Optimization of Penman Monteith transpiration rate by gas exchange and stem water transport measurement in grapevines

To assess the accuracy of the prediction of plant transpiration by the Penman-Monteith model, sap flow of grapevines was measured directly using the stem heat balance technique. Leaf transpiration was also estimated by extrapolating single leaf gas exchange measurements to the whole canopy. Two days in the month of September 1997 were considered, with progressively decreasing soil moisture content. The relative differences between calculated and measured transpiration rates were small, compared with the diurnal variations in global radiation. The response of sap flow measurements to environmental changes lagged behind that of transpiration rate estimates based on leaf gas exchange readings, and may be due to the existence of water capacitances in the plant.