Study on Water-Salt Transport and Irrigation Optimization in the Vadose Zone of Salt-Impacted Regions with Variable Permeability

Purpose Soil salinization critically constrains sustainable agricultural development in arid regions. Xinjiang represents the largest area of saline soil distribution in China. In Yuepuhu County, located in Southern Xinjiang, traditional salt-suppression irrigation methods demonstrate inefficiency while exacerbating secondary salinization. Field investigations have indicated that the vadose zone in the study area contains a layer of colloidal clay with varying depths and thicknesses, exhibiting variable permeability. Methods This study employs a combination of field investigation sampling and testing, physical experiments, and numerical simulations to reveal the laws of water-salt transport in the vadose zone with variable permeability soil layers, elucidate the dual retardation mechanisms of variable permeability layered structure on water-salt transport, analyze the comprehensive salt control effects of collaborative irrigation regimes, and propose the optimal irrigation regime. Results The results indicate that along the groundwater flow direction (NW-SE) in the study area, the vadose zone soil structure progressively shifts from a single-layer structure to double and triple layers. The low-permeability clay layer progressively shallows and thickens, generating a west-to-east spatial pattern of increased permeability variation. The variable permeability layered structure governs the vertical distribution difference of salinity via the dual blocking mechanisms of "water-blocking" and "salt-blocking", inducing a significant asymmetric response of soil salinity dynamics above and below the variable permeability interface to irrigation regulation. Conclusions This study proposes a stratified salt‐control strategy integrating high-frequency, low-volume freshwater drip irrigation with the engineered modification of aquitard, laying the foundation for the precise irrigation regime adapted to variable permeability layered structure in arid regions.