When the true sap velocity is negative or very low, the uncertainty inV 23 will be greater than that forV 12 and V DRM will equalV 12, whereas at high velocities,σ 12 >σ 23 andV DRM will equal V 23. This selection procedure avoids the need to specify an arbitrary value of sap velocity at which to switch from one estimate to the other, as is required in the approach of Pearsall et al. (2014).
2.1.4 Identifying the optimal time window for averaging sap velocity estimates
In the HRM, V 12 is computed as the average of repeated instantaneous estimates during a fixed period after the heat pulse. ICT International suggests a 40 s time window, beginning 60 s after the pulse and ending at 100 s. The uncertainty-based approach of the DRM provides an objective way to identify the optimal time window: namely, by choosing the period that minimizes intrinsic uncertainty in sap velocity, computed as the standard error of the mean estimated velocity (based on the theoretical uncertainty inV DRM, not the standard deviation of a sample of instantaneous estimates):