Evapotranspiration over Land from a Boundary-Layer Meteorology Perspective
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Evapotranspiration over Land from a Boundary-Layer Meteorology Perspective J. Cuxart1
· A. A. Boone2
Received: 5 January 2020 / Accepted: 9 July 2020 © Springer Nature B.V. 2020
Abstract The precise determination of evapotranspiration rate is challenging because it is a quantity that is difficult to measure and to parametrize. Direct estimates include the determination of the change of mass of a volume of soil and vegetation that evapotranspirates using lysimeters, or direct measurements of turbulent water vapour fluxes by eddy-covariance systems. Parametrized estimates that make use of the Monin–Obukhov similarity theory use vertical gradient measurements of temperature and moisture at one point, and line or area averages by means of scintillometers operating at high frequency. A relation for the evapotranspiration from well-watered surfaces was initially developed by Penman and later expanded for vegetated surfaces and for heterogeneous croplands. A popular simplified expression was obtained by Priestley and Taylor. The current challenge is to find expressions for the evapotranspiration in non-saturated conditions, which are common in arid and semi-arid climates, and for heterogeneous terrain. In numerical models, the estimated actual evapotranspiration over land is obtained as the result of the explicit representation of the different involved sub-processes taking place in the soil and the canopy, using so-called land-surface models. Usually these mechanisms are described in a simplified manner and rely on a number of adjustable parameters. The improvement of such descriptions relies in the availability of experimental measurements to make the physical models more complete and robust. Keywords Evapotranspiration · Eddy-covariance systems · Lysimeters · Heterogeneous surfaces · Land-surface models
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J. Cuxart [email protected] A. A. Boone [email protected]
1
University of the Balearic Islands, Palma, Majorca, Spain
2
CNRM - Université de Toulouse, Météo-France/CNRS, Toulouse, Occitanie, France
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J. Cuxart, A. A. Boone
1 Introduction 1.1 Evapotranspiration as a Key Process in the Earth System The evapotranspiration rate is the flux of water vapour from the surface to the atmosphere, traditionally expressed in kg s−1 m−2 or an equivalent unit such as mm day−1 or mm year−1 , in which 1 mm represents 1 kg m−2 . We will denote it through the variable E as the sum of the evaporation from the soil (E g ) and the transpiration of plants (E tr ), E = E g + E tr
(1)
Water evaporation requires energy, denoted by the latent heat flux λE, which is the product of the enthalpy of vaporization of water λ and E, expressed in W m−2 . From an energy point of view, evapotranspiration is the most important non-radiative process transmitting heat from the surface to the atmosphere globally, and it is larger than direct heating of the air due to the surface sensible heat flux (Bosilovich et al. 2011). Furthermore it is the branch of the water cycle transporting water upwards feeding clouds and hence the origin
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