Study On The Distribution Of Soil Water,salt,and Heat In Drip Irrigation Cotton Fields Under Freeze-thaw Conditions And Optimization Of Winter Irrigation System

Xinjiang is the largest cotton production base in China.With the intensification of the contradiction between water supply and demand and the continuous expansion of soil secondary salinization,it has seriously affected the sustainable development of the cotton industry.In recent years,the "drip irrigation during the growth period/drip irrigation under film and winter irrigation during the leisure period" model has been adopted to promote watersaving practices and suppress soil salinity in cotton production.However,improper winter irrigation can result in water resource wastage,salt accumulation in the soil,and subsequent adverse effects on the emergence of cotton seedlings in the following year.The crucial factor contributing to this problem is often the neglect of the freeze-thaw effect on the soil.Understanding the impact of soil freezing on water and salt transport,as well as the distribution of soil water and salt during the freeze-thaw period in cotton fields before and after winter irrigation,is of significant importance for efficient water resource utilization and effective control of soil salinization.It also plays a crucial role in the development of a well-designed and sustainable winter irrigation system.This article presents a two-year study conducted at the Key Irrigation Experimental Station in Korla,Xinjiang.The research involved indoor soil column tests and field freeze-thaw tests to examine water and salt transport in frozen soil under various conditions,including initial water content,initial salt content,soil bulk density,irrigation temperature,and mineralization of infiltration water.The study also analyzed the water and salt content in freeze-thaw soil before and after winter irrigation,considering different soil textures,groundwater depth,and winter irrigation water volume.Furthermore,the distribution pattern of water and salt was analyzed.The study utilized the Water Heat Transport Model(SHAW)to simulate the distribution of water,heat,and salt under different irrigation regimes.The main research findings are as follows:(1)The study conducted indoor soil column freezing tests to measure the cumulative infiltration amount,infiltration rate,soil water content,and salt content in both unfrozen and frozen soil at a depth of 30 cm.The effects of various factors such as initial water content,initial salt content,soil bulk density,irrigation temperature,and mineralization of infiltration water on the soil infiltration characteristics of unfrozen and frozen soil were analyzed using the Philip infiltration model.The experimental results indicate that the infiltration patterns of unfrozen and frozen soil remain consistent under different conditions of initial water content,initial salt content,soil bulk density,irrigation temperature,and infiltration water salinity.The accumulated infiltration amount shows a rapid increase in the initial stage of infiltration and gradually reaches a stable level with the passage of time.However,the infiltration rate of frozen soil is significantly lower than that of unfrozen soil,suggesting that frozen soil inhibits the infiltration process.(2)As the initial water content,initial salt content,soil bulk density,and mineralization degree of infiltration water increase,the cumulative infiltration amount and infiltration rate decrease within the same infiltration time.Conversely,with the increase of irrigation temperature,the cumulative infiltration amount and infiltration rate increase within the same infiltration time.The initial water content,initial salt content,soil bulk density,irrigation temperature,and the degree of mineralization of infiltration water play a significant role in the downward movement of the soil freezing front and greatly influence the distribution of water and salt in unfrozen and frozen soil at a depth of 0-30 cm.(3)Through field freeze-thaw experiments,the study examined the influence of soil texture,groundwater depth,and winter irrigation amount on the transport of water,salt,and heat in freeze-thaw soil.It was observed that compared to silty loam soil,loam soil exhibited higher storage capacity in the 0-40 cm and 0-100 cm soil layers after the freeze-thaw period following winter irrigation of 2250 m3/ha.Furthermore,different groundwater depths and winter irrigation volumes had a significant impact on soil water storage,desalination rate,and salt return rate before sowing in the following year.Deeper groundwater levels hindered the accumulation of soil water and salt in the topsoil layer,while higher winter irrigation quotas effectively leached out soil salt.(4)The SHAW model proves to be effective in simulating the patterns of soil water,heat,and salt transport during the freeze-thaw period in the Xinjiang region.Specifically,the model demonstrates high accuracy in simulating soil temperature,with R2 values ranging from 0.898 to 0.997 across different soil depths.However,the simulation accuracy of soil moisture and salinity is slightly lower due to the influence of phase change in soil moisture,with R2 values of 0.769 and 0.726 respectively,representing the minimum values.Under different winter irrigation systems,it is observed that a larger winter irrigation quota leads to a higher unfrozen water content in the 60 cm and 100 cm soil layers during the freezing period.Winter irrigation carried out in late November results in the highest soil moisture content during the freezing period across different soil layers.The impact of different winter irrigation systems on soil temperature is relatively small.Additionally,a larger winter irrigation quota contributes to a lower soil salt content.When winter irrigation is conducted on late November,the soil experiences the highest relative desalination rate,indicating a significant leaching effect on salt.(5)Considering the current water resource situation in Xinjiang and the effects of different winter irrigation systems on soil water,heat,and salt prior to cotton planting,it is recommended to implement the following winter irrigation system:a winter irrigation quota of 2600 m3/ha,winter irrigation scheduled for late November,an irrigation duration of 2-3 days,and a frequency of winter irrigation once.