Improvement of the trunk heat balance method including measurement of zero and reverse sap flows

This paper deals with the theoretical analysis, numerical simulations and experimental measurements of temperature field for sap flow calculation based on xylem volume heating and is focused on the improvement of the trunk heat balance (THB) method. This method has been successfully applied for many years in different field conditions and tree species. When using the classical equations applied by this method, calculation of sap flow (Fw) was done considering heat losses by sap flow and other losses expressed as so-called “fictitious flow” (Fw.fic), which is visible in the recorded data (Fw.rec). The aim of this work was to improve the accuracy of the THB method and properties of the measuring system to enable measurements of zero and reverse sap flow. The improvement of the accuracy was achieved by: (1) a more accurate evaluation of the fictitious flow similarly to the process used when working with the heat field deformation (HFD) method, (2) by considering of Fw.fic dependence on the recorded sap flow Fw.rec. The improvement of sensor properties was done by the modification of sensor geometry (from vertical to horizontal) in order to allow for the measurement of zero and reverse sap flows.