UNSATURATED HYDRAULIC CONDUCTIVITY FROM TRANSIENT MULTISTEP OUTFLOW AND SOIL-WATER PRESSURE DATA

Soil water retention and unsaturated hydraulic conductivity functions [K(theta)] estimated by the inverse solution technique through minimiz ation of differences between measured and simulated transient outflow may be nonunique and differ from independently measured soil hydraulic data. Numerical and experimental studies have shown the benefit of us ing simultaneously measured soil water pressure head in the estimation of the soil water retention curve by the inverse technique. In this e xperimental study, soil water pressure head and transient cumulative o utflow measured simultaneously are used to estimate K(theta). An alter native method for the direct measurement of K(theta) from transient mu ltistep outflow experiments was adopted. Desorption experiments were c arried out for disturbed Yolo silt loam (fine-silty, mixed, nonacid, t hermic Typic Xerorthent), Panoche loam (fine-loamy, mixed [calcareous] , thermic Typic Torriorthent), Hanford sandy loam (coarse-loamy, mixed , nonacid, thermic Typic Xerorthent), and Oso Flaco fine sand columns. The optimized K(theta) values agreed well with the directly measured data for all soils, except the sand. Additionally, soil hydraulic func tions so obtained for the Panoche loam agreed well with those determin ed using the evaporation method. Measured infiltration in a column of the Panoche loam matched numerical results using optimized parameters as determined from a sorption multistep experiment. The addition of so il water pressure head values in the optimization procedure provides u nique parameters for the unsaturated hydraulic conductivity functions under our experimental conditions.