A mass-conservative procedure for time-stepping in models of unsaturated flow

In numerical models of moisture in the unsaturated zone, it is frequently desirable to choose the matric (or pressure) head, rather than the moisture content, as the dependent variable. Two important advantages of this choice are that it facilitates modeling of soil systems that are locally saturated and that it provides a state variable that is always continuous in space, regardless of soil inhomogeneities.