| Soil moisture is an important component of the terrestrial hydrological cycle linking soil-vegetation-atmosphere,and is an important factor affecting terrestrial ecosystems.In the context of global vegetation greening,it is important to investigate the impact of vegetation greening on global soil moisture to maintain ecosystem stability and protect the sustainability of ecological restoration projects.In order to quantitatively assess the impact of vegetation greening on global soil moisture,this paper constructs a linear model using MODIS NDVI data and GIMMS NDVI data,and extends the GIMMS NDVI time span to 2020.And we analyze the spatial distribution,trend and correlation of soil moisture and vegetation from 1982 to 2020.Secondly,this paper tests the abrupt change of the global vegetation greening at the regions with increasing trend of NDVI from 1982 to 2020,so as to obtain the abrupt change time point of the global vegetation greening.Again,this paper constructs a random forest model of vegetation-soil moisture on an pixel scale,and predicts the soil moisture changes caused by vegetation greening on a daily scale to quantitatively evaluate the impact of vegetation greening on global soil moisture changes.Finally,this paper analyzes the differences in the effects of vegetation greening on global soil moisture change through different climate zone conditions and different topographic conditions.The main conclusions drawn in this paper are as follows.（1）The global soil moisture from 1982 to 2020 is mainly decreasing,and the area with decreasing trend of global soil moisture at different depths accounts for about 64% of the total area.the NDVI is mainly increasing,and the area with increasing trend accounts for about 68.92%of the total area.Soil moisture is negatively correlated with NDVI,and the areas with negative correlation are mainly located near the equator,and the area with negative correlation increases as the depth of soil layer increases.（2）From the global scale,the effect of vegetation greening on the mean soil moisture varies with the time scale.At the monthly scale,the mean soil moisture value decreases in June to October and increases in January to April and November to December due to vegetation greening.At the seasonal scale,the mean soil moisture values decreased during the vegetation growing season,summer and autumn,and increased in spring and winter.At the annual scale,the greening of vegetation led to a global trend of decreasing and then increasing soil moisture.Vegetation greening in general led to a decrease in the coefficient of variation of soil moisture.At different soil depths,vegetation greening led to a decrease in the coefficient of variation of soil moisture in the 0-7 cm and 7-28 cm soil layers,and an increase in the coefficient of variation of soil moisture in the28-100 cm and 100-289 cm soil layers.（3）Under different climatic zones,the effect of vegetation greening on soil moisture showed large differences.In the tropics,vegetation greening generally led to an increase in mean soil moisture,while in the arid climate zone areas,vegetation greening led to a decrease in mean soil moisture.In the subtropical,temperate,cold temperate and polar zones,the greening of vegetation led to an overall decrease in mean soil moisture at the monthly and seasonal scales,and a decrease followed by an increase in mean soil moisture at the annual scale.The difference between different climatic zones is mainly reflected in the decrease of soil moisture coefficient of variation in the 0-7cm and 7-28 cm soil layers,and the decrease of soil moisture coefficient of variation in temperate,coldtemperate and polar zones is mainly concentrated in summer,while the rest of climatic zones have stable changes.（4）Under different topographic conditions,vegetation greening in low elevation topography mainly led to the decrease of the mean value of soil moisture in the shallow layer（28-100 cm and 100-289 cm）,while in high elevation topography,vegetation greening led to the increase of the mean value of soil moisture in the deep layer（0-7 cm and 7-28 cm）.Within the year,vegetation greening mainly led to a decrease in the mean monthly and seasonal soil moisture values in the first half of the year and an increase in the mean monthly and seasonal soil moisture values in the second half of the year.At the annual scale,vegetation greening resulted in decreasing mean soil moisture values in low plains and very high mountain terrain,increasing mean soil moisture values in low and medium mountain terrain,and stable changes in mean soil moisture values in hilly and high mountain terrain during 1993 to 2018.Vegetation greening in general led to a decrease in the coefficient of variation of soil moisture in all terrains,with a decrease in the coefficient of variation of soil moisture in the 0-7 cm and7-28 cm soil layers and an increase in the coefficient of variation of soil moisture in the 28-100 cm and 100-289 cm soil layers at both monthly and annual scales.The overall decrease in the coefficient of variation of soil moisture in all soil layers was caused by the greening of vegetation at the seasonal scale.The results of this paper quantitatively assessed the effects of vegetation greening on soil moisture variation at the global scale,and the results can provide some theoretical references and suggestions for the sustainable development of global ecosystem conservation and ecological restoration projects. |
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