To fully characterize the water and energy balances of a field crop it
is necessary to separate soil evaporation from total canopy evapotran
spiration. An alternative to measuring soil evaporation directly is to
measure the vapor exchange at the soil surface in order to assess mas
s transfer there. The objective of this research was to develop a meth
od for measuring vapor exchange coefficients beneath plant canopies. T
he system consists of a Mariotte siphon water supply apparatus and a f
elt-covered vapor source plate, both of which were tested under labora
tory and field conditions. A detailed description of the devices, thei
r construction and operation, and a sample data set are presented. The
system is suitable for long-term field installation and is capable of
accurately measuring vapor transfer rates over short time intervals.