Abstract

Stony soils that have a considerable amount of rock fragments are widespread around the world. However, experiments to determine effective hydraulic properties of stony soils (SHP), 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 high local heterogeneity. Therefore, effective properties of stony soils especially in unsaturated hydraulic conditions are still not well understood. An alternative approach to evaluate the SHP 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 rock fragment contents, f. Soils with volumetric stone contents from 11 to 37 % were created by placing impermeable spheres in the form of rock fragments in a sandy loam soil. Time series of local pressure heads in various depths, mean water contents and fluxes across the upper boundary were generated in a virtual evaporation experiment. Additionally, a multi-step 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 SHP for different volumes of rock fragments. 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 rock fragments. However, the reduction of 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.

Highlights

  • Stony soils are soils with a considerable amount of rock fragments and are widespread in mountainous and forested watersheds around the world (Ballabio et al, 2016; Novák and Hlaváčiková, 2019)

  • Rock fragments in soil are particles with an effective diameter of larger than 2 mm (Tetegan et al, 2015; Zhang et al, 2016). Their existence in soil influences the two constitutive soil water relationships known as soil hydraulic properties (SHP) i.e. water retention curve (WRC), and hydraulic conductivity curve (HCC) (Russo, 1988; Durner and Flühler, 2006)

  • The profile is shown for the steady state flux situation with a pressure head of -100 cm and the stony soil with 28.5 % rock fragment content

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Summary

Introduction

Stony soils are soils with a considerable amount of rock fragments and are widespread in mountainous and forested watersheds around the world (Ballabio et al, 2016; Novák and Hlaváčiková, 2019). Two approaches have been dominant in identifying the hydraulic behavior of stony soils: I) Experimental setups with the aim of measuring SHP of stony soils in the field or in controlled systems in the laboratory (Cousin et al, 2003; Dann et al, 2009; Grath et al, 2015; Beckers et al, 2016, Naseri et al, 2019), and II) Development of empirical, physical or physico-empirical approaches to scale hydraulic properties of background soil based on the volumetric content of rock fragments (f) and their characteristics (Novák et al, 2011; Naseri et al, 2020). This result is in agreement with Novák et al (2011) who reported a higher reduction in conductivity compared to a reduction that is proportional to the rock fragments content

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