Can the volume ratio of coarse to fine particles explain the hydraulic properties of sandy soil?

Hydraulic conductivity (Ks) and effective porosity (ϕeff) for saturated water flow are essential hydraulic properties for describing fluid and chemical transport in soil and groundwater systems. Typically, Ks is predicted by pedotransfer functions of soil texture and total porosity or ϕeff. This study shows that a more conceptual approach that uses a volume-weighted ratio of coarser (part of the sand fraction) to finer (clay and organic matter) particles alongside total porosity could explain variations in both Ks and φeff in intact 100-cm3 samples of 20 sandy surface and subsurface soils with <10% fines (clay + organic matter). The Ks function used was a simple power-law function of the volume-weighted coarse/fine particle ratio with two calibration parameters [A and pore network connectivity (PNC)]. The value of the power-law exponent (PNC) in the calibrated function was 1.8, similar to power-law exponents for gas diffusivity and air permeability in unsaturated soil (1.5–2). The second calibration parameter (A) probably depends on the soil classes under consideration, the Ks measurement method, and the sample scale. A sensitivity analyses showed that both Ks and ϕeff (taken as the volume content of pores larger than 30 μm, that is, drained at –10 kPa of soil water matric potential) are especially sensitive to organic matter content. Besides the water transport parameters, water retention under dry conditions was also closely correlated with the volume-weighted fines content. Therefore, the volume ratio concept seems to be a promising platform for the development of simple, accurate functions for the hydraulic properties of coarse-textured soils.