Articles | Volume 371
12 Jun 2015
 | 12 Jun 2015

Wind effects on leaf transpiration challenge the concept of "potential evaporation"

S. J. Schymanski and D. Or

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.

Short summary
The common use of "potential evaporation" to estimate actual evapotranspiration or to describe the suitability of a given climate for plant growth may lead to wrong conclusions about the consequences of climate change on plant growth and water relations. Wind speed in particular can have opposite effects on potential evaporation and transpiration from plant leaves. Therefore, we recommend to avoid using the concept of potential evaporation in relation to plants and transpiration from leaves.