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

Abstract

Thermal dissipation probe (TDP) method (Granier, 1985) is widely used to estimate tree transpiration (i.e., the water evaporated from the leaves) because it is simple to build, easy to install, and relatively inexpensive. However, the universality of the original calibration has been questioned and, in many cases, proved to be inaccurate. Thus, when the TDP is used in a new species, specific tests should be carried out. Our aim was to propose a new method for improving the accuracy of TDP on trees in the field. Small hazelnut trees (diameter at breast height 5 cm) were used for the experiment. The response of TDP sensors was compared with a reference water uptake measured with an electronic potometer system provided with a high precision liquid flow meter. We equipped three stems where we measured the sap flow density, the sapwood area (by using fuchsine), the total tree water uptake (reference), and the main meteorological parameters during summer 2018. Results confirmed that the original Granier's calibration underestimated the effective tree transpiration (relative error about -60%). We proposed a new equation for improving the measurement accuracy within an error of about 4%. The system proposed appeared an easier solution compared to potted trees and particularly suitable for orchards, thus contributing to improve the irrigation management worldwide.

Keywords: hazelnut; trees; water management.

Figure 1

Experimental setup: (1) Potometer with the first water reservoir, (2) porous layer, (3) second water reservoir, (4) connection pipe, (5) high precision flow meter sensor, (6) sap flow sensors, (7) data acquisition system, (8) temperature and relative humidity sensor.

Figure 2

Average sap flow densities from two probes per tree in day 177, 198, and 240 of summer 2018. Bars represent the standard error. VPD (hPa) appears in the background.

Figure 3

Reference sap flow density (black line), original thermal dissipation probe (TDP) output, Granier’s equation (red line), corrected TDP output, corrected equation (green line) and VPD (grey line), for the three performed tests.

Figure 4

Relationship between k Equation (1) and reference sap flow density (RSD: Black dots: Day 177; grey dots: Day 198; white dots: Day 240). The red line is the species-specific calibrated model Equation (5), the yellow line represents the original Granier calibration.

Figure 5

Relationship between the reference sap-flow density (RSD, from liquid flow meter) and the sap flow calculated with the Granier’s equation (Fd, left) and the new corrected equation (Fd_c, right). Different colors show different stems at day 177 (red), 198 (green), 240 (blue).

Figure 6

Transversal section of one of the stems perfused with a 0.05% aqueous solution of Acid Fuchsine. The whole area appeared active and around 25% was constituted of parenchymatic rays (white stripes).