TARGAS-1 Operation Manual V. 1.02
151
𝒆𝒆
𝒊𝒊𝒊𝒊
is defined as the partial pressure of water vapor of reference air supplied to the cuvette, but not yet
inside the cuvette, and therefore uninfluenced by the cuvette stirring fans or the leaf itself.
𝒆𝒆
𝒊𝒊𝒊𝒊
partial
pressure is determined by the H
2
O IRGA during Reference phase.
𝒆𝒆
𝒎𝒎𝒐𝒐𝒐𝒐
is defined as the partial pressure of water vapor in the air inside the cuvette, surrounding the leaf.
This air is both highly mixed by the stirring fans and influenced by transpiration water vapor from the leaf.
𝒆𝒆
𝒎𝒎𝒐𝒐𝒐𝒐
partial pressure is determined by the H
2
O IRGA during Analysis phase. As related to the calculated
values in the TARGAS-1 display:
𝒆𝒆
𝒊𝒊𝒊𝒊
=
𝑯𝑯𝟐𝟐𝑯𝑯𝑯𝑯
𝒆𝒆
𝒎𝒎𝒐𝒐𝒐𝒐
=
𝑯𝑯𝟐𝟐𝑯𝑯𝒂𝒂
Leaf Temperature
Calculate leaf temperature (
𝑻𝑻
𝒎𝒎𝒆𝒆𝒂𝒂𝒍𝒍
) from the energy balance
The Energy Balance technique estimates leaf temperature by equating energy flux into the leaf with
energy flux out of the leaf. The model includes incident solar radiation, leaf re-radiation, convective heat
transfer, and transpiration. (Note: the energy balance estimate for leaf temperature is one option on
TARGAS-1, the other option is to use chamber temperature.)
(A.6)
From Parkinson, 1983, the energy balance technique gives the difference between air and leaf
temperature as:
∆𝒐𝒐
=
�
𝑯𝑯 − 𝝀𝝀
×
𝑬𝑬
�
𝟎𝟎
.
𝟗𝟗𝟑𝟑
×
𝑴𝑴
𝒂𝒂
×
𝑪𝑪
𝒑𝒑
𝑯𝑯
𝒃𝒃
�
+ [
𝟒𝟒𝟒𝟒
× ((
𝑻𝑻
𝒄𝒄
+
𝟐𝟐𝟐𝟐𝟑𝟑
)
𝟑𝟑
)]
�
where:
𝑯𝑯
= incident radiation absorbed by the leaf
𝝀𝝀
= latent heat of vaporization of water
𝑬𝑬
= transpiration rate
𝑴𝑴
𝒂𝒂
= molecular weight of air
𝑪𝑪
𝒑𝒑
= specific heat at constant pressure
𝑯𝑯
𝒃𝒃
= boundary layer resistance to water vapor transfer, empirically determined for each cuvette by
the pseudo-leaf (filter paper) method. 0.93 converts it to that for heat transfer.
𝟒𝟒
= Stefan Boltzmann constant
𝑻𝑻
𝒄𝒄
= cuvette air temperature
𝑯𝑯
is calculated from the photon flux incident on the cuvette (
𝑸𝑸
), taking into account the ratio of infra-red to
visible radiation and typical reflection/absorption factors by the leaf:
