Distribution Characteristics And Numerical Simulation Of Soil Water And Salt Under Brackish Water Of Drip Irrigation Under Film In Cotton Field

Water resources are the key elements of agricultural production and ecological conservation in arid areas.The unique climatic environment and topographic conditions in the arid inland area lead to the scarcity of fresh water resources in this area,but there are abundant underground unconventional water resources（brackish water and salt water resources,etc.）,which have been gradually used as irrigation water sources in agricultural production and ecological environment conservation.It has achieved remarkable results in alleviating the shortage of agricultural irrigation water and ecological maintenance water in the arid area,and plays an important role in the efficient utilization of regional water resources.Aiming at the salt tolerance threshold of cotton and the distribution characteristics of soil water and salt under brackish water and saline water irrigation,and the response of cotton growth to gradient irrigation of water sources with different degrees of mineralization,through the pit test of drip irrigation of cotton under water source film with different degrees of mineralization from 2020 to 2021(Different gradient designs:CK,A,B,C,D and E,1,2,3,4,5 and 6 g·L^-1 in turn),analyze the distribution and migration law of soil water and salt in cotton irrigated with different salinity water sources,reveal the changes of soil p H and eight ion contents under different salinity water source irrigation,clarify the response of cotton growth characteristics to different salinity water source irrigation,and determine the mineralization threshold of cotton irrigated with different salinity water sources;Based on the theory of soil water and solute movement,the HYDRUS numerical model is applied to simulate the dynamics of soil water and salt,obtain and verify the law and data of soil water and salt movement,and obtain the long-term effect of the continuous change law of soil water and salt with time and space,which provides a theoretical basis for the further application of drip irrigation technology under film of unconventional water resources in arid areas of China.The main conclusions are as follows:（1）Under brackish water（salt water）irrigation,the soil moisture content fluctuates up and down with the irrigation date,and the average soil moisture content of each treatment in 0-20 cm soil layer is between12.43%～15.35%;The soil moisture content of treatment D and E in 20-40 cm soil layer was significantly higher than that of other treatments,and the average soil moisture content of each treatment was 14.61%～19.17%.Soil salinity gradually accumulated with the delay of irrigation period.The average soil conductivity of 0-20 cm and 20-40 cm soil layers were 0.73 d S·m^-1～1.92 d S·m^-1 and 1.23 d S·m^-1～2.57 d S·m^-1,respectively.During the two-year experiment,there was little difference in soil moisture,while soil salt further accumulated in the second year.In the second year,the average soil conductivity of 0-100 cm soil layer of CK-E treatment increased by 9.98%,12.72%,11.67%,14.70%,15.65%and 17.30%respectively compared with the first year.（2）The soil pH value of each treatment fluctuated between 8.03～8.48,and the soil was alkaline.There was no significant difference in soil p H value during the two-year test.In the second year,the ion contents further accumulated.The average contents of SO₄^2-and Na⁺in E treatment during cotton growth period were4.95 g·kg^-1 and 2.51 g·kg^-1,which increased by 15.65%and 20.09%respectively compared with E treatment in the first year.At the end of the second year,the soil water-soluble salt content was further accumulated compared with the first year.CK-E treatment accumulated 0.76 g·kg^-1,0.63 g·kg^-1,0.86 g·kg^-1,1.11 g·kg^-1,1.44 g·kg^-1 and 1.46 g·kg^-1 respectively.In the boll opening stage,the main mg equivalent percentages of Na⁺+K⁺and SO₄^2-mg equivalent percentages are 80%～100%and 80%～100%respectively.With the increase of irrigation salinity,the cations Na⁺,Mg²⁺,Ca²⁺and anions SO₄^2-and Cl^-gradually increase.（3）In 2020,the plant height of cotton in treatment D and E was significantly different from that in other treatments（P<0.01）,which were 59.00 cm and 53.33 cm respectively.The stem diameter,main root length,leaf chlorophyll content and leaf area index of cotton in treatment E were 8.46 mm,18.30 cm,42.40 SPAD and 4.25 respectively,which were significantly different from those in other treatments（P<0.05）.The cotton yield reached the peak value of 5379.24 kg·ha^-1 in treatment B.the difference of irrigation water use efficiency（IWUE）between treatments was consistent with that of cotton yield.The IWUE under CK treatment was 10.48,while that under E treatment was only 8.60.There was no significant difference in cotton growth during the two-year experiment.In the irrigation area lacking fresh water,using 1～4 g·L^-1brackish water to irrigate cotton field will not aggravate soil salinization.It can be determined that the water source with 4 g·L^-1 salinity is the cotton irrigation threshold in the study area.（4）MAE≤2.04,MRE≤16.98%and RMSE≤2.26 for simulated and observed soil moisture of 0-100cm soil layer at narrow and wide rows;MAE≤0.39 d S·m^-1,MRE≤17.74%,RMSE≤0.43 d S·m^-1 of simulated and observed soil salinity;The MAE of soil moisture simulation value and observation value at different points before and after irrigation in flowering and boll period is 0.93～2.08,MRE is 9.41%～16.13%,RMSE is 1.26～2.38,the MAE of soil salt simulation value and observation value is 0.16 d S·m^-1～0.20 d S·m^-1,MRE is 7.59%～10.30%,RMSE is 0.17 d S·m^-1～0.23 d S·m^-1.The simulated values of soil moisture and salt are in good agreement with the observation values,It can be used in actual cotton fields.