Optimization of Hydraulic Functions from Transient Outflow and Soil Water Pressure Data

Abstract

AbstractInverse solution techniques currently used for estimating unsaturated soil hydraulic functions from laboratory outflow experiments use cumulative outflow only in combination with initial and final soil water pressure head values. Additional soil water information is needed to improve the estimation procedure and to minimize uniqueness problems. It was the objective of this study to experimentally explore the feasibility of using both cumulative outflow and soil water pressure head data in the inverse solution for laboratory determination of soil hydraulic functions using both one‐step and multistep outflow experiments. Soil water pressure head was measured with a microtensiometer and pressure transducer. Desorption experiments were performed under both pressure and suction for the following soils: Yolo silt loam (fine‐silty, mixed, nonacid, thermic Typic Xerorthent), Panoche loam (fine‐loamy, mixed [calcareous], thermic TypicTorriorthent), Hanford sandy loam (coarse‐loamy, mixed, nonacid, thermic Typic Xerorthent), and Oso‐Flaco fine sand (coarse‐loamy, mixed Typic Cryorthod‐fine‐loamy, mixed, mesic Ustollic Haplargid). Water retention curves optimized from cumulative outflow alone were compared with those obtained from outflow and soil water pressure head measurements for both one‐step and multistep outflow experiments. Computer optimization of the retention curve by the inverse solution technique using transient outflow experiments was greatly improved when cumulative transient outflow data were combined with simultaneously measured soil water pressure head data. Pressure and suction experiments yielded equally good results for one‐step and multistep desorption. Moreover, the addition of soil water pressure head data resulted in unique parameter values for the optimized soil hydraulic functions under our experimental conditions.