Translating snowmelt and freeze–thaw legacies into irrigation–drainage zoning of water–salt constraints in seasonally frozen oasis croplands

Study region The Manas River Basin in northwestern China is a snowmelt-fed, seasonally frozen oasis that depends strongly on mountain snowmelt for irrigation. High evaporative demand, shallow groundwater, drainage heterogeneity, and water-saving irrigation create spatially variable salinity, sodicity, and water-status constraints, making it suitable for examining associations between antecedent cold-season hydroclimatic conditions and late-season soil water–salt management. Study focus We asked whether a management-oriented water–salt constraint index (WSCI) can integrate moisture, salinity, alkalinity, sodicity, carbonate status, and texture to identify dominant constraints; whether snow, snowmelt, and freeze–thaw covariates add predictive information beyond surface, terrain, groundwater, and climate conditions; and whether the resulting maps support irrigation–drainage zoning. We constructed weighted-average (WSCI_WA) and dominant-constraint-sensitive (WSCI_DE) formulations from 1250 topsoil samples and predicted them using multisource covariates under 10-fold spatial block cross-validation. Extreme gradient boosting (XGBoost) best predicted WSCI_DE: coefficient of determination (R²) = 0.761, root mean square error (RMSE) = 0.108, mean absolute error (MAE) = 0.086, and residual prediction deviation (RPD) = 2.05. New hydrological insights for the region Removing snow, snowmelt, and freeze–thaw predictors reduced R² to 0.707 and RPD to 1.85, indicating non-redundant statistical information for 2024 late-season topsoil WSCI prediction. Moderate, high, and very high relative constraints covered 38.7%, 19.6%, and 3.2% of cropland, respectively. Salinity was dominant, while sodicity, carbonate alkalinity, water-status imbalance, and texture created distinct management needs. The framework translates cold-season signals into relative priorities for leaching, drainage, sodicity mitigation, irrigation adjustment, and compound remediation.