A pH‐Based Pedotransfer Function for Scaling Saturated Hydraulic Conductivity Reduction: Improved Estimation of Hydraulic Dynamics in HYDRUS

Abstract

Soil‐specific pedotransfer functions can be incorporated into the HYDRUS model. Electrolyte concentration reduces the adverse effect of pH and Na on soil structural stability. Clay content is important in governing soil hydraulic reduction dynamics due to pH. Hydraulic conductivity is a key soil property governing agricultural production and is thus an important parameter in hydrologic modeling. The pH scaling factor for saturated hydraulic conductivity (Ks) reduction in the HYDRUS model was reviewed and evaluated for its ability to simulate Ks reduction. A limitation of the model is the generalization of Ks reduction at various levels of electrolyte concentration for different soil types, i.e., it is not soil specific. In this study, a new generalized linear regression model was developed to estimate Ks reduction for a larger set of Australian soils compared with three American soils. A nonlinear pedotransfer function was also produced, using the Levenberg–Marquardt optimization algorithm, by considering the pH and electrolyte concentration of the applied solution as well as the soil clay content. This approach improved the estimation of the pH scaling factor relating to Ks reduction for individual soils. The functions were based on Ks reduction in nine contrasting Australian soils using two sets of treatment solutions with Na adsorption ratios of 20 and 40; total electrolyte concentrations of 8, 15, 25, 50, 100, 250, and 500 mmolc L−1; and pH values of 6, 7, 8, and 9. A comparison of the experimental data and model outputs indicates that the models performed objectively well and successfully described the Ks reduction due to the pH. Further, a nonlinear function provided greater accuracy than the generalized function for the individual soils of Australia and California. This indicates that the nonlinear model provides an improved estimation of the pH scaling factor for Ks reduction in specific soils in the HYDRUS model and should therefore be considered in future HYDRUS developments and applications.

Hysteresis in soil hydraulic conductivity as driven by salinity and sodicity – a modeling framework

“…While numerous experimental studies have led to models describing declines in K s due to salinity and sodicity, we are unaware of any model that considers differences between degradation and rehabilitation (McNeal and Coleman, 1966;McNeal, 1968;Yaron and Thomas, 1968;Lagerwerff et al, 1969;Russo, 1988;Ezlit et al, 2013;Ali et al, 2019). That is, all existing models assume that these two processes are strictly reversible, meaning decreases in K s can be restored simply by changing the chemical properties of the soil water (McNeal and Coleman, 1966;McNeal, 1968;Yaron and Thomas, 1968;Lagerwerff et al, 1969;Russo, 1988;Ezlit et al, 2013).…”

“…Clay dispersion, for instance, is widely understood to be irreversible (Goldenberg et al, 1983;Yaron et al, 2008). For this reason, a significant reduction in K s , i.e., a decrease of more than 15 %-25 % compared to its initial level, is considered a threshold beyond which a soil will suffer irreparable damage (Quirk and Schofield, 1955;McNeal and Coleman, 1966;Rengasamy et al, 1984;Cook et al, 2006;Bennett et al, 2019). Likewise, the experimental evidence that has considered degradation and rehabilitation together suggests that changes in K s are defined by hysteresis (Dane and Klute, 1977), the exact nature of which can be expected to vary on a soil-specific basis (Levy et al, 2005).…”