Automated exponential feedback weighting method for subtraction of heat losses from sap flow measured by the trunk heat balance method
Removal of heat losses from the measuring point is one of the crucial issues during processing of data from the sap flow sensors, including the trunk heat balance method (THB). Heat losses (here referred to as 'baseline') are usually estimated at the time of presumably no sap flow. Three main issues may decrease the accuracy of the estimation of baseline: (i) existing sap flow in the night, which did not stop due to ongoing night transpiration or xylem refilling, (ii) natural temperature gradients between heated and reference part of the measuring point, and (iii) long-time (i.e. seasonal) changes in the heat losses due to stem growth or changes in the stem heat conductivity. Therefore, neither the estimation of baseline each night, nor its estimation as a lowest value in a specific (i.e. 2-week period) provide the correct result. Experienced users may be able to estimate the baseline manually (taking into consideration i.e. vapor pressure deficit) but this is a subjective and lengthy approach. Here we present an automated baseline subtraction based on the Exponential Feedback Weighting method. This method considers the nighttime sap flow, but it also removes outliers due to natural temperature gradients. We compared various averaging intervals of this method for the removal of baseline, from one day (i.e. estimation at every night) to 28 days in 1-day increments, estimation at the beginning and at the end of the season and manual subtraction by user (considering the weather data). Best results were provided by the 5-days weighting average, which was closest to the manual subtraction, and also provided enough flexibility for changes in baseline due to changes in the stem heat conductance.
Kučera, J., Vaníček, R. and Urban, J. (2020). Automated exponential feedback weighting method for subtraction of heat losses from sap flow measured by the trunk heat balance method. Acta Hortic. 1300, 81-88
sap flow, trunk heat balance method, baseline subtraction, heat losses