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.

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. The concept of potential evaporation is used commonly in
catchment and water balance studies to estimate actual evapotranspiration

Physically based definitions of potential evaporation consider all or a
subset of the following atmospheric variables or various transformations
their-of:

sky radiation

air temperature

vapour concentration

wind speed.

Therefore, in the present study, we challenge the assumption that common definitions of potential evaporation provide a meaningful measure for the sensitivity of transpiration to wind speed and potentially other components of atmospheric forcing. The objective of the present study is to analyse how transpiration from a leaf scales with evaporation from a wet surface with similar properties under the same variations of atmospheric conditions.

In order to consider all relevant atmophseric forcing variables explicitly,
and to avoid ambiguities between a “reference crop” and an actual plant, we
adopt a detailed physically based leaf transpiration and energy balance model
published previously

The model was described in detail by

The leaf energy balance is determined by the dominant energy fluxes between
the leaf and its surroundings, including radiative, sensible, and latent
energy exchange (linked to mass exchange). In this study we focus on
steady-state conditions, in which the energy balance can be written as:

Following

The sensible heat flux or the total convective heat transport away from the
leaf is represented as:

The latent heat flux (

Dependence of the leaf-air water vapour concentration difference
(

Due to their reliance on leaf boundary layer properties, the one-sided heat
transfer coefficient (

For given atmospheric forcing including shortwave irradiance
(

Modelled leaf boundary layer resistance compared to observations by

Modelled latent heat fluxes and leaf temperatures compared to
observations by

Simulated relative transpiration (latent heat flux at low stomatal
conductance (0.001 m s

The experimental conditions were reported as 70–75 % relative humidity and
100–200 W m

In order to evaluate the effect of stomatal conductance on the transpiration
rate under various conditions, we plotted relative transpiration rates, i.e.
transpiration rate at low stomatal conductance (

Model simulations and predictions of a decrease in transpiration rates with
increasing wind speed were consistent with the experimental results presented
by

The analogy with the representation of actual evapotranspiration as a
fraction of potential evaporation, in which the reduction coefficient
represents soil moisture limitations, is equivalent to assuming that the
ratio between potential and actual evapotranspiration is constant as long as
surface properties do not change. Figure

The use of potential evaporation to estimate actual evapotranspiration or to describe the suitability of a given climate for plant growth confounds different effects of atmospheric drivers on transpiration from plant leaves versus evaporation from wet surfaces and may lead to wrong conclusions about the consequences of climate change on plant growth and water relations. This is most pronounced for effects of wind speed, which increases potential evaporation, but may reduce actual transpiration. Therefore, we recommend to avoid using the concept of potential evaporation in relation to plants and transpiration from leaves, and rather address the effects of different atmospheric drivers on plant water use separately.

The heat transfer coefficient (

Symbols, standard values and units used in this paper. All area-related variables are expressed per unit leaf area.

For sufficiently high wind speeds, inertial forces drive the convective heat
transport (forced convection) and the relevant dimensionless number is the
Reynolds number (

In the absence of wind, buoyancy forces, driven by the density gradient
between the air at the surface of the leaf and the free air dominate
convective heat exchange (“free” or “natural convection”). The relevant
dimensionless number here is the Grashof number (

For

The average Nusselt number under forced convection was calculated as a
function of the average Reynolds number and a critical Reynolds number
(

Due to their reliance on leaf boundary layer properties, the heat transfer
coefficient (

The heat and mass transfer coefficients described above can be seen as the
reciprocals of flow resistances,

The concentration difference in Eq. (

In order to simulate steady state leaf temperatures and the leaf energy
balance terms using the above equations, it is necessary to calculate

The authors are grateful to Francis Chiew for inviting this contribution to the Session “Hydrologic Non-Stationarity and Extrapolating Models to Predict the Future” at the IUGG 2015 General Assembly, and to Jai Vaze and an anonymous reviewer for helpful comments.