| Soil moisture is an important factor affecting vegetation growth and drought in desert steppe areas,and it plays a decisive role in the grassland ecosystem.The study of the time-series changes of soil moisture and its response to precipitation and other influencing factors is great significant for revealing the laws of the timeseries changes of soil moisture,the land-atmosphere coupling in the desert steppe and the prediction of soil moisture model.In this study,the southern edge of the desert steppe in Inner Mongolia was used as the research area.A ground observation system was set up on the bottom and on the slope of a typical slope to observe soil moisture and meteorological factors for a long time.Using mathematical statistical analysis,Pearson correlation analysis,wavelet analysis,cross wavelet and wavelet coherence analysis,autocorrelation analysis,crosscorrelation analysis and so on,the characteristics of intra-annual and inter-annual changes of soil moisture in different slope positions were clarified,and the characteristics of soil moisture were explored.The main results are as follows:(1)The seasonal variation of soil moisture is obvious,and the soil moisture at the bottom of the slope is generally greater than its on the slope.Soil moisture and its volatility decrease with the increase of soil depth,and soil moisture has obvious seasonal differences.The amplitude of soil moisture changes in spring,summer and autumn more,and its in winter is small.The seasonal changes of soil moisture on the slope and bottom basically follow the pattern of "low value in winter-rising in early spring-stable high value in spring-falling in late spring-fluctuating in summer – fluctuating rising in early autumn-falling rapidly in late autumn".Compared with the uphill slope,the changes at the bottom of the slope are relatively lagging behind.Meanwhile,at the bottom of the slope,the soil moisture does not change significantly in summer and autumn with a high value and fluctuates.Temperature(atmospheric temperature and soil temperature)has different effects on soil moisture at different stages.During the low-value stable period in winter,the rising period in early spring,and the falling period in late autumn,temperature and soil moisture are significantly positively correlated;in the late spring falling period and the high-value stable period in spring,there is a significant negative correlation.The influence of air temperature and soil temperature on soil moisture is not obvious in summer.(2)The inter-annual variation of soil moisture from 2009 to 2019 showed a fluctuating and significantly increasing trend.At the bottom of slope,the soil moisture fluctuated with a large period of 63 months and a small period of 18 months.In the shallow soil layer,the 63-month cycle of soil moisture changes more obviously,which is the first main cycle.With the deepening of soil,the 18-month cycle oscillation gradually strengthens,which becomes the first main cycle.The main oscillation period of soil moisture in each layer on the slope was 63 months without other periodic changes.Through cross-wavelet and wavelet coherence analysis,there is a 10-15 month conjugate cycle between precipitation and soil moisture from June 2010 to February 2015,and other conjugate cycles are sporadically distributed in 1-6 month cycles,which shows the randomness of the influence of precipitation on soil moisture.Soil moisture and precipitation on the slope also have a 10-15 month conjugate cycle,which is regional.With the increase of soil depth,the influence of precipitation on soil moisture gradually weakened in the conjugate period of 10-15 months.The effects of soil temperature and air temperature on soil moisture are similar,and there is a conjugate period of10-15 months in the whole time series.(3)Soil moisture has a high time series autocorrelation.In general,the influence of soil moisture on soil moisture in the later period is less than two months.Compared with the surface soil,the influence of deep soil moisture on the later soil moisture can last longer.The disturbance of other soil moisture influencing factors such as atmosphere is the main reason for weakening the autocorrelation of surface soil moisture.The effect of soil moisture on the bottom of the slope lasts longer than that on the slope.(4)Soil moisture has a significant negative correlation with soil bulk density and a significant positive correlation with soil porosity.On the whole time series scale,the effects of air temperature,precipitation and soil temperature on soil moisture have a significant positive correlation.The three factors have scale effect on soil moisture.In the process of soil moisture decrease,the influence of air temperature and soil temperature plays a dominant role,while in the process of soil moisture increase,the influence of precipitation plays a dominant role.The influence of temperature and precipitation on soil moisture has a certain lag,and the correlation between soil moisture and precipitation 2-3 days ago is the greatest.At the bottom of the slope,the influence of precipitation and temperature on soil moisture can last for 3 months.On the slopes,however,the effects of precipitation last only two months.Conclusion: Soil moisture in desert steppe varies significantly on intraannual and inter-annual scales,and is highly sensitive to the changes of atmospheric factors,especially in winter.Precipitation is the main supplement of surface soil moisture in this region,and evaporation and transpiration,which are dominated by soil temperature,are the main dissipative processes that reduce soil moisture.Under the influence of concentrated precipitation and persistent dissipation in desert steppe,soil moisture showed a multi-cycle fluctuation with a period of 63 months and a period of 18 months.In terms of the autocorrelation of soil moisture and the sustainability of its response to influencing factors,the hysteresis and persistence of soil moisture at the bottom of the slope are stronger. |
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