| Soil salinization is the main obstacle restricting the agricultural development of Hetao irrigation area,and soil salt accumulation generated by seasonal freeze-thaw is an important part of soil salinization.Since the implementation of the water-saving renovation project,the annual water intake has been reduced by 20%,coupled with planting structure,land rectification,water fees and other reasons,resulting in a significant reduction in autumn watering area and a corresponding increase in spring irrigation area.The autumn watering area has decreased from 530,000 hm2 at the peak to about 400,000 hm2 in recent years.Spring irrigation farmland is affected by planting factors and farmers’ planting intentions,mainly distributed in the form of flower arrangements,and the area varies greatly.After autumn irrigation to spring irrigation of the following year,the soil undergoes a freeze-thaw period,which is a very complex process,which is accompanied by physical,physical,chemical,mechanical phenomena and sub-processes,mainly including water,heat transfer,water phase transition and salt accumulation.This raises new questions about the distribution of water and salts,which require further analysis.In this study,field monitoring was carried out on the freeze-thaw period of adjacent farmland under local autumn watering conditions under the background of Yichang irrigation area,which has the largest decrease in autumn irrigation area in recent years.Through the combination of field sampling and indoor experimental analysis,the temperature variation characteristics and the temporal and spatial changes of soil cross-section water and salt in local autumn irrigation between autumn and non-autumn irrigation(spring irrigation the following year)were studied,and the water and salt migration rules of freeze-thaw irrigation and non-irrigated farmland were further clarified,and the suitable spring irrigation system of the following spring irrigated farmland was analyzed,which provided a theoretical basis for the implementation of autumn and spring irrigation in the irrigation area.And HYDRUS was used to simulate soil water and salt transport under different spring irrigation quotas in spring irrigation farmland.The main findings are as follows:(1)The average temperature of each soil layer in autumn and spring irrigation farmland decreased to varying degrees one week after local autumn watering compared with the week before autumn watering,and the decrease rate of autumn watered farmland was 0.38 °C-1 °C;The decrease in spring irrigation farmland ranged from 1.28 °C to 2.38 °C.During the freezing process,it took 50 days for the autumn watered farmland to fall below 0 °C from 0 °C to 60 cm soil layer from the surface,while the spring irrigation farmland lasted 82 days.In the ablation stage,it took 55 days for the autumn irrigated farmland to rise from 0°C to 60 cm and 0°C for the soil layer to 0°C,while the spring irrigated farmland lasted 41 days.(2)Before local autumn irrigation,the groundwater level rise rate from October 21 to October 24 was: autumn-irrigated farmland > autumn-spring irrigated farmland junction> spring-irrigated farmland;From October 25 to October 27,the groundwater table rose at the following rates: spring irrigated farmland> autumn irrigated farmland junction> autumn irrigated farmland.From October 28 to October 29,the groundwater level rose at the following rates: autumn-irrigated farmland> autumn-spring irrigated farmland junction>spring-irrigated farmland.During the freezing period I.,the water level of spring irrigated farmland was higher than that of autumn-irrigated farmland during the three periods of December 2-December 4,December 15-December17 and December 21-December 23.(3)After partial autumn irrigation,after undergoing the freeze-thaw process,before spring irrigation in the following year,the salt content of the soil layer of the spring irrigated farmland was 1.60 times that of the autumn-irrigated farmland.Compared with before autumn watering,the overall desalination of 1 m soil was 58.35%.17.29% desalination of 0-60 cm soil layer in spring irrigated farmland;80-100 cm soil layer salt 41.59%.For spring-irrigated farmland,after the freeze-thaw process,only the upper layer of salt is "transported" to the lower layer,so that the overall salt content of the soil layer increases.Compared with the previous autumn watering farmland,the water content of autumn irrigation farmland increased by 2.96 times that of spring irrigated farmland,while the water content of 0-100 cm soil layer in spring irrigated farmland increased by 8.62% on average,which was mainly affected by the seepage of irrigation water.(4)After spring irrigation of autumn-watered farmland in spring irrigation in 2021 and 2022,the salt content of 0-10 cm and 10-20 cm soil layers increased to varying degrees.Among them,the increase of 0-10 cm soil layer was significantly greater than that of 20-10 cm soil layer.In 2022,the salt content of 0-10 cm and 10-20 cm soil layer increased by 1.20 g/k and 0.51g/kg,respectively.In 2021,the salinity content of 0-10 cm and 10-20 cm soil increased by 1.16 g/kg and 0.3 g/kg,respectively.After spring irrigation in2021 and 2022,the moisture content of 0-40 cm soil layer in autumn-watered farmland decreased to varying degrees compared with before spring irrigation,and the 0-10 cm soil layer decreased by 13.31% and 10-40 cm soil layer decreased by only 5.69% in 2021.In 2022,it will decrease by 18.34% and6.32% respectively.(5)Through the simulation analysis of the moisture and salt of the farmland on the west side of the spring irrigation farmland,it can be obtained that since the west side farmland is used as the adjacent farmland of the local autumn irrigation farmland after autumn watering,the east side of the spring irrigation farmland belongs to the adjacent autumn irrigation farmland,considering that the local sunflower planting growth period is generally not irrigated,and in order to achieve the purpose of water saving and salt control,it is considered that 110 mm-130 mm in this simulation is suitable for the irrigation system on the west side of the spring irrigation farmland in this experimental scenario;170 mm-190 mm is suitable for the irrigation system on the east side of the spring irrigation farmland in this experimental scenario. |
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