Effet de la longueur de la portée des sondes de dissipation thermique de type Granier sur des mesures de densité de flux de sève

Abstract

Abstract• Granier-type thermal dissipation sensors measure sap flux density (u) by using the temperature difference between the heater and the reference probe. To detect u correctly, heat must not be transferred to the reference probe by thermal conduction. The distance across which heat can be transferred by conduction is important for the span length of a sensor and spacing of a number of sensors.• To validate span lengths and spacing of sensors, we used numerical simulations to calculate the potential distance across which heat can be transferred by conduction. We compared measurements with an original and a modified sensor for a Japanese red pine (Pinus densiflora) from December 2004 to May 2005. The span length of the original and the modified sensor is 15 and 4 cm, respectively.• Numerical simulations showed that span length and spacing of Granier sensors should be more than 10 cm for trees in which u ceases for a few hours before the predawn period. The modified sensor underestimated u by 18-46% in winter (December-March) because its reference temperature was increased by heat transferred by conduction. The modified sensor measured u correctly in warm seasons, and only underestimated the annual amount of transpiration by 6%. Mots-clés :technique de dissipation thermique / altération du capteur / conduction thermique / sous-estimation de la densité de flux de sève Résumé -Effet de la longueur de la portée des sondes de dissipation thermique de type Granier sur des mesures de densité de flux de sève.• Les capteurs de dissipation thermique de type Granier mesurent la densité du flux de sève (u) en utilisant la différence de température entre le radiateur et la sonde de référence. Pour détecter u correctement, la chaleur ne doit pas être transférée à la sonde de référence par conduction thermique. La distance à travers laquelle la chaleur peut être transférée par conduction est importante pour la longueur de la portée d'un capteur et l'espacement de nombreux capteurs.• Pour valider les longueurs des portées et l'espacement des capteurs, nous avons utilisé des simulations numériques pour calculer la distance potentielle à travers laquelle la chaleur peut être transférée par conduction. • Des simulations numériques ont montré que la longueur de la portée et l'espacement des capteurs Granier devraient être supérieurs à 10 cm pour les arbres dans lesquels u cesse pendant quelques heures avant la période précédant l'aube. Le capteur modifié sous-estime u de 18 à 46 % en hiver (Décembre-Mars), car sa température de référence a été augmentée par la chaleur transférée par conduction. Le capteur modifié mesurée u correctement pendant les saisons chaudes, et seulement sous-estime la transpiration annuelle totale de 6 %.

A simple calibration improved the accuracy of the thermal dissipation technique for sap flow measurements in juvenile trees of six species

“…However, a trade-off exists between minimizing errors associated with heating the reference probe at night and errors caused by thermal gradients (Lu et al 2004). Iida and Tanaka (2010) reported no difference in F S estimates made during the growing season on Pinus densiflora trees when probes were spaced 4 or 15 cm apart, suggesting that substantial probe separation is not necessary. Our calibration compensated for any potential interaction that might have occurred between the heater and reference probes, suggesting another reason why species-specific calibrations are important for improving accuracy.…”

“…This 4 cm spacing distance is similar to the 5 cm distance used by Granier (1985) to develop the original a and b parameter values for the thermal dissipation technique. In a study of the effect of spacing on the accuracy of thermal dissipation measurements, Iida and Tanaka (2010) found that sensor spacing (4 or 15 cm) did not impact mean daily F S during the growing season. If spacing between the heater and reference probe was to influence F S estimates, it would likely do so by influencing DT max as heat should move upward when transpiration is occurring and only influence the reference probe when transpiration is not occurring or is very low.…”

Calibration of Granier-Type (TDP) Sap Flow Probes by a High Precision Electronic Potometer

“…Externally, the probes are provided with a coil winding of an insulated resistance constantan wire (copper–nickel alloy). Although in many works this distance was changed [27,39], we decided to keep it at 10 cm to avoid underestimation of sap flow [40]. Before to insert the sensors in the stem, their windings are inserted in a metal sleeve shaped as an aluminum micro-tube, 2 mm in diameter [41] that makes the thermal exchange with the wood more uniform.…”

Interrelationships among dry season leaf fall, leaf flush and transpiration: insights from sap flux measurements in a tropical dry deciduous forest

“…The power supply for the heater of each sensor was controlled by the regulator (GN‐P; Environmental Measurement Japan, Co., Ltd., Tokyo, Japan), and the outputs of the sensor were sampled every 10 s, with 10‐min averages recorded using data loggers (CR1000; Campbell Scientific). F D was then calculated using a procedure described in the literature (Iida et al , ; Iida and Tanaka, ), with a correction applied (Clearwater et al , ) in cases when SW was less than the sensor length (Table ). The main objective of measuring the sap flux density was to determine the seasonal changes in transpiration, the timing of leaf fall and flush, and the relative importance of transpiration during the dry season on an annual scale for each tree.…”