[Insert Fig. 8 here]
4 Discussion
4.1 DRM vs. other heat-pulse based methods
The double-ratio method (DRM) presented here was more robust than other
common heat pulse methods (HRM, CHPM and Tmax) across a wide range of
sap velocity, in both theoretical and experimental tests. In theoretical
tests, the DRM produced the least bias and smallest variance among the
four methods. In experimental tests, the DRM accurately tracked diel
trends in tree water use (as measured by lysimetry) from 0-7 kg
h-1 or 0-100 cm h-1.
The applicable upper limit for the HRM can be roughly estimated from
Eqn. 4 , given the resolution of temperature sensors and peak
rise in temperature. For example, if we assume:
- that the temperature rise at the proximal probe
(δ 1) is reliable if its output is at least 3
times the standard deviation of temperature noise due to sensor
resolution (approximately 0.005 K in our experiment), so that values
of δ 1 below 0.015 K are excluded; and
- that the largest observed value of the temperature rises at the first
downstream probe (δ 2) is 1.0 K,
then with a probe spacing of 1.5 cm and k = 0.0025
cm2 s-1, the upper limit of velocity
that the HRM can estimate is,