWT Methods". See notes below on the solver settings.

That is all you need to do. The next time you run the model, you will be running MODFLOW-NWT. We do have a windows DLL version, as well as the USGS command line versions in both 32-bit and 64-bit flavors.

Just a note on the solver settings. There are a lot of settings but one thing I really like about the implementation of this NWT solver is the use of an OPTIONS variable that can be SIMPLE, MODERATE, COMPLEX, or SPECIFIED. If you choose one of the first 3 settings you eliminate most of the settings and NWT will configure itself. We are defaulting to MODERATE in Vistas but we'll see how that goes. As we gain more experience we may change that. Anyway, if you choose one of those first three settings then you only need to set the following variables in the solver:

HEADTOL, FLUXTOL, MAXITEROUT, THICKFACT, LINMETH, IPRNWT, and IBOTAV

These variable names are listed on the GV dialog and they are all on that first tab mentioned above.

One other note on MODFLOW-NWT is the specification of dry cell head. We have set up GV so that cells will not show up as dry. This is essentially the same thing that MODFLOW-Surfact does. If you would rather see dry cells, there is a checkbox on Model|MODFLOW2005|Options - LPF Options tab called "IPHDRY". Turn that option on if you want to see dry cells. There is one other option there that is for MODFLOW-NWT which will reduce pumping rates in cells that become desaturated. See the MODFLOW-NWT manual for more information.

We have noticed some lingering "stack overflow" problems with both MODFLOW2005 and MODFLOW-NWT so if your model crashes please send us the GWV file so we can figure out the cause. We have tried to test this on large problems but there is no guarantee that we caught all of the problems. This is a long story - corner me at the MODFLOW conference in a couple of weeks and I'll bore you with it.

Here is the link. It is not live yet on the USGS Groundwater Software page but should be soon.

water.usgs.gov/nrp/gwsoftware/modflow_nwt/ModflowNwt.html