Three different methods of measuring the transpiration in young potted
walnut frees (Juglans regia L, cv Lara) were compared over a 115-d pe
riod in 1990. Weighing of the pots was used as the reference for compa
rison with the sap flow rate given by a thermal method and the rate of
transpiration by gas exchange determination. Several sap flow meters
were installed at different heights on the same tree in order to estim
ate the storage of water between them. A good calibration of the sap f
low measurement method was obtained in those trees which had been subj
ected to a short period of severe water stress, leading to an excellen
t agreement with hourly or daily values of transpiration by weighing.
Gas exchange determination systematically underestimated transpiration
. Nevertheless if seems possible to improve gas exchange determination
of transpiration by: i) accurate calibration of all the sensors; and
ii) introducing a pressure correction in the calculation of water vapo
ur density in the air. Although the accuracy of the sap flow method wa
s high (3%), it was not possible to assess within tree water movement
correctly.