Global surface soil moisture dynamics in recent 38 years according to remote sensing observation

Soil moisture is an important variable for terrestrial surface system as its temporal and spatial changes greatly affect the global water and energy cycle. How global soil moisture responds to global environmental change still has not been revealed. Satellite remote sensing has made global soil moisture observation over long time series possible. The data of this study is obtained from European Space Agency (ESA) climate change initiative (CCI) combined soil moisture (SM) data set. After correcting of abnormal pixel values by interpolation, the trend and variation distribution of global surface soil moisture in 1979-2016 years is identified by cubic stepwise regress at annual and quarterly scales. Globally, our study suggests that the changes of surface soil moisture is not monotonous but presents cubic polynomial trend that decreasing to increasing to decreasing (down-up-down), as the breaking points appear in 1990 and 2009 respectively. On the quarterly scale, global average surface soil moisture in the third quarter shows monotonically increase, while in other quarters, it presents nonlinear trend that down-up trend in the second quarter and down-up-down trend in the first and fourth quarters. On the annual scale, significant changes have taken place in 1/3 areas of the land. There is a decreasing trend in northern Asia and some coastal regions of the Americas with 20% land areas, and the eastern and central Asian regions have shown an obvious upward trend significantly. The regions with monotone increasing soil moisture account for the most areas in all quarters. Since spatial distributions of surface soil moisture changes are very different between quarters, nonlinear changes with downward trend cover more areas on the annual scale. On the whole, global surface soil moisture has increased comparing to 1990s. However, decreasing trends existed in the next 10 years. Meanwhile, environmental change in some regions where experienced vibrant changes between quarters should be paid close attentions.