Examining the impacts of salt precipitation on soil hydraulic properties at the field lysimeter scale

Previous bench-scale investigations have demonstrated that salt precipitation reduces soil saturated hydraulic conductivity (K_s) due to the clogging effect. However, this conclusion may be confounded by the boundary effects inherent to the physical model. While existing field-scale studies have primarily focused on water-solute migration by directly assuming that salt precipitation reduces K_s, systematic investigations examining how salt crystallization alters soil hydraulic properties remain scarce. This study employed a time-windowed inverse method to analyze one set of data from four lysimeters supplied through the bottom with NaCl solution at concentrations of 3, 30, 100, and 250 g/L under field condition, aiming to examine: (1) whether the salt precipitation impacts the soil hydraulic properties; and (2) whether the degree of this impact depends on the water salinity. Results in each column showed that the inverse-derived K_s unexpectedly increased by more than 50 % at the intermediate time and then decreased to its early-time value. This trend in inverse-derived K_s showed a strong positive correlation with the ambient evaporation rate. Based on measurements of bottom fluxes and ambient evaporation, these opposing trends in inverse-derived K_s are primarily ascribed to the actual K_s change caused by the salt precipitation, rather than variations in salt crust-soil surface hydraulic connectivity (which also affect effective K_s). These findings highlight the need for future experiments to investigate salt precipitation-induced soil pore structure changes under varying evaporation intensities and across multiple scales.