Technical note on long-term probe misalignment and proposed quality control using the heat pulse method for transpiration estimations

Abstract. Whole-plant transpiration is a crucial component in the hydrological cycle and a key parameter in many disciplines like agriculture, forestry and ecology. Sap flow measurements are one of the most widely used methods to estimate whole-plant transpiration in woody species due to its wide application range and its ready automation for continuous data readings. Several different methods have been developed and adjusted to different climatic conditions and wood properties. However, the scientific literature also identifies several sources of error in the method that needs to be accounted for; misalignment of the probes, wound to the xylem, thermal diffusivity and stem water content. This study aims to integrate probe misalignment as a function of time to improve readings during long-term measurements (> 3 months). We conclude that even when geometrical misalignments errors are small, the introduced corrections can imply an important shift in sap flow estimations. Additionally, we propose a new set of statistical information to be recorded during the measurement period to use as a quality control of the heat ratio readings obtained from the sensors. By using relative standard deviation and slope as quality indicators we concluded that no general time limit can be decided for all sensors but should rather be determined from individual performance over time.