Effective hydraulic properties of 3D virtual stony soils identified by inverse modeling

Abstract

Abstract. Stony soils that have a considerable amount of rock fragments (RFs) are widespread around the world. However, experiments to determine the effective soil hydraulic properties (SHPs) of stony soils, i.e., the water retention curve (WRC) and hydraulic conductivity curve (HCC), are challenging. Installation of measurement devices and sensors in these soils is difficult, and the data are less reliable because of their high local heterogeneity. Therefore, effective properties of stony soils especially under unsaturated hydraulic conditions are still not well understood. An alternative approach to evaluate the SHPs of these systems with internal structural heterogeneity is numerical simulation. We used the Hydrus 2D/3D software to create virtual stony soils in 3D and simulate water flow for different volumetric fractions of RFs, f. Stony soils with different values of f from 11 % to 37 % were created by placing impermeable spheres as RFs in a sandy loam soil. Time series of local pressure heads at various depths, mean water contents, and fluxes across the upper boundary were generated in a virtual evaporation experiment. Additionally, a multistep unit-gradient simulation was applied to determine effective values of hydraulic conductivity near saturation up to pF=2. The generated data were evaluated by inverse modeling, assuming a homogeneous system, and the effective hydraulic properties were identified. The effective properties were compared with predictions from available scaling models of SHPs for different values of f. Our results showed that scaling the WRC of the background soil based on only the value of f gives acceptable results in the case of impermeable RFs. However, the reduction in conductivity could not be simply scaled by the value of f. Predictions were highly improved by applying the Novák, Maxwell, and GEM models to scale the HCC. The Maxwell model matched the numerically identified HCC best.

Rock fragments influence the water retention and hydraulic conductivity of soils

“…For more information about their advantages, constraints, and assumptions, we refer to Naseri et al. (2019, 2020, 2022) and the references therein.…”

“…More comprehensive models such as Maxwell, Novák, and general effective medium theory (GEM), not only consider the volumetric content of RFs, but also their shape, orientation, and soil type (Naseri et al, 2020;Novák et al, 2011;Peck & Watson, 1979;Zimmermann & Bodvarsson, 1995). Recently, these models have been evaluated for an ideal case of spherical RFs included in a homogeneous sandy loam background soil by three-dimensional numerical modeling (Naseri et al, 2022). For more information about their advantages, constraints, and assumptions, we refer to Naseri et al (2019Naseri et al ( , 2020Naseri et al ( , 2022 and the references therein.…”

“…Recently, these models have been evaluated for an ideal case of spherical RFs included in a homogeneous sandy loam background soil by three-dimensional numerical modeling (Naseri et al, 2022). For more information about their advantages, constraints, and assumptions, we refer to Naseri et al (2019Naseri et al ( , 2020Naseri et al ( , 2022 and the references therein.…”

“…The error of each model was quantified by the RMSD and mean deviation (MD) between the modeled and measured data points of the common log of HCC. The critical volumetric RF content in the GEM model was set as 0.84 with the shape parameter t = 1.26 for rounded RFs (Naseri et al, 2022), and in the model of Novák, a value of 1.2 was used for α K . Novák et al (2011) estimated that values of α K vary between 1.1 and 1.32 depending on the soil texture, diameter, and number of RFs in the stony soil.…”

3–4D soil model as challenge for future soil research: Quantitative soil modeling based on the solid phase

“…Virtual soils can be used for educational purposes (e.g., Grunwald et al., 2016) and experiments, which cannot be performed in real situations. A recent example is the analysis of effects of content and local distribution of stones in soil samples on soil hydraulic properties (Naseri et al., 2022) or effects of chemical heterogeneity by pyrite distribution in overburden spoil piles on the long‐term acidification of groundwater in mining areas (Gerke et al., 2001). Other applications are model‐based studies of tillage‐induced features on flow and transport in arable soils in response to varying boundary conditions (e.g., Schelle et al., 2013; Schlüter et al., 2012).…”