Determining Soil Hydraulic Properties from One‐step Outflow Experiments by Parameter Estimation: II. Experimental Studies

Abstract

Unsaturated hydraulic properties of four soils of varying particle size distributions were evaluated by determining values of the five parameters in van Genuchten's (1980) hydraulic model. Saturated conductivities (Ks) and saturated water contents (θs) were directly measured and values of residual water content (θr) and the parameters α and n were evaluated by a nonlinear inversion method to minimize various objective functions. Method I uses an objective function involving sums of squared deviations between measured cumulative outflow with time Q(t) for one‐step pressure desorption and numerically simulated outflow from saturation to a final pressure head h = − 10 m. Method II supplements Q(t) data with the measured equilibrium water content θ at h = − 150 m, while Method III employs equilibrium θ(h) data only. Method I yields the most accurate description of Q(t) and the independently determined hydraulic diffusivity D(θ). A fair description of θ(h) is obtained within the range of the one‐step experiment but at lower θ predictions are less reliable especially for finer‐textured soil. Method II extends the range of validity of the predicted θ(h) to lower θ with generally small effects on predicted D(θ) and Q(t). Method III gives the best description of θ(h) but at the expense of accuracy in Q(t) and D(θ). Implications for routine evaluation of soil hydraulic properties are discussed.

Effect of unrepresented model errors on estimated soil hydraulic material properties

“…Despite decades of research, direct identification of these properties is time-consuming and near to impossible at larger scales. Therefore, indirect identification methods, such as inversion (Hopmans et al, 2002;Vrugt et al, 2008a), have been successfully applied to evaluate experiments starting from lab-scale (e.g., Parker et al, 1985;Van Dam et al, 1994;Šimůnek et al, 1998;Schneider et al, 2006) up to field-scale studies (e.g., Wollschläger et al, 2009;Huisman et al, 2010). Due to the multiscale heterogeneity of the soil hydraulic material properties (Nielsen et al, 1973;Gelhar, 1986;Cushman, 1990;Vogel and Roth, 2003), effective material properties have to be identified directly at the scale of interest.…”

Field-scale apparent hydraulic parameterisation obtained from TDR time series and inverse modelling

“…Comprehensive reviews are provided, e.g., by Hopmans andŠimůnek (1999) and Vrugt et al (2008). Significant effort has been spent to improve the methods from the original One-Step Outflow (Parker et al, 1985), through Multi-Step Outflow (van Dam et al, 1994), all the way to evaporation experiments (e.g., Simůnek et al, 1998;Schneider et al, 2006). As a result, the hydraulic properties of a soil sample can now be determined rather accurately and over a wide range of hydraulic states.…”

Bayesian estimation of the hydraulic and solute transport properties of a small-scale unsaturated soil column

“…Similar studies have been carried out in the past using both smallscale laboratory soil column experiments (Abbaspour et al, 2001(Abbaspour et al, , 2004Kool et al, 1985;Parker et al, 1985) as well as for more elaborate field scale settings (Hosseini et al, 2011;Kohne et al, 2006;Si and Kachanoski, 2000;Vrugt et al, 2006), in which minimization of the maximum likelihood objective function was used for estimating the unknown parameters. Various studies have used stochastic methods for studying fluid flow and contaminant transport processes using synthetic experimental data (Franssen et al, 2003a, Fu andGómez-Fernandez, 2009), including particle filters for real-time estimations (Franssen et al, 2003b;Li et al, 2012;Vrugt et al, 2013;Xu et al, 2013;Zhou et al, 2011).…”