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 minimization 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 using simultaneously measured soil water pressure head in the estimation of the soil water retention curve by the inverse technique. In this experimental study, soil water pressure head and transient cumulative outflow measured simultaneously are used to estimate K(theta). An alternative method for the direct measurement of K(theta) from transient multistep outflow experiments was adopted. Desorption experiments were carried out for disturbed Yolo silt loam (fine-silty, mixed, nonacid, thermic 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 functions so obtained for the Panoche loam agreed well with those determined 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 soil water pressure head values in the optimization procedure provides unique parameters for the unsaturated hydraulic conductivity functions under our experimental conditions.