Abstract
Abstract. Transpiration is commonly conceptualised as a fraction of some potential rate, driven by so-called "atmospheric evaporative demand". Therefore, atmospheric evaporative demand or "potential evaporation" is generally used alongside with precipitation and soil moisture to characterise the environmental conditions that affect plant water use. Consequently, an increase in potential evaporation (e.g. due to climate change) is believed to cause increased transpiration and/or vegetation water stress. In the present study, we investigated the question whether potential evaporation constitutes a meaningful reference for transpiration and compared sensitivity of potential evaporation and leaf transpiration to atmospheric forcing. A physically-based leaf energy balance model was used, considering the dependence of feedbacks between leaf temperature and exchange rates of radiative, sensible and latent heat on stomatal resistance. Based on modelling results and supporting experimental evidence, we conclude that stomatal resistance cannot be parameterised as a factor relating transpiration to potential evaporation, as the ratio between transpiration and potential evaporation not only varies with stomatal resistance, but also with wind speed, air temperature, irradiance and relative humidity. Furthermore, the effect of wind speed in particular implies increase in potential evaporation, which is commonly interpreted as increased "water stress", but at the same time can reduce leaf transpiration, implying a decrease in water demand at leaf scale.
Highlights
Potential evaporation is a measure for atmospheric water demand, i.e. how much water can be evaporated from a wet surface under given atmospheric conditions
Taking a relative humidity of 70 %, assuming that the wind tunnel experiments were conducted under the same air temperature as the growth conditions of the plants (20 ◦C), and taking into account the ranges of stomatal conductances estimated by Dixon and Grace (1984), we simulated the responses of latent heat flux and leaf temperatures of the different species to variations in wind speed by choosing constant values for absorbed shortwave radiation (Rs) and stomatal conductance to best represent the data points in the original study (Fig. 3 in Dixon and Grace, 1984)
Our simulations predicted a decrease in both latent heat flux and leaf temperature for all experiments, which is consistent with the experimental results, except for P. sylvestris, where an initial increase in transpiration was observed, followed by a decrease at higher wind speeds
Summary
Potential evaporation is a measure for atmospheric water demand, i.e. how much water can be evaporated from a wet surface under given atmospheric conditions. In contrast to evaporation from wet surfaces, transpiration is commonly controlled by plant stomata, which, by gradually opening and closing, impose a varying resistance on the surface-to-air vapour transfer. Despite this fundamental distinction, it is a surprisingly widely adopted assumption that transpiration scales with “atmospheric evaporative demand” or potential evaporation. Barella-Ortiz et al (2013) analysed the sensitivity of different formulations of potential evaporation to climate change, and found large differences in climate sensitivities, depending on the processes included in the formulations They concluded that those formulations that represent the most complete consideration of physical processes contributing to evaporation are most robust and reliable for the characterisation of the impact of climate change on surface processes
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