Study On Soil Water Deep Percolation Dynamic Characteristics And Its Relationship With Rainfall Of Mu Us Sand Land

Water is an extremely important ecological limiting factor for vegetation construction in sandy areas.Soil water deep percolation plays an important role in groundwater resources and vegetation resistance to seasonal or annual drought in sandy areas.At present,due to the limitation of research means,there are still some deficiencies in quantitative study of soil water deep percolation and its dynamic characteristics in sandy areas.In this study,the deep percolation process at 50,100 and 200 cm layers in the mobile sandy land of Mu Us Sandy Land was monitored continuously and in real time by the soil water percolation recorder,and its dynamic and distribution characteristics were analyzed.The soil water situation in the process of percolation was studied,too.The relationship between soil water content and soil water storage and deep soil water percolation was found out,and the dynamic of soil water percolation at different layers of mobile sand land through model simulation was predicted.Combining with the deep soil water percolation,the supply of rainfall to the deep soil water of sandy land with different vegetation types was analyzed.It provided theoretical basis for water cycle,water resources assessment,rational development and utilization of sand land and ecological environment construction.The main results and conclusions of this study are as follows:(1)Dynamic and distribution characteristics of deep soil water percolation:the order of percolation at the three sand layers was 50 cm>100 cm>200 cm,and the year 2016 was a rainy year,with percolation accounting for 69.96%,67.52%and 50.36%of the rainfall respectively,which was more than twice as much as that in 2017(normal rainy year).Generally speaking,the deeper the percolation process was,the less obvious the response to rainfall was.The percolation in the first half of the year was mainly affected by the previous year's rainfall,while in the second half it was affected by the current year's rainfall.It could be considered that the period from June of the previous year to May of the second year was a period of deep percolation.At the same time,with the deepening of the soil layer,the number of days with less percolation intensity and the cumulative amount of percolation were increasing,while the number of days with larger percolation intensity and the cumulative amount of percolation were decreasing.(2)The relationship between deep soil water percolation and fluctuation of soil water content and soil water storage:the precipitation in the growing season in 2016 was 1.81 times of that in the same period of 2017.The average soil water content was not significantly different from 2017,but its variation range,the influence range of wetting front and the variation range of soil water storage are obviously larger than that of 2017.In 2016,the percolation at the three sand layers was 4.53 times,5.53 times and 5.22 times of that in 2017,respectively.The distribution of the percolation had a good correspondence with the wetting front of the soil water content and the fluctuation of water storage in the same period.Deep soil water percolation was significantly correlated with soil water content in both upper and lower layers.The minimum and average soil water content increased with the increase of soil water percolation intensity,but there was no obvious regularity in the change of maximum soil water content.At the same time,under the condition that the capillary water holding capacity was not reached,the percolation was produced at all three sand layers.(3)Changes of soil water percolation and soil water storage under typical rainfall conditions:after a single day of 51.4 mm rainfall,the maximum percolation rates at the three layers were 16 hours,43 hours and 116 hours respectively,which clearly showed the lag and delay of the response of deep percolation to rainfall.After a single day of 24.9 mm rainfall,the percolation rates at 200 cm did not change significantly,and the water which accounted for16.06%of rainfall percolated into this layer.The change of soil water storage was more closely related to it.The influence of continuous rainfall process on percolation process was superimposed,and the later percolation was the result of accumulation of multiple rainfall processes.With the deepening of soil layer,the effect of single rainfall intensity and duration on deep percolation was less obvious.(4)Soil water percolation process and rainfall recharge difference in sandy land of different vegetation types:during the monitoring period,the proportion of percolation to rainfall at 200 cm of the plot of Artemisia ordosica?Salix cheilophila?and Pinus sylvestris var.mongolica was Artemisia ordosica>Salix cheilophila>Pinus sylvestris var.mongolica.Taking mobile sandy land as blank control,Salix cheilophila consumed the most water and Artemisia ordosica consumed the least in 2016,while Artemisia ordosica consumed the most and Salix cheilophila consumed the least in 2017.After 51.4 mm rainfall in a single day,no percolation occurred in the fixed sandy land of Salix cheilophila and Pinus sylvestris var.mongolica.Except for the fixed sandy land of Pinus sylvestris var.mongolica,continuous rainfall resulted in percolation at 200 cm in other plots.Generally speaking,rainfall could supplement more water storage of 0-200 cm soil and the soil below 200 cm in mobile sand land;it could supplement less water storage of 0-200 cm soil in semi-shrub Artemisia ordosica fixed sand land,but it had a certain supplement below200 cm.it could supplement less water storage of 0-200 cm soil and the soil below 200 cm in shrub Salix cheilophila fixed sand land,but Salix cheilophila could maintain water balance through its own regulation;it could supplement more water storage of 0-200 cm soil but less of the soil below 200 cm in shrub Pinus sylvestris var.mongolica fixed sand land,and Pinus sylvestris var.mongolica could maintain the basic water balance through its own regulation in low-rainfall years.(5)The correlation and fitting of soil water deep percolation with rainfall and soil water:Pearson correlation analysis showed the deeper the soil,the weaker the dependence of soil water deep percolation on rainfall,and the stronger the dependence on soil water content and water storage.Rainfall was the main cause of soil water change,so rainfall was always closely related to the amount of deep percolation.The correlation coefficients between monthly cumulants were the largest,and the correlation coefficient between the water storage on the same day and the percolation on the next day was higher.The fitting effect between rainfall and soil water storage and deep soil percolation was the best by using cubic equation of one variable.The R~2of monthly rainfall fit the monthly percolation at 50 cm,the monthly percolation at 50 cm fit the monthly percolation at 100 cm,and the the monthly percolation at 100 cm fit the monthly percolation at 200 cm were 0.906,0.870 and 0.962,respectively.The R~2of the previous day's water storage fitting to the percolation at the corresponds soil layer were 0.125,0.430 and 0.856,respectively.Only the fitting effect of the previous day's water storage of 100-200 cm and the percolation at 200 cm soil layer was ideal.Through the fitting equation,it could be estimated and predicted by using rainfall and soil water storage to the deep percolation at 50,100 and 200 cm in mobile sand land,which provided a new idea for understanding the deep percolation of sand soil,and had important scientific significance for understanding the water cycle characteristics and water balance mechanism of sand land under rainfall conditions.