Combined Transient Method for Determining Soil Hydraulic Properties in a Wide Pressure Head Range

Simulation of water flow in unsaturated soils requires knowledge of the soil hydraulic properties. Two standard methods for the simultaneous determination of the water retention and hydraulic conductivity function in the laboratory are the multistep-outflow and the evaporation method. The multistep-outflow method provides information primarily in the pressure head range corresponding to relatively moist conditions, whereas the evaporation method has its highest information content in the medium to dry range. In the moisture ranges not covered by the experiments, an extrapolation of the soil hydraulic functions leads to very uncertain and possibly incorrect estimations of hydraulic properties. To obtain reliable estimates of soil hydraulic properties in a wider range of soil moisture contents, we combined both methods in a successive manner. The combined experiment starts with a multistep-outflow experiment, which is directly followed by an evaporation experiment. We tested this experimental design using synthetic data and laboratory measurements evaluated by inverse modeling. In the evaluation of the combined experiment, data points for the retention and conductivity functions were calculated from the evaporation experiment and included in the objective function for the inverse simulation of the multistep-outflow experiment. The combined evaluation led to greatly improved estimates of the hydraulic properties in a wide moisture range, circumvented the unreliable extrapolation beyond the pressure head ranges of the separate experiments, and significantly reduced the model error caused by such extrapolations.