Abstract:In order to understand the mechanism of soil moisture, heat and salt migration and explore the optimal strategy of combined winter and spring irrigations in the cotton field of Southern Xinjiang during fallow season, the SHAW model was calibrated and verified by using monitored soil moisture, heat and salt data of 0~80cm soil in the cotton test pits during two fallow seasons (2019—2020 and 2020—2021). Then 64 scenarios with eight winter irrigation treatments (0m/hm2, 600m/hm2, 1200m/hm2, 1800m/hm2, 2400m/hm2, 2700m/hm2, 3000m/hm2 and 3600m/hm2) and eight spring irrigation treatments (0m3/hm2, 300m3/hm2, 600m3/hm2, 900m3/hm2, 1200m3/hm2, 1800m3/hm2, 2100m3/hm2 and 2400m3/hm2) were simulated by SHAW to investigate the soil moisture, heat and salt transport behavior in fallow season under different winter and spring irrigation combinations. The results showed that SHAW model could reliably simulate the water, heat and salt transport in freezing-thawing soil in Southern Xinjiang during fallow period. The simulation accuracy of soil temperature was the highest, R2 of different soil depths was not less than 0.95, while poor for soil water content and salt content with the minimum R2 of 0.61 and 0.73, respectively. The unfrozen water content of different winter irrigation treatments was decreased significantly in the freezing period, and the unfrozen water content was decreased with the increase of winter irrigation quota. The greater the amount of winter irrigation was, the smaller the range of surface soil temperature in freezing period was. However, the larger the amount of winter irrigation was, the less fluctuation the soil surface temperature in frozen period and the slower the temperature increase in spring period were. However, the soil temperature tended to be similar after spring irrigation. The salt content of soil surface was decreased significantly after irrigation, but increased significantly during soil freezing period, and the increase of salt content in deep soil lagged behind that in surface soil. The frozen water in the soil melted in spring period which could leach the salt in topsoil, thus with supplement of a small amount of spring irrigation the surface soil salinity could decrease dramatically and satisfy the requirement of cotton germination. Considering the current situation of water resources in Southern Xinjiang and the influence of various combinations of winter and spring irrigations on soil moisture, heat and salt before sowing, the recommended watersaving irrigation mode was winter irrigation plus a small amount of spring irrigation to improve soil moisture, and the appropriate irrigation quota was winter irrigation of 2250m3/hm2 and spring irrigation of 300m3/hm2. The proposed strategy of combined winter and spring irrigations for salt leaching could provide theoretical basis for sustainable agricultural water utilization and soil secondary salinization control in Southern Xinjiang.
