Study On The Dynamic Change Of Soil Moisture Under Different Vegetation Coverage In Alpine Meadow

This paper took the Fenghuo mountain basin as the study area that located in headwaters of the Yangtze River.Three observation plots,the ratios of vegetation coverage classified 30%,68%and 92% respectively were established on the slope where is a confluence of Zuomaoxikongqu River and Dunzaijialong River.There are some parameters of soil and vegetation coverage that have investigated,and others such as the soil moisture,soil temperature,soil evapotranspiration and meteorological data have observed in order to analyze the impacts on the dynamic change of soil moisture in the Permafrost Active Layer related from Alpine Meadows coverage changes.There are mainly consists of four contents:the spatial distribution and change processing of the soil moisture,Coupling Relationship between soil moisture and soil temperature,the response of soil moisture variation to precipitation under different vegetation coverage,and dynamic change of the evapotranspiration and condensation water under different vegetation coverage.The research finding has shown as follows:(1)Define the freezing/thawing points according to the inflexion of soil moisture curve,the spatial distribution and change processing of the soil moisture for one year in the Permafrost Active Layer can be divided into four phases:high moisture content period,freezing period,low moisture content period and thawing period,respectively.There is much difference of the soil moisture's spatial distribution and change processing under different vegetation coverage.The freezing and thawing variation were delayed by high degree vegetation coverage,which can protect the permafrost and the damage of Permafrost by freeze-thaw erosion could be alleviated.(2)The cooperative relationship between soil moisture and soil temperature is remarkable and also their spatial distribution under different vegetation coverage are different.When the vegetation degradation is serious,the heat transfer in soil become more easily especially the shallow soil.The coupling relationship between soil moisture and soil temperature in the deep layer is more complicate than that in the shallow layer during freezing period.Furthermore,the bidirectiona freezing is confirmed from the top and bottom of permafrost horizon,and the degree is gradually weaken due to the alteration of coverage.Thawing is top-down process,and the difference of the depth from altered vegetation coverage is significant.The regression models were used to simulate the relationships between soil moisture and soil temperature,and they show exponential correlative model for both the freezing and thawing periods.The effectiveness of regression models for the thawing process is better than that for the freezing period.(3)There was a significant delayed response of soil moisture variation to precipitation under different vegetation coverage,and the hysteretic time is too long.When the vegetation degradation is serious,soil moisture within the depth of 20 cm response to precipitation will reach to a certain extent,but deep soil layer is rather dry so as the response is weak.Additionally,when the original plant community is preserved and coverage is higher,soil moisture capacity would be enhanced and the soil moisture content in deeper layers is greatly improved.As the depth on which soil moisture response to precipitation is deeper than 40cm,the change of soil moisture is distinct.Higher vegetation coverage can effectively improve the physical and chemistry characteristics of soil so that the precipitation infiltration is accelerated.The response of soil moisture to precipitation is also advanced by the activity of plant roots which could help to obtain preferential flow and water—absorbing.(4)The amount of evapotranspiration and condensation water from soil surface is greatly much more in the fast growth period than the fade period.During the observation period,the highest value of evapotranspiration and condensation water is in the meadow zone that holds 92%of land-cover,and the value become lower of which holds 30%of land-cover,the meadow zone holding 68%land-cover is lowest.At the same time,water vapor condensation of nights becomes intensively and the value where holds 30%of land-cover is close to that holds 68%of land-cover.Because of soil freezing, evapotranspiration decreased significantly and it is beneficial to soil moisture conservation.With the decrease of vegetation coverage,condensation water reduces significantly.Condensation water can compensate for evapotranspiration efficiently.The higher the vegetation coverage,the greater the compensation ability is.