Spatial-temporal Variation of Soil Moisture in China from Long Time Series Based on GLDAS-Noah

Soil moisture is a comprehensive reflection of soil moisture status and is an important parameter for land surface conditions. It is very important to study the distribution characteristics of soil moisture for ecological environment protection, scientific and rational utilization of soil water resources, and climate research. Using the soil layer humidity data sets of GLDAS-Noah v2.0 and v2.1, we analyzed the spatial-temporal distribution of soil moisture in China in a layer from 0 to 200 cm over 71 years from 1948 to 2018. Firstly, the Mann-Kendall trend test was used to analyze the trend of the changes and the spatial variation characteristics of soil moisture over the 71 years. Secondly, the coefficient of variation was used to analyze the temporal and spatial fluctuation of soil moisture in each layer of the study area over the 71 years. Finally, the Hurst index was used to predict the future trend of soil moisture changes in each layer. In addition, the correlation between soil moisture and the spatial-temporal variation of soil temperature in China was explored. The results show that the annual variation trend of soil moisture in the 0-200 cm soil layer has been consistent, that is, the soil humidity in most parts of east China has been decreasing, especially in northeast China, central China, the area surrounding the Yunnan Guizhou Plateau, and Taiwan Island, while it has been increasing in most of the western regions. Also, the change in soil layer humidity from 0 to 200 cm in southern China was greater than that in the northern region, and the humidity of the soil layer in the Pearl River Delta region was the most unstable. In addition, the spatial variation of soil moisture in the study area was relatively small from 1948 to 2001, but from 2002, the soil moisture throughout the study area became uneven. In the future, the trend of the change in soil moisture in most areas of China will remain consistent with that in the past 71 years, i.e., the soil in most parts of the east will gradually dry out and the soil moisture in most parts of the west will gradually increase; the soil humidity from 0 to 200 cm in most of the study area is inversely related to the soil temperature, and is mainly concentrated in northeast and central China, central and northern Inner Mongolia, the Qinghai Tibet Plateau, and Taiwan Island.