Optimal groundwater depth thresholds for sunflower in salt-affected farmland: A process-based modeling approach across hydrological years in the Hetao Irrigation District

In arid and semi-arid regions with shallow groundwater, soil salinization, water scarcity, and deterioration water quality are major constraints to regional agricultural development. Determining optimal groundwater depth (GWD) is essential for conserving water resources, controlling resalinization, and sustaining crop yields. However, it is not feasible to evaluate the results under various scenarios solely through field experiments. This study addresses this gap by integrating field experiments (2023–2024) with the agro-hydrological & chemical and systems simulator (AHC) to optimize GWD for sunflower cultivation under varying hydrological years and groundwater salinity in the Hetao Irrigation District (HID), China. The calibrated model simulated root zone water-salt dynamics and crop responses across scenarios, with multi-objective optimization (NSGA-II) deriving Pareto-optimal solutions for yield and salinity control. The results indicated that root zone bottom flux decreases with rising GWD, with upward water flux approaching zero at about 3 m depth and upward salt flux approaching zero at 1.5–1.9 m depth, respectively. The final yield of sunflowers was negatively correlated with groundwater mineralization, though this dependence weakens at deeper GWD (>1.8 m). The optimal GWDs for sunflower growth under dry, normal, and wet hydrological years with pre-sowing spring irrigation and rainfed conditions during the growth period were approximately 1.18–1.28 m, 1.23–1.32 m, and 1.37–1.45 m, respectively. These results demonstrate GWD’s pivotal role in regulating farmland water-salt distribution, with derived thresholds enabling sunflower production while reducing root-zone salt accumulation. • The decline of groundwater level is conducive to soil desalination. • A groundwater depth of 1.5–1.9 m: critical range for root zone salt accumulation and leaching. • The amount of soil salt accumulation in wet years is 4.4–55.2 mg cm -2 lower than that in dry years. • Optimal groundwater depth range is around 1.2 m（dry year）, 1.3 m（normal year）, and 1.4 m（wet year).