Spatial Optimization and Allocation of the Dry Drainage System in the Hetao Irrigation District Based on SahysMod

In the Hetao Irrigation District of China, land consolidation to expand cultivated areas has disrupted the regional water–salt balance, increasing soil salinization risks. This study investigates the spatial optimization of cultivated land and salt-accumulating wasteland, using the SahysMod model to simulate soil water–salt dynamics and develop multi-scenario plans. The objective is to identify optimal strategies for regulating the dry drainage system and controlling salt accumulation by optimizing three key parameters: cultivated land-to-wasteland area ratio, elevation difference between cultivated land and wasteland, and spatial layout schemes. The results show that the SahysMod model accurately simulates soil water–salt interactions. Under the current scenario, the root zone ECe of cultivated land is projected to reach 6.16 dS·m−1 by 2030, surpassing the salt tolerance threshold for sunflowers and threatening crop yield. The optimized scenario, which reduces the cultivated land-to-wasteland ratio from 14.41 to 12.97, increases wasteland area to 22.01 hm2 and raises the elevation difference from 20 cm to 40 cm, significantly improving salt accumulation efficiency. By 2030, the ECe in the root zone decreases to 5.37 dS·m−1, bringing soil conditions within the tolerance range for major crops in the region. Between 2021 and 2025, salt accumulation in cultivated land decreases dramatically from 19.08% to 5.60% under the optimized scenario, demonstrating effective early-stage salt control. However, from 2026 to 2030, the annual salt accumulation rate stabilizes at 24.88% (optimized) versus 25.20% (current), with a difference of only 0.32%. This finding reveals that while spatial optimization effectively mitigates short-term salt buildup, it has limited efficacy in preventing long-term salt accumulation. Spatial simulations suggest that a northern concentrated and southern patchwork wasteland layout enhances salt-accumulating capacity. These results demonstrate that spatial optimization of cultivated land and wasteland configuration alone is insufficient to fundamentally resolve soil salinization. Therefore, comprehensive measures, including drainage system improvements, soil amendments, and refined irrigation management, are necessary for sustainable salt management in arid irrigation regions.