| For many years,the unreasonable distribution of water resources in the lower reaches of the Tarim River has led to the deterioration of the surrounding hydrological conditions,the decline of groundwater level has led to the stress of vegetation growth,a large area of decline and death,and serious damage to the ecosystem.Although the lower reaches of Tarim River have been supplied by ecological water supply in recent years,it is still unable to fundamentally solve the problem of vegetation water shortage due to the limitation of linear water supply mode.The single plant and incomplete vegetation outside the response area are still facing severe water shortage.There are many kinds of vegetation in the lower reaches of Tarim River.Populus euphratica,as a representative plant community,plays an important role in restraining soil desertification and maintaining the ecological stability of the river bank.It is an irreplaceable natural resource in the ecosystem of Tarim River.Therefore,in order to solve the problem of vegetation water shortage,this paper studies the method of using column soil replacement technology to improve the ability of groundwater rising,and gradually realize the ecological self recovery of the lower reaches of Tarim River.The research results of fragile ecological environment protection have certain guiding significance.In this paper,the capillary effect of typical desert soil improved by fly ash in the vegetation growing area of the lower reaches of Tarim River was studied through laboratory experiments.The capillary water rising law of desert soil improved by fly ash was studied by vertical tube method,and the influence weight of fly ash content and initial moisture content was analyzed by projection pursuit regression(PPR);the improvement mechanism of desert soil pore structure was revealed by soil moisture characteristic curve,and the number of large and small pores in the improved desert soil was calculated by VG model;the relationship between capillary force and plant transpiration was analyzed The water lifting capacity under the pulling force is compared and analyzed.The main conclusions are as follows(1)Under the conditions of 0,10%,20%,30%,40%,50% fly ash content and 1%,2%,3% initial moisture content,18 groups of desert soil capillary water rise tests were carried out.The results show that:the capillary water rising process conforms to the power function relationship,the capillary water rising height of improved desert soil with 40% fly ash and 3% initial moisture content is the largest,which is215.8cm;the capillary water rising height of pure desert soil with 3% initial moisture content is the smallest,which is only 44.7cm.It shows that fly ash can improve the capillary effect of desert soil significantly,and can effectively improve the capillary water rising height.(2)The results of single factor analysis showed that the average capillary water rising height of improved desert soil was 198.5cm at 40%,which was 4.4 times of that of pure desert soil;when the initial moisture content was 1%,the average capillary water rising height of improved desert soil was 138.5cm;PPR analysis showed that the influence weight of fly ash content was 1,and the initial moisture content was 0.18.It shows that the content of fly ash is the main factor affecting the rise of capillary water.(3)With the increase of fly ash content,the water characteristic curve moves up as a whole,and it is better to use van Genuchten model to fit the water characteristic curve of improved soil;pore structure is an important factor affecting soil capillarity,and the proportion of capillary pores in desert soil increases from0.39% to 7.34% after adding fly ash.The results show that fly ash changes the pore structure of desert soil,enhances the water holding capacity of desert soil,and forms the optimal capillary pore structure in the soil structure when the content of fly ash is 40%.(4)The three stages of capillary water rising are affected by different pore structures.In the early rapid rising stage,the peak value of capillary water rising speed is determined by aeration pores,in the middle slow rising stage,capillary water rising height in the short term is determined by capillary pores,and in the later stable stage,capillary water rising duration and final rising height are determined by chemical adsorption pores in fly ash.(5)The results show that the cumulative increase of capillary water has a power function relationship with time and a linear relationship with height;the critical point of saturated zone and unsaturated zone of improved desert soil with 40% fly ash is 1.2m;under the combined action of capillary force and plant transpiration pull,the water lifting capacity of improved desert soil is stronger,and the smaller the soil replacement height is,the stronger the water lifting capacity is. |
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