Unreconciled effects of salinity on draining and wetting functions in unsaturated soils

Abstract

Abstract In a sequence of one‐step outflow–inflow experiments, this study examines the effects of salinity on soil hydraulic properties with the goal of developing a better understanding of how to predict salinity's impacts on soils. The results of this study are compared with estimates obtained using the Hydrus‐1D model for soil water and solute transport. The model assumes a decline in hydraulic conductivity (K), caused by solution chemistry, is constant over the entire hydraulic conductivity function [K(θ)] and equal to a constant reduction factor established at saturation. In contrast with the model assumptions, the results of this study show an increase in the solution chemistry's effect on K with decreasing soil moisture (θ). These findings demonstrate that current models that exclude solution chemistry, or models that assume a constant K reduction for the entire K(θ) function, may be overestimating drainage, recharge, or bare soil evaporation. This research used undisturbed soil cores and simulated irrigation water with decreasing electrical conductivity (EC) from 20 to 0.25 dS m–1 and sodium adsorption ratio (SAR) held constant. Changes in K(θ) were observed only after dropping below an EC of 1.5 dS m–1, which is consistent with past studies. At reduced soil water contents, however, K(θ) decreased by up to five orders of magnitude relative to native soils. A second experiment was conducted that increased the SAR from 4 to ≥20, held EC constant at 0.65 dS m–1, and displayed only moderate reductions in K(θ), although K was reduced proportionally more at reduced soil water contents. Therefore, based on the observed results, an integrated K(EC,θ) and K(SAR,θ) model is needed to better predict salinity effects on soil hydraulic properties and is increasingly important as freshwater resources decline.