[Insert Fig. 10 here]
The HRM requires a known value of k to correct errors due to
probe misalignment. Typically, a constant k is assumed. Burgess
et al. (2001) used the Tmax method (Eqn. 15) to calculate k . This
is a somewhat circular procedure because the Tmax method requires
knowledge of probe locations. It would be preferable to concurrently
solve for all three unknowns –∆x 1,
∆x 2 and k – as shown by Chen et al.
(2012). Such a calibration approach may not be achievable unless
constrained with assumptions or additional calculations (e.g. sap
velocity independently inferred via an empirical function in Chen et al.
(2012)). We suggest an alternative calibration procedure. First,
alignment is checked using the HRM method suggested by Burgess et al.
(2001). Second, the non-unique solution is constrained by comparing
velocities using the CHPM, which is independent of k . Finally,k is identified with the Tmax method under zero flow conditions
(see the misalignment calibration procedures in Methods S4 ).
Fig. 11 below compares such calibration results. When the
average solution is chosen from the HRM method, sap velocities estimated
using the DRM match well with those estimated using the CHPM. In this
scenario, any misalignment is well corrected (CHPM is sensitive to
misalignment) and k is appropriately estimated (DRM is sensitive
to k at low flow).
The three-probe approach used by the DRM allows diagnosis of
misalignment corrections. Combined with calculations based on the CHPM,
it also allows for detemination of both k and misalignment under
moderate conditions (e.g. typical day-time).