Influence of Stone Content on Soil Hydraulic Properties: Experimental Investigation and Test of Existing Model Concepts

Abstract

Core Ideas Unsaturated hydraulic conductivity of stony soils was determined in medium moisture range. Evaporation method works for stony soils, even if stone contents are high. Theoretical scaling models showed a good agreement with measurements for moderate stone contents. Model results and measurements differ markedly for soils with high stone contents. Studying the role of gravel, stones, or rock fragments on effective soil hydraulic properties (SHPs) is crucial for understanding and predicting soil water processes such as evaporation, redistribution, and water and solute transport through soils containing significant amounts of coarse inclusions. We conducted a laboratory study in which we investigated the effect of stones on the water retention and unsaturated hydraulic conductivity curves of soil–stone mixtures. Stony soils were created by packing predefined masses of soil particles (sand and sandy loam) with diameters <2 mm and crushed basalt (2–5 and 7–15 mm). The resulting mixtures ranged from 0 to 40% (v/v) stone content. The SHPs were determined with the simplified evaporation method. The measurements yielded plausible water retention and hydraulic conductivity curves across a wide moisture range. Results qualitatively showed the expected dependencies of SHPs on volumetric stone content, characterized by a reduction of soil water content and hydraulic conductivity across the whole pressure head range. Measured data suggested that coarse inclusions in soil tend to widen the effective pore‐size distribution. Prediction of SHPs of the stony soils, performed by fitting a flexible SHP model to the data of the background soil and scaling it with approaches from the literature, worked well for low stone contents. However, for volumetric stone contents of 25 and 40%, measured SHPs differed substantially from the properties predicted by simple scaling models.

Investigation of water retention functions of artificial soil-like substrates for a range of mixing ratios of two components

“…In other studies (Schindler et al 2015), no replicates are considered in order to verify the packing method or the variations in bulk densities in sampled replicates are not quantified (Deeb et al 2016). Furthermore, Naseri et al (2019) report "excellent agreement" (not quantified) of measurement results for three replicates packed into steel cylinders with a comparable packing method as used in this study. Figure 3, displaying all mean water retention curves within one plot, gives a first impression of how the water retention curves of a full mixing range of binary mixtures change depending on the mixing ratio.…”

“…A more reproducible way of mixture formulation is based on weighted compositions as Yilmaz et al (2018) use for constructing Technosols of different ratios of structural material and growing material or Vidal-Beaudet et al (2018) use for substrates for plant biomass production. Another promising approach is presented by Naseri et al (2019) who prepare mixtures of homogeneous ratios of soil and stones by matching up the weighted composition to a desired volumetric mixing ratio.…”

“…The technical mixing process, having the aim of achieving homogeneous mixtures, depends on sample sizes needed for each experimental method. In the before-mentioned studies, materials were sieved to a determined particle size (Deeb et al 2016;Vidal-Beaudet et al 2018;Rokia et al 2014) and wetted either before the mixing process 80% of the water content they show at 100 hPa or slightly moistened during the mixing process (Naseri et al 2019). The sample sizes prepared range from 0.25 to 20 L. Accordingly, the technical mixing process was realized by different means: manually, using a rotating shaker (Vidal-Beaudet et al 2018, Rokia et al 2014, and using a cement mixer (Deeb et al 2016).…”

“…The number of layers, the moisture state of the sample, and pressure applied are different in each study and usually adjusted in order to reach the desired bulk density. A more standardized way of packing samples was implemented by Naseri et al (2019), who use a constant weight, falling from a defined height in order to apply compaction strokes (seven strokes per layer, three layers). However, hydraulic comparability of soil-like substrate with differing organic contents is not given at a fixed bulk density, as compressibility decreases with increasing share of organic content (Paradelo and Barral 2013).…”

Studying Unimodal, Bimodal, PDI and Bimodal-PDI Variants of Multiple Soil Water Retention Models: I. Direct Model Fit Using the Extended Evaporation and Dewpoint Methods

“…HYPROP generates high-resolution data typically close to 100 water retention data points in the 0 to 100 kPa soil tension range plus an extra point close to the wilting point by considering the air-entry value as an additional measurement [37]. In recent years, the HYPROP system has been increasingly used and evaluated around the world [4,[38][39][40][41][42][43][44] and promising results have been reported. For instance, Bezerra-Coelho et al [38] evaluated the HYPROP system as applied to the van Genuchten [15] soil hydraulic functions for a wide range of soil textures using the HYDRUS-1D software package [45].…”