Evaluating spatial and temporal variations of soil water, heat, and salt under autumn irrigation in the Hetao Irrigation District based on distributed SHAW model

The Hetao Irrigation District (HID) is a typical seasonal frozen soil region in Inner Mongolia. Soil salinization is directly linked to the freeze-thaw cycle of soil water-heat-salt transport. Autumn irrigation is critical for water storage and salt washing, which can effectively prevent and control soil salination. However, less attention has been given to the spatial and temporal variations of soil moisture, heat, and salt throughout the freeze-thaw phase under autumn irrigation at the HID regional scale. Therefore, the distributed SHAW model was constructed, calibrated, and validated, and then used to simulate soil water-heat-salt transport during the freeze-thaw periods under autumn irrigation in the HID from 2000 to 2017. Subsequently, the suitability of planting spring wheat, spring maize and sunflowers was quantitatively determined based on the soil salt content at the time of spring sowing. The results indicated that the distributed SHAW model was effective in simulating the soil water-heat-salt dynamics in the HID. Within the year of freeze-thaw period, the 0–40 cm soil temperature and water content exhibited a similar “plunge-stabilization-recovery” trend, the 0–40 cm soil salinity during the ablation period increased by 5.68% compared to the initial freeze period. During the freeze-thaw period from 2000 to 2017, soil water content showed an increasing trend, soil temperature variability was stable, and soil salinity was desalinated. The spatial distribution of soil water content during the freeze-thaw period was higher in the east than in the west, ranging from 0.07 to 0.13 cm3/cm3. The soil temperature was relatively obvious, ranging from −0.8 to −2.2 ℃, with high soil temperature mainly concentrated in Dengkou county (DK). The soil salinity varied significantly, with most cultivated land falling into the lightly salted soil category with salinity of 2.1 - 2.3 g/kg, while all soils had less than 1.5 g/kg salinity and were non-salinized soils in Wulateqianqi (WQQ). Under existing autumn irrigation, the area suitable for spring wheat cultivation was mainly concentrated in WQQ, accounting for approximately 28.36%; the areas suitable for planting spring maize were mainly in DK, the northern part of Hangqinhouqi (HH), Wuyuan county (WY), and WQQ, accounting for approximately 76.09%; all the cultivated land was satisfied with sunflower cultivation in the HID. The research results provide valuable references for adjusting and optimizing autumn irrigation and cropping patterns in the HID.