Estimation of depth averaged unsaturated soil hydraulic properties from infiltration experiments

Soil hydraulic properties determined from laboratory experiments often are non-representative of field conditions. An inverse method was developed to estimate hydraulic properties of unsaturated uniform soil from intermediate- and field-scale infiltration data, where there is no significant preferential flow during the infiltration experiment. Four parameters, (α, n, Ks and θr) in van Genuchtenʹs model were estimated by numerical inversion of Richards equation. Measurements needed for the inversion method include: soil water tension versus time at one distance from the soil surface, the initial water content, and a final steady-state water content behind the wetting front. The objective function used for parameter optimization is constructed from two parts, one from the transient tension versus time curve, and another from the steady-state water content data. The second part of the objective function is defined as the water storage estimation error, and was calculated as the difference between the measured and predicted water storage profiles. The method was tested using data collected during a constant flux infiltration experiment in a 400 cm deep soil column. The sensitivity of parameter estimation and the uniqueness of the inverse problem were dramatically improved when the second part of the objective function was included. The proposed method is promising for estimating depth averaged in situ unsaturated hydraulic properties at the field scale without the need to collect excessive amounts of data.