Response Of Soil Moisture To Precipitation In Desert Steppe

Desert steppe is the driest grassland type and the first ecological barrier to prevent desert expansion.Precipitation is the main source of water for desert steppe vegetation.Climate and soil determine the landscape type of desert steppe.The system consisting of precipitation,surface cover and soil moisture is the most critical component of desert steppe ecosystem.Studying the response of soil moisture to precipitation has important theoretical value for regulating surface cover,scientifically recovering vegetation types,and promoting the effective conversion of precipitation to soil moisture.Taking three land cover types of desert steppe in Yanchi county of Ningxia--sand,native grassland and caragana woodland as research objects,precipitation and soil moisture data were obtained through years of observation by automatic weather station and soil moisture meter.We have also carried out a survey of land cover structure including vegetation structure,soil profile and physical properties.On this basis,using the orderly cluster method,Morlet wavelet analysis,wavelet and wavelet coherence analysis and correlation analysis,studied the interannual precipitation variability and distribution pattern,discussed the different land cover types soil moisture dynamic of space and time,influence factor,and the relationship of the precipitation in the time-frequency domain,analysis of three types of land cover in a different order of magnitude and the rule of the response of precipitation in different seasons.The main results are as follows:(1)Characteristics of precipitation.The average annual precipitation in Yanchi County fluctuated greatly from 1954 to 2017,and there was no obvious trend change on the whole.The annual distribution of precipitation has certain concentration or abnormal concentration,and the concentration tends to weaken.The precipitation in spring,summer,autumn and winter accounts for 16.42%,57.98%,24.24%and 1.37%of the total annual precipitation respectively.Precipitation events less than<5 mm account for more than 57%of the annual precipitation events,but their precipitation accounts for only 15.74%of the total precipitation.Precipitation events?10 mm occur only 6?10 times a year,but their precipitation accounts for more than 50%of the total annual precipitation.The period shocks around 38a,56a and 6a are the first,second and third major period with annual precipitation changes respectively.(2)Variation characteristics of soil moisture.The spatial characteristics,temporal characteristics and types of soil moisture changes of the three land cover types of sand,native grassland and caragana woodland are both consistent and different.The vertical changes of soil moisture in the three land cover types are divided into soil moisture weak use layer(0-20 cm),soil moisture use layer(20-150cm in sand,20-200 cm in native grassland and caragana woodland)and soil moisture regulation layer(150-250cm in sand,200-250 cm in native grassland and caragana woodland).The soil moisture in sand is basically increasing from top to bottom on the profile.The soil moisture in native grassland and caragana woodland is basically increasing,decreasing and increasing.The soil profile is stable from top to bottom.The seasonal variation of soil moisture is divided into stable period,accumulation period,regression period and recovery period.In the shallower soil layer,the monthly average soil moisture changes obviously in a small period of about 9 months,and gradually disappears with the deepening of the soil layer,and the larger period oscillation in the deep soil layer of 23?36 months is obvious.The soil moisture of the three land cover types has a very significant or significant negative correlation with bulk density,a very significant or significant positive correlation with total porosity,a positive correlation with capillary porosity and saturated water content,and a significant negative correlation between soil moisture and coarse sand content in native grassland and caragana woodland.(3)Response of soil moisture to precipitation.Different grades of single precipitation have significant effects on soil moisture.For the same cover type,the soil moisture in the 0-10 cm soil layer responds rapidly to precipitation and fluctuates greatly,and grade ?-? precipitation can cause soil moisture changes in this layer.For different cover types,the response of sand to all precipitation is the fastest and most intense,and the soil layer affected by precipitation is the deepest.caragana woodland responded to precipitation faster than native grassland,but the depth of soil layer was the same.Three land cover types have different responses to precipitation in different seasons.Take a single rainfall of 11?14 mm as an example,in spring,the response depth of soil moisture to precipitation is the shallowest,all within 10 cm soil layer.In summer,soil moisture increases the most after precipitation.In autumn,the depth of soil moisture response is the deepest,and it can be well maintained after soil moisture increases.Soil moisture and precipitation of different land cover types have two relatively significant resonant periods,which are resonant period of 5-8 months and 9-16 months respectively.Soil moisture of 0-20 cm soil layer in sand,0-10 cm soil layer in native grassland and caragana woodland changes at the same time or slightly lags behind(7 days)precipitation.The deeper the soil layer,the slower the response of soil water content to precipitation,and the more complicated the resonance relationship between soil water content and precipitation in deep soil layer.Conclusion:The precipitation in Yanchi County has typical pulsation and dispersion characteristics of precipitation in arid and semi-arid regions.The precipitation is a comprehensive result of large precipitation events in the year,large interannual variation and multi-cycle fluctuation.The soil moisture changes of the three land cover types of sand,native grassland and caragana woodland show the characteristics of sand being the most sensitive to precipitation,caragana woodland being the second and native grassland being the slowest.The soil moisture of the three land cover types has significant response to summer precipitation and the deepest response level to autumn precipitation.Precipitation is the main source of soil moisture for the three land cover types.Heavy rainfall events dominate soil moisture,and there is a resonant relationship between the two for 5?8 months and 9?16 months.