| Hetao Irrigation Area is an important food producing area in China.Soil salinization is a key influencing factor for sustainable development of the irrigation area.During the rapid development of agriculture,people thought of heavy irrigation and light drainage.The secondary salinization of soil poses a greater threat to farmland.Therefore,the application and development of farmland drainage technology is very important.subsurface drainage technology,as the most direct and effective measure to improve salinized soil,plays an important role in crop growth and agricultural income.To understand the soil water and salt transport law and soil improvement effect in the process of subsurface drainage is of great significance to the prevention and management of saline soil in the irrigation area.In view of the serious soil salinization in irrigation area,and field positioning monitoring and plot experiments were carried out.Study the spatial and temporal variation of soil water,fertilizer,and salt in the experimental area,and use principal component analysis to evaluate the dominant factors affecting soil salinization.Subsurface drainage technology is used to improve and control salinized soil,and to study the soil water and salt transport law and the effect of soil salinization improvement.DRAINMOD model was used to simulate the drainage process of underground and open trench drainage methods and crop yield.The main findings are as follows:(1)The coefficient of variation of the moisture content of the topsoil(0-20,20-40cm),except for 0-20cm(9.779%)in June,is between 12.384%and 19.667%,which is medium variability.Deep soil(40-100cm)has the smallest coefficient of variation of water content,ranging from 3.513%to 9.757%,which is weak variability.The coefficient of variation of surface soil salinity is between 100.845%?129.279%,which belongs to strong variability.The coefficient of variation of deep soils ranged from 83.685%to 98.853%,which was medium variability.The coefficients of variation of soil nutrients were between 32.954 and 69.869,all of which belonged to moderate variability.With the increase of soil layer,the variability of soil water,fertilizer and salt weakened.The semivariograms of the spatial variability of soil water,fertilizer,and salt in each layer of the study area are well-fitted,and the spatial correlations are between 0.038%and 20.408%,all less than 25%,indicating a strong spatial correlation.It can be considered that it is mainly affected by structural factors,random factors account for a small total variation,and the spatial variability caused by autocorrelation is strong.(2)The principal component analysis was used to analyze soil salinization.The analysis results showed that the cumulative variance contribution rate of the first two principal components was 86.44%,and Na+,Cl-,TS,Ca2+,and Mg2+were closely related to the first principal component.These five indicators are closely related to soil salinization and represent the status of soil salinization in a practical sense;Among the second principal component factor variables,K+and HCO3-have higher loads than other variables,and K+ and HCO3-affect soil salinity to a certain extent by affecting soil alkalinity.Therefore,the degree of soil salinity is reduced,and appropriate methods are adopted to reduce Na+,Cl-,Mg2+and Ca2+in the soil.The focus is on reducing Na+and Cl-content.(3)Using subsurface drainage technology to improve salinized soil,the quantitative analysis results of the transformation process of soil "high-salt heterogeneity-low-salt homogeneity" showed:After a leaching cycle,the effects of salt drainage in spring irrigation and autumn irrigation are obvious,and the salt removal amounts are 743.59 and 904.65t.The growth period is a state of salt accumulation,which is 1527.84 t.It is in a state of desalination throughout the year,with a total desalinization of 120.39 t.The average amount of desalination is 408.1kg/hm2.The area of severely salinized soil and saline soil decreased,the area of severely salinized soil decreased to 30.62%,and the area of saline soil decreased to 2.39%.The light and moderate salinized soil increased by 14.83%and 52.15%.The heavy salinized soil contributed the most to the soil desalination rate in the experimental area,which was 91.57%.As the salinity decreases after irrigation,The area of lightly salinized soil and moderately salinized soil increased,and the contribution ratios were-3.24%and-31.48%.The desalination rate of each salt isolate in the soil is expressed as Cl->Na+>SO42->Mg2+>Ca2+>HCO3->K+.The Cl-and Na+content decreased from 7.43 and 4.66 g/kg to 3.68 and 2.58 g/kg,decreased by 50.67%and 44.68%,K+decreased the least,only by 7.14%.The content of Cl-and Na+ions in the experiment was large,and the soil salt isolates developed in a uniform direction after leaching.After irrigation and leaching in the experimental area,the soil was mainly moderately salinized,and the soil salinity was still serious.Cl-and Na+are easily soluble in water and have a weak affinity with the soil.In subsequent experiments,the irrigation cycle and frequency should be increased to rinse the salt and reduce the degree of soil salinization.(4)DRAINMOD was used to simulate the changes of groundwater depth and drainage volume in different drainage methods.The simulation accuracy is high.The relative error RE is 6.02 to 11.15,the correlation coefficient R is as high as 0.89 to 0.99,and the efficiency coefficient NS is 0.87 to 0.96.The simulation of subsurface drainage is slightly better than open drainage.The groundwater fluctuation in the field is mainly during the irrigation and precipitation periods,and the groundwater depth is between 1?1.8m.The model simulation slightly overestimates the depth of groundwater,and the relative error is large,which affects the overall correlation.Therefore,the DRAINMOD model can be used as a simulation tool for field hydrological processes in the study area.(5)The 0-40cm soil layer is the main root distribution layer of the crop.The soil moisture content after spring irrigation under pipe treatment and open ditch treatment are 22.08%and 23.31%.After spring irrigation,both can meet the water required for normal growth and development of crops.Because the planted crop is oil sunflower,it is only irrigated once during the growing period.After irrigation during the growing period,the water content in the open trench treatment is 22.77%,and the water content in the pipe treatment is 21.36%.Although the water content of the pipe treatment is lower than that of the open trench treatment,it does not affect the provision of effective moisture for the later stages of crop growth,and does not affect the final yield.The soil desalination rate of the subsurface drainage treatment was significantly better than that of the open trench drainage(P<0.05).The spring irrigation and autumn irrigation water volume is relatively large,and the desalination rate is relatively high.The 0-100cm underground pipe drainage and open ditch drainage spring irrigation desalination ratios are 57.96%and 36.57%,respectively.Irrigation during the growth period leached a portion of the salt to the 60-100 cm soil layer,and the soil desalination rates of the 60-100 cm soil layer in the dark pipe drainage and open trench drainage were-2.6%and-3.09%,respectively.The closer the horizontal distance to the subsurface pipe(open ditch),the higher the soil desalination rate,and the soil desalination rate is the smallest,Large difference in subsurface drainage.The soil desalination rates of spring irrigation,growth period and autumn irrigation dark pipe drainage at the horizontal pipe distance of 0m are as high as 61.94%,4.47%and 67.77%,The soil desalination rates at the middle point of the pipe were 57.96%,3.54%,and 63.54%,respectively.The soil desalination rates of open ditch drainage at the horizontal distance of 0.4m were ashigh as 40.01%,4.27%,and 39.91%,and the soil desalination rates at the middle point of open ditch were 33.8%,2.55%,and 36.22%. |
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