Relating Macropore Flow to Soil Parametric and Morphological Variables

Core Ideas A new parameter, “slope of macropore flow (s)”, was introduced The parameter s allowed the identification of soil variables controlling macropore flow Soil clay content was the key parameter controlling macropore flow The parameter s may be used as a soil physical indicator in environmental studies Preferential flow is an important soil hydraulic property as it controls water flow and solute transport in saturated and near‐saturated conditions. Preferential flow occurs in a small range of soil water potentials and soil structure predominantly controls soil water flow; it is thus challenging to model preferential flow using soil parametric variables. The aim of this study was to identify soil properties that affect macropore‐type preferential flow. Thirty undisturbed soil samples were collected from the topsoil of a paddy field and adjacent grassland, and hydraulic conductivity (K) was measured at saturation and near saturation at different tension (h) values. Soil parametric and morphological features were then recorded in the laboratory. To evaluate correlations between K(h) and different soil properties, the log‐log slope of K(h) at near saturation (s) was obtained for each sample by plotting logK versus logh. The values of s were then correlated with soil parametric and morphometric properties. Values of Ks ranged from 1.2 × 10−6 to 7.5 × 10−4 cm h−1 with a mean of 3.77 × 10−4 cm h−1 and standard deviation (SD) of 2.14 × 10−4 cm h−1. Application of 7 cm tension resulted in an approximately 80‐fold decrease in Ks. Values for s ranged from –3.64 to –0.98 with a mean of –2.32 and SD of 0.77. These results demonstrated that the soil clay content, cation exchange capacity, specific surface area, coefficient of linear extensibility, rupture resistance, and plasticity were negatively correlated with s, while BD, structure size, structure grade, pore number and pore size were positively correlated with s. A correlation analysis further revealed that soil clay content was the key variable controlling s in the studied soils. These findings suggest that s can be used as a physical soil indicator that links environmental impact, water, and solute dynamics in near‐saturated and saturated soils.