Lagged surface soil salinization responses to groundwater dynamics in the Yinchuan Plain: Stage dependence and spatial differentiation postprint

Abstract: Shallow groundwater may influence surface soil salinization in arid irrigated plains, but the timing and spatial form of this association remain uncertain. Consequently, this study aimed to identify the monthly timing, seasonal asymmetry, and spatial differentiation of surface soil salinization responses associated with shallow groundwater dynamics. We combined monthly groundwater depth records from 116 monitoring wells (January 2021–August 2024) in the Yinchuan Plain, northwestern China, with multiple other datasets, including groundwater hydrochemical data, surface soil salinity samples, multi-source remote sensing images, model interpretability diagnostics, response-time sensitivity tests, and representative profile observations. A hierarchical classifier was trained using surface soil salinity samples collected in August 2022 and tested externally using independent samples collected in August 2025. The resulting monthly salinization class sequence was used as a model-generated regional indicator. Shallow, relatively saline groundwater was concentrated in the lacustrine-alluvial plain (shallow), whereas severe salinization was mainly found in the localized shallow groundwater areas. A regional class area analysis showed a decrease in the severe salinization area proportion accompanied by an increase in the moderate salinization area proportion within 1.00–3.00 months after the onset of summer irrigation-related groundwater shallowing. Approximately 4.00–5.00 months after the onset of winter irrigation-related groundwater shallowing, a delayed winter–spring resalinization signal was detected. Sensitivity tests showed that the separation between earlier summer responses and later winter responses persisted across the tested cycle subsets and operational settings. The water-salt coupling zone accounted for 56.97% of the classified area, and three response-based groundwater control forms were identified: threshold-like depth, water-salt sensitive response zone, and groundwater-control instability form. These findings support salinity management that considers both the subregional setting and seasonal stage. The predicted 4.00-to-5.00-month delay in winter–spring resalinization should be understood as an integrated winter–spring resalinization indicator rather than a direct groundwater transit time. More broadly, this response-based paradigm extends regional research on groundwater-related surface soil salinization beyond static depth thresholds by explicitly accounting for temporal lags, seasonal stages, and hydrogeological heterogeneity.