Category: Groundwater Modeling Software

(Originally published on March 12, 2013)

Sometimes it is helpful to compare the results of a SEAWAT model with a SWI model or vice versa. In this post, I will discuss converting from SEAWAT to SWI, which is the easier transition.  SWI employs a simplifying assumption which means that it needs fewer inputs than SEAWAT.  Both SWI and SEAWAT are built on the MODFLOW structure, which means it is relatively easy to transition one to the other.  The process, here, involves replacing SEAWAT’s modified MT3D files with a SWI file.

When SEAWAT’s starting conditions include a sharp interface, as is the case in many of the distribution example problems, it is very straightforward to transfer to SWI.  The .BNT file in the SEAWAT model contains starting concentrations under the SCONC variable.  Fresh water will look like this, “0.000,” and salt water will look like this, “35.000.”  Thus, the top elevation values of the cells containing salt water could be used to describe the initial surface position of the salt water interface in SWI.  This should be entered in the SWI file as the ZETA variable.  The effective porosity (the SSZ variable  in SWI), can also be taken from SEAWAT.  In SEAWAT, effective porosity is listed under the PRSITY variable in the BNT file.

Something to keep in mind is that one of the major advantages of using SWI over SEAWAT is that SWI requires less vertical discretization than SEAWAT, because the SWI density layers can be defined inside of a cell, rather than on its faces.  Thus, to truly take advantage of SWI, some reduction of vertical discretization of the SEAWAT model may be necessary, but doing this correctly will require a certain finesse.

In this blog post, I will review two recent experiments in saltwater intrusion modeling calibration.  In 2004, W. Barclay Shoemaker published an article with Ground Water in which he discussed using UCODE in conjunction with SEAWAT.  UCODE is an inverse modeling program, published by the USGS to aid in parameter estimation and sensitivity analysis.  SEAWAT is a numerical flow model with density-dependent simulations capacities.  Both of these codes require a certain level of expertise to run, and while most hydrogeologists would be comfortable with the principles behind them, actually executing the codes correctly takes some investment of time.  These pragmatic challenges may be why UCODE has not been used in conjunction with SEAWAT very often.  However, as Shoemaker explains in his article: “As computing power increases and expertise grows, modeling studies will likely use both inverse methods and density-dependent ground water flow simulations to solve complicated water resource or ground water contamination problems (Shoemaker 2004).”

Shoemaker’s results demonstrated why UCODE can be so advantageous to saltwater intrusion modeling.  Among other benefits, UCODE helped illustrate the relative importance of different model parameters, and the types of observations the result in decreased correlation.

The second experiment was a modeling effort conducted by the USGS and the Georgia Department of Natural Resources.  In order to develop a regional-scale model of saltwater intrusion in coastal Georgia, the researchers constructed models in MODFLOW and SUTRA concurrently.  The MODFLOW model (MODFLOW can only modeled constant density fluid flow) was used to develop and calibrate the initial grid characteristics, using parameter estimation tools.  Once the MODFLOW model was constructed, a parallel SUTRA model was developed of the same region based on the developed parameter values.  The SUTRA code is able to simulate variable density flow.  When the researchers were satisfied with the calibration of the SUTRA model, they began refining the grid (Payne et al 2003).

Payne, D.F., Provost, A.M., & Voss, C.I. (2003). Parallel Development of MODFLOW and SUTRA Models in Coastal Georgia, South Carolina, and Florida: An Approach to Study Regional Ground-Water Flow and Local Saltwater Intrusion.  Proceedings of the 2003 Georgia Water Resources Conference, held April 23–24, 2003, at the University of Georgia. Kathryn J. Hatcher, editor, Institute of Ecology, The University of Georgia, Athens, Georgia.

Shoemaker, W.B. 2004. Important Observations and Parameters for a Salt Water Intrusion Model. Ground Water. 42(6):829-840.