Global spatiotemporal trend of satellite-based soil moisture and its influencing factors in the early 21st century

Soil moisture (SM) plays a significant role in water, energy, and carbon cycles, and is a key variable related to multiple sustainable development goals. The frequent occurrence of extreme events in the 21st century complicates the understanding of terrestrial SM change and its drivers under diverse environmental conditions, particularly in the global scope. Here we globally explored the spatial-temporal trend of satellite-based SM (ESA CCI SM) and its possible drivers under a variety of environmental factors (land cover, soil texture, terrain, and vegetation coverage) during 2000-2021. Results indicate that global SM has generally declined with a rate of -0.10 x 10-3 m3m- 3 yr- 1 which is mainly dominated by the drying trend of the southern hemisphere. The moisture replenishment role of precipitation dominates in areas with relatively high SM values, and the positive influence of vegetation and precipitation on SM is prominent in regions where SM values are relatively low. But vegetation has a pronounced negative impact on SM north of 60 degrees N and in southeastern China. Compared with precipitation and vegetation, topsoil temperature has less impact on SM globally. Land cover types, soil properties, elevation, and vegetation cover influence SM variability and its response to climate and vegetation to varying degrees. For example, the overall wetting trend of SM is unique under loam soils and high elevations (> 1000 m) in terms of different soil texture and elevations. The increase in vegetation cover within a certain amount is accompanied by the wetting of SM, but in densely vegetated regions such as forests, the water retention effect of vegetation on SM is weakened. An evaluation of 'Dry gets drier, wet gets wetter' (DDWW) paradigm from the SM viewpoint shows the paradigm summarizes 41.06% of areas with significant SM changes. Precipitation replenishment and vegetation water retention profoundly impact the 'wet gets wetter' and 'dry gets wetter' patterns. Temperature is a strong influencing factor of the 'wet gets drier' pattern, while the reduction of vegetation and precipitation jointly affect the 'dry gets drier' pattern. These findings can deepen the understanding of SM changes in relation to climate, ecology, and land surface features, particularly in the 21st century with profound changes of the environment.