Global Scale Intercomparison of Critical Soil Moisture Estimation Pathways and Their Influence on Hydrological Regime Dynamics and Land-Atmosphere Interactions

Abstract

Hydrological regimes are fundamental for analyzing land-atmosphere (LA) interactions. These regimes are delineated into water- and energy-limited regime by estimating their critical soil moisture $\left({\theta }^{\ast }\right)$ threshold using the evaporative fraction $\left(EF\right)$, temporal rate of soil moisture $\left(\theta \right)$ drying $\left(d\theta /dt\right)$, and diurnal soil temperature range $\left(d{T}_{\text{soil}}\right)$. However, the impact of inconsistencies stemming from ${\theta }^{\ast }$ on our understanding of LA feedbacks has not yet been examined. The present work, for the first time, conducts a comprehensive intercomparison of ${\theta }^{\ast }$, regime properties (persistence, chances of transitioning, and temporal similarity), and LA coupling strength estimated using covariability of $EF$, $d\theta /dt$, and $d{T}_{\text{soil}}$ with $\theta$. We find that ${\theta }_{d{T}_{\text{soil}}}^{\ast }$, ${\theta }_{EF}^{\ast }$ and ${\theta }_{d\theta /dt}^{\ast }$ are in close agreement in semi-arid and sub-humid climatic regions characterized by herbaceous vegetation. $d{T}_{\text{soil}}$ results in higher persistence of water-limited regime and less frequent regime transitioning compared to $EF$ and $d\theta /dt$. However, ${\theta }_{d{T}_{\text{soil}}}^{\ast }$ identifies similar regimes for >80%of the time in 84% of regime transitioning regions. The tipping potential of regimes identified by ${\theta }_{d{T}_{\text{soil}}}^{\ast }$, ${\theta }_{EF}^{\ast }$ and ${\theta }_{d\theta /dt}^{\ast }$ is lowest during the dry seasons, which increases during the summer. $d{T}_{\text{soil}}$ identifies stronger LA coupling over western USA, south-central Australia and peninsular India, compared to $EF$ and $d\theta /dt$. Central Asian grasslands are identified to have mild LA coupling using $d{T}_{\text{soil}}$, while $EF$ and $d\theta /dt$ do not indicate notable coupling. Overall, $d{T}_{\text{soil}}$ represents a viable proxy for $EF$ and $d\theta /dt$ toward enhancing the understanding of local LA coupling with potential applications in assessing ecosystem responses, evolution of dry extreme events, and land surface modeling.