A new perspective for transpiration estimation using an extended framework of isohydricity linking to drought-driven changes in VPD and leaf area

The framework of isohydry or anisohydry, which is usually defined as the sensitivity of leaf water potential (Ψ_L) to soil water potential (Ψs), has been rapidly adopted to solve a range of eco-hydrologic problems. While its reliability to describe the impacts of land-atmosphere interaction and seasonal phenology on plants has been recently questioned. In this study, we propose an expansion of the modern isohydricity framework to coordinate the dynamics of Ψ_L derived from vapor pressure deficit (VPD) and leaf area index (A_L), respectively. The contributions of VPD and A_L to the sensitivity of Ψ_L to Ψs are calculated and further evaluated using the FLUXNET dataset, as to validate the applicability of the extended concept. Then, we suggested a new method to calculate transpiration based on the new framework to establish relationship between Ψ_L and Ψs at ecosystem scale. Our results illustrate that the new framework is reasonable for describing the dynamics of Ψ_L and provides a promising potential for transpiration estimation.