Preprints
https://doi.org/10.5194/hess-2020-590
https://doi.org/10.5194/hess-2020-590

  21 Nov 2020

21 Nov 2020

Review status: this preprint is currently under review for the journal HESS.

Long-term relative decline in evapotranspiration with increasing runoff on fractional land surfaces

Ren Wang1,2, Pierre Gentine3,4, Jiabo Yin5, Lijuan Chen1,2, Jianyao Chen6, and Longhui Li1,2 Ren Wang et al.
  • 1School of Geographical Sciences, Nanjing Normal University, Nanjing 210023, China
  • 2Key Laboratory for Geographical Environment Evolution, Ministry of Education, Nanjing, 210023, China
  • 3Earth and Environmental Engineering Department, Columbia University, New York, NY 10027, USA
  • 4Earth Institute, Columbia University, New York, NY 10025, USA
  • 5State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, China
  • 6School of Geography and Planning, Sun Yat-sen University, Guangzhou 510275, China

Abstract. Evapotranspiration (ET) accompanied by water and heat transport in the hydrological cycle is a key component in regulating surface aridity. Existing studies on changes in surface aridity have typically estimated ET using semi-empirical equations or parameterizations of land surface processes, which are based on the assumption that the parameters in the equation are stationary. However, plant physiological effects and its response to a changing environment are dynamically modifying ET, thereby challenging this assumption and limiting the estimation of long-term ET. In this study, the latent heat flux (ET in energy units) and sensible heat flux were retrieved for recent decades on a global scale using machine learning approach and driven by ground-based observations from flux towers and weather stations. The study resulted in several findings, namely that the evaporative fraction (EF) – the ratio of latent heat flux to available surface energy – exhibited a relatively decreasing trend on fractional land surfaces; In particular, the decrease in EF was accompanied by an increase in long-term runoff as assessed by precipitation (P) minus ET, accounting for 27.06 % of the global land areas. The signs were indicative of reduced surface conductance, which further emphasized that land-surface vegetation has major impacts on regulating the water and energy cycles, as well as aridity variability.

Ren Wang et al.

 
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Ren Wang et al.

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Short summary
Assessment of changes in global water cycle has been a challenge. This study estimate long-term global evapotranspiration (ET) for recent decades using machine learning and ground observations. The results show that normalized ET experienced a relative decline on fractional land, and the decline was typically accompanied by an increase in long-term runoff in 27.06 % of the global land areas. The finding emphasized that surface vegetation have a great impact in regulating water and energy cycles.