Deep Percolation Risk and Optimal Irrigation Threshold for Sunflower Under Mulched Drip Irrigation: A HYDRUS-2D Modeling Study

Deep percolation induced by excessive irrigation represents a critical bottleneck limiting water use efficiency in sunflower fields under mulched drip irrigation in arid regions. However, systematic understanding of stage-specific allocation of water losses remains limited. This study conducted a field experiment on oilseed sunflower with three irrigation levels (300, 450, and 600 mm) in Shihezi, Xinjiang. The two-dimensional finite-element software, version 2.05 (HYDRUS-2D model) was calibrated and validated to simulate water balance components in the 0–60 cm soil profile across growth stages. The model demonstrated satisfactory performance, with coefficient of determination (R2) ≥ 0.70, Root Mean Square Error (RMSE) of 0.025–0.039 cm3 cm−3 and Nash–Sutcliffe Efficiency (NSE) of 0.574–0.822. Results showed that evaporation responded weakly to irrigation amount (increasing by only 1.3 mm from 300 to 600 mm), while deep percolation increased nonlinearly, with the high-irrigation treatment exhibiting a percolation rate approximately five times that of the low-irrigation treatment. Crop water consumption exhibited pronounced stage-specific differences: during the seedling stage, evapotranspiration (92.29 mm) was derived almost entirely from soil evaporation; and from budding to filling (S2–S5), cumulative root water uptake accounted for 75–78% of seasonal total, while percolation losses during this period represented 91.5–96.3% of total deep percolation. An irrigation amount of 450 mm optimized the trade-off between grain yield and water use efficiency. A “small and frequent” irrigation strategy during S2–S5 is recommended to minimize deep percolation and enhance irrigation water use efficiency.