The three governing equations in HST3D are coupled through the interstitial pore velocity, the dependence of the fluid density on pressure, temperature, and solute-mass fraction. The modified equations are more tightly coupled and have better stability for the numerical solutions. The basic source-sink term represents wells. A complex well flow model may be used to simulate specified flow rate and pressure conditions at the land surface or within the aquifer, with or without pressure and flow-rate constraints. Boundary condition types offered in HST3D include specified value, specified flux, leakage, heat conduction, an approximate free surface, evapotranspiration, and two types of aquifer influence functions.
All boundary conditions can be functions of time.
Four techniques are available in HST3D for resolution of the finite-difference matrix equations. A restart option is available in HST3D for storing intermediate results and restarting the simulation at an intermediate time with modified boundary conditions. This feature can also be used as protection against computer failure. Input and Output may be represented as tables of dependent variables and parameters, zone contour maps, and plots of dependent variables versus time. This equation is a combination of the conservation of total fluid mass and Darcy's Law for porous media.