Stomatal response to decreased relative humidity constrains the acceleration of terrestrial evapotranspiration

Mingzhong Xiao,Leonardo Montagnani,Lutz Merbold,Jason Beringer,Dongdong Kong,Zhongbo Yu,M Altaf Arain,Xihui Gu,Mana Gharun,Sebastian Wolf,Nina Buchmann,Annalea Lohila,Lukas Hörtnagl,Ivan Mammarella,Beniamino Gioli,Vincenzo Magliulo

doi:10.1088/1748-9326/ab9967

Abstract

Terrestrial evapotranspiration (ET) is thermodynamically expected to increase with increasing atmospheric temperature; however, the actual constraints on the intensification of ET remain uncertain due to a lack of direct observations. Based on the FLUXNET2015 Dataset, we found that relative humidity (RH) is a more important driver of ET than temperature. While actual ET decrease at reduced RH, potential ET increases, consistently with the complementary relationship (CR) framework stating that the fraction of energy not used for actual ET is dissipated as increased sensible heat flux that in turn increases potential ET. In this study, we proposed an improved CR formulation requiring no parameter calibration and assessed its reliability in estimating ET both at site-level with the FLUXNET2015 Dataset and at basin-level. Using the ERA-Interim meteorological dataset for 1979–2017 to calculate ET, we found that the global terrestrial ET showed an increasing trend until 1998, while the trend started to decline afterwards. Such decline was largely associated with a reduced RH, inducing water stress conditions that triggered stomatal closure to conserve water. For the first time, this study quantified the global-scale implications of changes in RH on terrestrial ET, indicating that the temperature-driven acceleration of the terrestrial water cycle will be likely constrained by terrestrial vegetation feedbacks.

Highlights

On a global scale, approximately two-thirds of the annual precipitation over land originates from terrestrial evapotranspiration (ET) (Oki and Kanae 2006)
Net radiation resulted in the most important driver of ET representing the energy source for ET itself (figures 1(b) and (c)). Both the multiple regression and Random Forest methods indicated that relative humidity (RH) is a more important driver than temperature (figures 1(b) and (c)) highlighting its importance especially since a sharp decrease in RH has been widely observed at global scale in recent years (Simmons et al 2010, Willett et al 2014)
The scatter plot of RH in relation to the normalized measured actual ET and estimated potential ET (refer to equation (6) for details on the estimation method) illustrates that the normalized actual ET generally increased with increasing RH, while the normalized potential ET generally decreased with increasing RH (figure 1(d))

Summary

Introduction

Approximately two-thirds of the annual precipitation over land originates from terrestrial evapotranspiration (ET) (Oki and Kanae 2006). There is evidence of increasing water vapour content at both the surface and upper tropospheric levels (Trenberth et al 2005); numerous previous studies have suggested surface relative humidity (RH: ratio of actual water vapour pressure to saturated water vapour pressure) to be constant (Dai 2006, Willett et al 2008). Recent studies have indicated that the actual atmospheric water vapour content does not always increase at exactly the same rate as saturated vapour pressure. Increasing evidence suggests that the decrease in terrestrial RH are due to the greater warming over land than the ocean (Byrne and O’Gorman 2016) and are associated with a concurrent decline in the soil moisture over vast regions in the terrestrial biosphere (Byrne and O’Gorman 2016, Zhou et al 2019). Details about the patterns of reduced RH still remain unclear (Vicente-Serrano et al 2018)

Objectives

Methods

Results

Discussion

Conclusion