Transport of heat, water, and salts in freeze-thaw soils under flood irrigation: Experiment and simulation☆

The seasonal freeze-thaw irrigation district faces severe soil and secondary salinization, which is closely related to the water, heat, and salt transport in freezing soils under flood irrigation/irrigation-induced ice cover. A flood irrigation experiment was conducted in lysimeters at the Yonglian Station, Hetao Irrigation District, in December 2020 (late irrigation). The three treatment lysimeters were irrigated with 270, 135, and 35 mm of water (TL270, TL135, and TL35), and no irrigation was performed in the control lysimeter (CL). An in-situ observation of soil water, heat, and salt in the lysimeters was then carried out until May 2021. The applicability of the SHAW model was also investigated by scenario analysis in which the irrigation was regarded as rainfall, snowfall, and no irrigation, respectively. The results showed that compared with the initial water storage before irrigation, the final water storage in the 0-40 cm soil layer decreased by 40.0 %, 29.2 %, 21.7 %, and 19.6 % in TL35, TL135, CL, and TL270, respectively. Although soil surface evaporation increased in lysimeters with irrigation, the rapid freezing of infiltrating water cooled by melting the frozen layer significantly reduced percolation losses during the early freezing period, resulting in better water conservation in TL135 and TL270. Compared with the initial salt storage, the final salt storage in the 0-40 cm soil layer decreased by 64.4 % in TL270, and increased by 84.6 %, 44.1 %, and 17.8 % in CL, TL135, and TL35, respectively. The irrigation of over 135 mm was sufficient to melt the frozen soil layer, allowing irrigation water to be retained in the deeper soil layer. As the soil continues to freeze, the prolonged freezing and significant matric potential gradient between the upper freezing front and the lower moist soil drive soil water and salt to move upward. However, it was insufficient to leach down the soil salt, eventually accumulating salt during the early freezing period. Although the water efficiency for saving water (WUEwc of 52.4 %) and salinization prevention (WUEsp of 54.3 %) at 35 mm is not the best, considering the water resource limitations and the crop's salt tolerance, 35 mm is acceptable. This indicates that late irrigation has the potential for further water savings beyond the recommended quota in early irrigation. The SHAW model is less effective in simulating the soil water, heat, and salt transport in the three treatment lysimeters due to the neglection of the detailed subprocesses such as energy equilibrium of ponded water and frozen soil, ice cover forming, soil deformation, etc. Future studies on integrating the subprocesses during autumn irrigation in freezing soils with the SHAW model and developing a proper autumn irrigation regime based on it are recommended.