IN-SITU ESTIMATION OF SOIL HYDRAULIC FUNCTIONS USING A MULTISTEP SOIL-WATER EXTRACTION TECHNIQUE

Estimation of the retention and unsaturated hydraulic conductivity fun ctions is essential to effectively provide input for water flow and tr ansport simulation and prediction. A parameter optimization procedure is shown as a promising tool to estimate inversely these hydraulic fun ction parameters from transient soil matric potential and cumulative s oil solution extraction measurements; Sensitivity analyses from synthe tic data generated from forward numerical model simulations showed tha t optimum tensiometer locations will depend on soil type. Experiments were carried out in both a laboratory column (Columbia sandy loam) and in the field (Yolo silt loam). In both cases a series of vacuum extra ction pressures was applied to a ceramic soil solution,sampler, and cu mulative soil solution extraction volume and matric potentials at vari ous positions near the extraction device were monitored as the soil so lution was extracted. In the laboratory a zero-flux boundary condition was maintained at the bottom of the column, whereas matric potential measurements were used in the field to define the lower boundary. In b oth the field and laboratory experiments, flow at the upper boundary w as zero. Cumulative extraction volume and matric potential data were i ncluded in the objective function to be minimized to estimate the hydr aulic function parameters. We determined that the optimized solution w as sensitive to the contact between the ceramic ring and the surroundi ng soil. By also optimizing the hydraulic resistance of the ceramic ri ng bf the extraction device, optimization improved the fit between mea sured and optimized flow variables. Comparison of the optimized with t he independently measured hydraulic functions indicated that the in si tu estimation using a multistep extraction procedure can provide accur ate soil hydraulic data.