Abstract
Soil water retention parameters are critical to quantify flow and solute transport in vadose zone, while the presence of rock fragments remarkably increases their variability. Therefore a novel method for determining water retention parameters of soil-gravel mixtures is required. The procedure to generate such a model is based firstly on the determination of the quantitative relationship between the content of rock fragments and the effective saturation of soil-gravel mixtures, and then on the integration of this relationship with former analytical equations of water retention curves (WRCs). In order to find such relationships, laboratory experiments were conducted to determine WRCs of soil-gravel mixtures obtained with a clay loam soil mixed with shale clasts or pebbles in three size groups with various gravel contents. Data showed that the effective saturation of the soil-gravel mixtures with the same kind of gravels within one size group had a linear relation with gravel contents, and had a power relation with the bulk density of samples at any pressure head. Revised formulas for water retention properties of the soil-gravel mixtures are proposed to establish the water retention curved surface models of the power-linear functions and power functions. The analysis of the parameters obtained by regression and validation of the empirical models showed that they were acceptable by using either the measured data of separate gravel size group or those of all the three gravel size groups having a large size range. Furthermore, the regression parameters of the curved surfaces for the soil-gravel mixtures with a large range of gravel content could be determined from the water retention data of the soil-gravel mixtures with two representative gravel contents or bulk densities. Such revised water retention models are potentially applicable in regional or large scale field investigations of significantly heterogeneous media, where various gravel sizes and different gravel contents are present.
Highlights
A large proportion of soils containing rock fragments are present in the world due to soil evolution and erosion [1,2]
In order to quantify the effects of content and size of rock fragments on soil water retention and develop a new method for determining the water retention curves (WRCs) of soil-gravel mixtures, especially for mixtures with weathered gravels and for WRCs at high-suction range, we investigated WRCs of a loess soil and gravels mixtures with various gravel contents and gravel sizes
The weathering degree of the rock fragments was defined by observing their weathering characteristics in the field and observing the surface fissures and coarseness under a magnifier in the laboratory according to the Geotechnical Engineering Handbook [31]
Summary
A large proportion of soils containing rock fragments are present in the world due to soil evolution and erosion [1,2]. The highly variable gravel content or size in soilscape greatly increases the variability of the soil properties [3,4,5]. Knowledge of soil water retention curves (WRCs) is a prerequisite for modeling the fluxes of water and solutes in the vadose zone and it is necessary to determine their spatial variability [6,7,8]. Several reports noted that water held by gravel cannot be neglected in the determination of water retention properties due to the significant porosity of gravel and the changed pore-size distribution [14,15]. The ironstone gravel contained a large amount of available water ranging from 0.03 cm cm23 to
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