Validation of an analytical method for determining soil hydraulic properties of stony soils using experimental data

Soil unsaturated hydraulic properties are an important input for simulations of water and solute movement in the vadose zone. However, in the case of soils containing rock fragments, direct measurement of unsaturated hydraulic properties remains difficult. Recently, an analytical method was proposed (Ma et al., 2009) for determining soil hydraulic properties from horizontal water absorption experiments. Here, we test if this method could be used in the determination of the hydraulic properties of stony soils with a set of experiments. The results show that the method can predict accurately and quickly the hydraulic parameters of stony soils using the cumulative infiltration volume and the rate of wetting front advancement during water absorption into a horizontal soil column. The influence of rock fragment content on some soil hydraulic properties including air entry suction, saturated hydraulic conductivity and the shape coefficients of hydraulic functions was further evaluated. Both air entry suction and shape coefficient show a large range of variation with rock fragment content. Globally, the saturated hydraulic conductivities of stony soils decreased with increasing rock fragment content. However, when the volumetric rock fragment content is about 0.08 cm3 cm−3, the stony soil displayed a greater saturated hydraulic conductivity compared to the fine earth at 95% confidence level. The relationship between saturated hydraulic conductivity and rock fragment content is not accurately estimated by different equations relating saturated hydraulic conductivity of stony soils to that of non-stony soils. These findings imply that other factors than the reduction of cross sectional area for water flow influence the hydraulic properties of stony soils.

Research Highlights
► A new method to estimate hydraulic parameters was proved applicable to stony soils.
► Soil hydraulic parameters (hd, n and Ks,T) vary with rock fragment content.
► The hydraulic conductivities of stony soils cannot be modeled well by present methods.
► The influences of stones cannot be fully attributed to reduced pore volume.