We are frequently asked about making measurements of transpiration with the AP4.It is possible to convert from conductance readings to transpiration readings, and the basic equation used to define conductance will do this (see the AP4 manual, page 85):
E = cv (wvdc – wvdl)
where E = flux density of water vapour, in g m-2 s-1,
cv = conductance in velocity units, m s-1,
wvdc = water vapour density, at the cup RH and temperature, g m-3.
wvdl = water vapour density, at the leaf RH and temperature, g m-3.
However, this measurement will only apply to the conditions within the cup itself. It cannot be used as a basis for calculating the transpiration loss from a plant or canopy because the leaf temperature, air temperature, vapour pressure deficit and boundary layer conductance will all be different in the cup from in the field. We don’t include the option to measure transpiration rate in the AP4 so that users are not tempted to make this extrapolation.
In fact, although the ability to take a reading in transpiration units is built into the LI-COR LI1600, they also strongly recommend that it is not used as a basis for calculating in situ transpiration loss, and instead make the calculation based on the diffusion conductance. The paper published by Dale McDermitt at LI-COR (McDermitt, D.K., 1990.Sources of Error in the Estimation of Stomatal Conductance and Transpiration from Porometer Data.HortScience 25: 1538-1548) contains a very good explanation of how to do this.