Improving Heat-Pulse Methods to Extend the Measurement Range Including Reverse Flows

摘要

Traditional heat-pulse methods are not well suited to measuring either very high or very low sap flows and few methods can measure reverse flow. We have analysed two new methods that potentially extend the measurement range. Both methods can be used by modifying the analysis algorithm and adjusting the probe positions of common heat-pulse methods, with no change to existing equipment. The first method we will refer to as the Symmetrical Gradient method (HPSG). It consists of averaging the temperature difference signal of two probes (ΔT-dar) that are equidistant from the heater. The second method we will refer to as the Symmetrical Derivative method (HPSD). It uses the same symmetrical probe configuration. However, the analysis is based on the maximum rate of change of the temperature difference curve (i.e. the derivative, ΔT'). We use computer modelling to show that these two indicators (ΔT-dar and ΔT'max) are proportional to the heat-pulse velocity across a wide range of positive and negative flows. Hence, both metrics can be used to determine the actual sap flux density with acceptable measurement errors and good measurement sensitivity. We present results from field experiments on a willow tree (Salix alba L.) that was set up to compare our new methods against other heat-pulse techniques. We show that HPSG and HPSD both provide reliable data across a very wide range of flows. We are currently working on other field experiments to further refine our use of HPSG and HPSD to estimate tree transpiration and even to observe sap flow in roots.