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Numerical modeling of karst water systems is a challenge due to vastly different flow regimes in the solution channels and the porous medium, and because of the complex interactions between them. Recent developments with unstructured grid models have enabled robust simulation of such systems due to full coupling of the system of equations and the interactions, within a flexible gridding framework that allows these domains to be discretized independently of each other. These capabilities have been incorporated into the MODFLOW-USG code, which has been developed and released in the public domain by the United States Geological Survey. Further developments to MODFLOW-USG have continued and have also been released in the public domain. The significant features that enhance simulations in karst environments include various optional turbulent flow formulations, solute transport capability and a density-dependent flow simulation option that can be used to model transport of nonaqueous phase liquids (NAPLs) or freshwater–seawater interactions, for example. A dual-porosity-flow and dual-porosity-transport formulation is also included to further expand model scale and flexibility. A physically based representation of flow and transport within the karst conduits (channels) and porous medium eliminates the need for various surrogate modeling approaches in karst, thus enabling solutions to some of the common problems that groundwater professionals face when dealing with groundwater resources management in coastal karst areas. A demonstrative example shows the impact of karst on the saltwater system in a coastal aquifer. [ABSTRACT FROM AUTHOR]

Groundwater discharge through evaporation due to a shallow water table can be an important component of a regional scale water balance. Modeling this phenomenon in irrigated regions where soil moisture varies on short time scales is most accurately accomplished using variably saturated modeling codes. However, the computational demands of these models limit their application to field scale problems. The MODFLOW groundwater modeling code is applicable to regional scale problems and it has an evapotranspiration package that can be used to estimate this form of discharge, however, the use of time-invariant parameters in this module result in evaporation rates that are a function of water table depth only. This paper presents a calibration and validation of the previously developed MOD-HMS model code using lysimeter data. The model is then used to illustrate the dependence of bare soil evaporation rates on water table depth and soil moisture conditions. Finally, an approach for estimating the time varying parameters for the MODFLOW evapotranspiration package using a 1-D variably saturated MOD-HMS model is presented. [ABSTRACT FROM AUTHOR]

The objective of this paper is to present an overview of the fundamental equations governing transport phenomena in compressible reservoirs. A general mathematical model is presented for important thermo-mechanical processes operative in a reservoir. Such a formulation includes equations governing multiphase fluid (gas-water-hydrocarbon) flow, energy transport, and reservoir skeleton deformation. The model allows phase changes due to gas solubility. Furthermore, Terzaghi's concept of effective stress and stress-strain relations are incorporated into the general model. The functional relations among various model parameters which cause the nonlinearity of the system of equations are explained within the context of reservoir engineering principles. Simplified equations and appropriate boundary conditions have also been presented for various cases. It has been demonstrated that various well-known equations such as Jacob, Terzaghi, Buckley-Leverett, Richards, solute transport, black-oil, and Biot equations are simplifications of the compositional model. [ABSTRACT FROM AUTHOR]