SAP FLOW MEASUREMENTS IN GRAPEVINES - SCOPE AND LIMITS OF HEAT BALANCE AND GRANIER-TYPE SENSORS

The original calibration of Granier was extended from a sap flux density of 140 10^-6 m³ m^-2 s^-1 to around 225 10^-6 m³ m^-2 s^-1 without any error and to 380 10^-6 m³ m^-2 s^-1 when accepting an error of up to 7%. In contrast to the heat balance method a time lag of around 20 min between calculated and real flow is evident, which was attributed to the thermal mass of the sensor itself. The time lag and the consequently dampened response of the system caused a very low accuracy over short time periods reducing the value for detailed plant physiological investigations. However, when integrating over longer time intervals, much of the errors cancelled out. For daily values the maximum error was within +/-10% and after a period of 89 days only 1.5% error remained. This method is thus best suited for long term measurements of total water use.

The heat balance system proved to be a very responsive system reflecting even short term fluctuations in sap flow of 10 min. However, as even 10–20 year old xylem vessels transport significant amounts of water, large inhomogeneities in sap temperature developed in the stem. Consequently only very small water fluxes, where a relatively homogeneous temperature profile was achieved, were measured correctly. The more the flow rate increased, the larger the resulting inhomogeneity was. At high flow rates the calculation of the heat balance resulted in an overestimation of 50 – 100%. The heat balance method is thus not suited for thicker stems of grapevines due to significant water conduction even in very old xylem vessels.

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