Modeling soil hydraulic properties for a wide range of soil conditions

This paper presents equations for estimating soil hydraulic properties such as bulk density (Db), field capacity (FC), permanent wilting point (PWP), and saturated hydraulic conductivity (Ksat) from soil composition (sand, silt, clay, organic matter, coarse fragments), and with increasing soil depth, for a wide range of natural soil types and conditions. The equations, derived from New Brunswick and Nova Scotia soil survey reports, were evaluated and extended to international databases, e.g., ISRIC (Db, FC, PWP) and UNSODA (Ksat). Equation derivation was guided by theoretical considerations and regression techniques, to generate realistic Db, FC, PWP and Ksat values with sand, clay, organic matter content and soil depth as the only predictor variables, each with low inter-coefficient correlations. The results ensure that Db ≤ Dp (soil particle density), FC ≤ SP(soil saturation point), PWP ≤ FC, and Db, FC, PWP, Ksat > 0. The best-fitted Db, FC, PWP, log10(Ksat), data variations are represented as follows: R2 = 0.83, 0.96, 0.65 and 0.79, respectively. In addition, the associated root-mean-square-residuals are similar to published values achieved with other approaches. The equations can be used to quantify expected changes in available soil water, Db, FC, PWP, log10(Ksat), and with varying organic matter content and texture. Examining the ISRIC data revealed systematic coefficient variations for the Db equation by soil order.