Study On Water And Salt Dynamic Response And Drainage Reuse Of Heavy Saline-Alkali Land Under The Condition Of Dark Pipe Drainage In Yellow River Diversion Irrigation Area

Ordos is located in the southwest of Inner Mongolia,and its saline-alkali land area accounts for 14.01%of the total saline-alkali land area in Inner Mongolia,the problem of soil salinization seriously restricts the agricultural development in this area.Underground pipe drainage technology is widely used in the improvement of saline-alkali land,which plays an important role in the normal growth of crops.In addition,the exploitation and utilization of saline water resources for agricultural production and improvement of saline-alkali land has become one of the important ways under the condition of the shortage of water and fresh water resources.This thesis carried out 2 years in a row of underground pipe drainage monitoring test,To study the spatial and temporal distribution and spatial structure characteristics of soil water and salt under the condition of underground drainage,analyzing the effect of groundwater on soil salinity,using the principle of water and salt balance to evaluate the improvement effect of dark pipe drainage on severely saline-alkali land,Drainmod model was used to simulate the field hydrological and salinity process in this area,and the main factors affecting the drainage,groundwater depth and soil desalination rate of the test area were considered under the condition of changing the buried depth,spacing and diameter of the hidden pipes.In addition,according to the composition of drainage ions,saline water with different salinity was prepared for indoor freezing irrigation experiments to analyze the influence of the irrigation conditions on soil water infiltration characteristics and desalination effect,and to explore the most suitable water quality for freezing irrigation and the feasibility of drainage recycling.The main conclusions are as follows:(1)With the increase of soil depth,the variation coefficient of soil water content decreased,and the variation coefficient of soil water and salt in two years were in the range of 10%-100%,belonging to medium variation.When Gaussian model was used to fit the spatial structure of soil water content and soil salt in two years,the correlation coefficients were high.The soil water content in 0-20cm surface is greatly affected by the spatial structure factors and random factors,while the water and salt in 20-100cm soil is greatly affected by the spatial structure factors.Under the laying of hidden pipes,the soil salt content showed a surface accumulation type,and the salt distribution showed a phenomenon of"higher in the south and lower in the north".(2)Compared with the non-underground area,the underground water depth in the underground area increases by about 35%,and the groundwater mineralization in two years decreases by 18.9%and 40.8%respectively.In non-dark pipe area,the ion content in all soil layers in 2020 will increase by 2%-16%compared with that in 2019.In the dark tube area,the content of various ions in the 20-100cm soil layer in 2020 was 2%-30%lower than that in 2019,indicating that the laying of the light and dark tube could reduce the content of ions in the deep layer.Influenced by the content of HCO₃^-in groundwater,the content of HCO₃^-in deep soil is higher than that in surface soil,and soil salinity has a negative correlation with groundwater depth and a positive correlation with groundwater salinity.(3)When the spring irrigation amount is 5.25×10⁴/m³in 2019,the amount of salt drainage by spring irrigation is 30.31t,and the amount of salt drainage by hidden pipe after spring irrigation is 11.32t.When the spring irrigation amount is 3.97×10⁴/m³in 2020,the amount of salt drainage by spring irrigation is 22.90t,and the amount of salt drainage by hidden pipe after spring irrigation is 4.85t.The critical irrigation and drainage ratio of the experimental area is 18.76,and the irrigation and drainage ratio in two years is greater than the critical irrigation and drainage ratio,and the soil presents the phenomenon of desalination.(4)Under the condition of dark pipe drainage,after two years of Yellow River irrigation,the content of HCO₃^-in the soil increased by 56.0%and 46.2%,respectively,while the content of other ions decreased to SO₄^2->K⁺+Na⁺>Cl^->Mg²⁺>Ca²⁺,SO₄^2-,K⁺+Na⁺and Cl^-in the soil were largely leached.After irrigation,the contents of various ions in groundwater and drainage increased,and the increase rates of salinity in drainage in 2019and 2020 were 3.43 and 8.11 times of that in groundwater,respectively.Therefore,increasing irrigation water can effectively promote the leaching and discharge of SO₄^2-,K⁺+Na⁺and Cl^-in the soil,but at the same time,attention should be paid to the soil alkalization caused by HCO₃^-in Yellow River irrigation.(5)DRAINMOD model is used to simulate the drainage,groundwater depth and soil desalination rate under different buried depth,spacing and pipe diameter.It is found that the influence of buried depth on drainage,groundwater depth and soil desalination rate is more sensitive than that of buried distance.Considering the economic and other conditions,it is considered that the drainage effect,groundwater depth and soil desalination rate are relatively good when the distance between buried pipes is 18m and the buried depth of buried pipes is 1.4m.(6)The saline freezing irrigation method was used to carry out an experimental study on drainage recycling and reuse,and the soil water infiltration and salt leaching effects of saline freezing irrigation and direct saline irrigation were compared.It was found that the soil infiltration rate under salt water freezing irrigation was higher than that under salt water direct irrigation,and the soil moisture content was lower than that under salt water direct irrigation.Under freezing irrigation,the higher the salinity,the faster the soil infiltration rate and the smaller the soil moisture content.Under 150mm irrigation amount,the depth of soil salt reduction was 0-30cm,greater than that under 100mm irrigation amount,the depth of soil salt reduction was 0-20cm.Under the irrigation amount of150mm,the desalination effects of 10g/L saline ice irrigation and 3g/L brine water direct irrigation were the same,and the desalination rates of 0-30cm soil were 80.30%-85.70%and 81.40%-87.33%,respectively.At the end of the experiment,the soil salt content at50-60cm was about 12.5g/kg.When 3g/L brackish water was used for freezing irrigation,the desalination effect of 0-60cm soil was the best.The desalination rate of 0-30cm soil was 87.80%-91.13%,and the desalination rate of 50-60cm soil was 9.1%at the end of the experiment.Therefore,the freezing irrigation with briny water from the dark pipe discharged into the drainage ditch can achieve a faster soil infiltration rate and a better desalination effect on 0-60cm soil,it can realize the recycling of drainage.