where formula_4 is the volumetric fraction of the total porosity occupied by the porous medium, formula_5 is the internal fluid exchange rate (e.g. surface water, wells, and tile drains), formula_6 is the external fluid outside of the model domain, formula_7 is the saturated water content, formula_8 is the degree of saturation, formula_9 is the hydraulic conductivity tensor, formula_10 is the relative permeability of the medium calculated as a function of saturation, formula_11 is the pressure head, and formula_12 is the elevation head.