Unsaturated Hydraulic Conductivity from Transient Multistep Outflow and Soil Water Pressure Data

Abstract

Soil water retention and unsaturated hydraulic conductivity functions [K(6)] 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 AT(6). An alternative method for the direct measurement of Jf(8) 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 Af(9) 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.

Inverse estimation of soil hydraulic properties under oil palm trees

“…Eching et al (1994) in their study used, in addition to the cumulative drainage data, also water contents as input for the inversion in order to optimize the soil water retention and hydraulic conductivity curves. The initial condition was given by measured initial water contents of both the stemflow and throughfall areas.…”

Unsaturated hydraulic conductivity from nuclear magnetic resonance measurements

“…Faster and more flexible alternatives have been sought, the most popular of which is inverse modeling of transient flow data to determine the P c (S w ) and k rw (S w ) functions simultaneously by optimization (i.e., least squares estimation of the parameters in Brooks-Corey, van Genugthen-Mualem or other empirical models of the hydraulic functions) [Kool et al, 1987]. This approach has been applied to data obtained from onestep [e.g., van Dam et al, 1992] and multistep outflow [e.g., Eching et al, 1994;Nützmann et al, 1998], evaporation [e.g., Romano and Santini, 1999] and centrifuge experiments [Šimůnek and Nimmo, 2005]. Posed as an inverse problem, however, determination of soil hydraulic functions is complicated by issues of parameter nonuniqueness and identifiability.…”

Determining Soil Hydraulic Properties

“…For these reasons, the multistep method was suggested, where the pressure at the boundary is changed in several small steps (van Dam et al, 1990). Eching et al (1994), van Dam et al (1994), Crescimanno andIovino (1995), andZurmühl (1996) demonstrated for the van Genuchten/Mualem model that the multistep method is superior to the one-step method because the hydraulic parameters are less correlated.…”