Spatiotemporal dynamics of global surface and rootzone soil moisture: a comprehensive assessment from dominant factors, impact pathways, and deficit probability

Soil moisture (SM) plays a vital role in the global water and carbon cycles. The intensification of climate change in the 21st century has complicated the understanding of variations in surface soil moisture (SMsurf) and root-zone soil moisture (SMroot) and their driving mechanisms. Therefore, this study provided a comprehensive assessment of SMsurf and SMroot from the perspectives of spatiotemporal characteristics, dominant factors, impact pathways, and deficit probability during the period 2001–2021. The results indicated that although SMsurf and SMroot exhibited broadly similar spatial patterns, their fluctuation periods, amplitudes, and frequencies differed markedly in hotspot regions. Numerical simulation experiments based on Random Forests revealed the mean–state SM driving factors, showing that atmospheric water demand was the primary cause of aridity in both global SMsurf and SMroot, with effects of 8.9 × 10 -5 m 3 /m 3 /year and 7.5 × 10 -5 m 3 /m 3 /year, respectively, while vegetation and precipitation exerted strong but contrasting effects. The Partial Least Squares Structural Equation Modeling further revealed that vegetation primarily mediated the indirect effects of climate on SM. Copula functions revealed that precipitation, SPEI, and vegetation dynamics were the primary risk factors for both SMsurf and SMroot deficits. Under the same conditions, the conditional probability of SM deficits induced by these factors is 2–3 times higher than that associated with other drivers. Overall, these findings will provide important insights for global water resource management and the sustainable development of ecosystems.