A Holistic Multi-Index Approach to Quantify Land Feedback Strength Across Evapotranspiration Regimes

Soil moisture (SM) is a critical Earth system variable that regulates the cyclicity of water, energy, and carbon, through which SM determines the evolution and thermodynamic state of the atmosphere. Land and atmospheric is tightly coupled in the water-limited regime (WLR), while the coupling strength diminishes in the energy-limited regime (ELR). Specifically, in response to progressive SM drying in the WLR, SM fractionates the net insolation into a greater proportion of sensible heat flux (SHF) and a smaller amount of latent heat flux (LHF), owing to the depletion of moisture. This phenomenon results in reduced land surface cooling, increased air temperature, expansion of the boundary layer, and subsequently enhances the land-atmosphere feedback. Further continuation of SM depletion leads to dry hydroclimatic extremes such as droughts and heatwaves. Consequently, understanding regime-specific coupled water-energy dynamics is fundamental to comprehending such extremes.

We propose a new metric called Land Feedback Strength (LFS) that combines three indices: sensitivity index (SI), variability index (VI) and regime persistence index (RPI). This formulation over the past attempts facilitates to effectively characterise the important components of LFS, which holistically quantify the terrestrial leg of land-atmospheric coupling. SI quantifies the responsiveness of SM to surface energy partitioning and is defined as the slope between SM and EF (LHF/LHF+SHF) in the WLR. Specifically, we observe higher SM sensitivity in semi-arid and sub-humid regions than in wet regions, indicating that the landscape rapidly responds to SM losses and begins influencing the atmosphere instantaneously. In addition, VI quantifies the sufficiency of SM to act as a dominant forcing and is calculated as the ratio of the standard deviation of SM in the WLR to WLR and ELR. While strong coupling is expected where higher sensitivity and sufficient SM variation are present, the coupling strength is exacerbated with the increasing persistence of the WLR. Thus, the RPI is formulated to indicate the likelihood of a landscape remaining in the WLR within a certain period. Furthermore, to quantify the LFS, we initially delineate global regimes using the coverability of SM and EF data pairs during drydowns.

This study’s findings indicate the following: (1) the highest sensitivity is observed during the dry seasons, whereas sensitivity is lowest during the summer; (2) SM variability is predominantly confined to WLR during winter and spring, with approximately equal variability in both regimes noted during autumn, and variability predominantly occurring in ELR during summer; (3) ELR is prevalent during summer in response to precipitation pulses, WLR and ELR demonstrate comparable likelihood in autumn, and WLR becomes predominant during winter and spring; (4) consequently, LFS is at its lowest during summer, increases in autumn, and further intensifies in winter; (5) LFS has facilitated the identification of two groups of strong coupling hotspots – with relatively higher intensity over the western USA and Austrian shrubland, African and Brazilian savannah, and lower intensity over Sahelian grassland, and peninsular India (6) LFS is found to be higher in semi-arid and sub-humid regions or savanna and grassland areas than forested or humid regions.