Articles
CAN WE IMPROVE HEAT-PULSE TO MEASURE LOW AND REVERSE FLOWS?
Article number
951_1
Pages
19 – 29
Language
English
Abstract
Thermal methods, including heat-pulse, have been used to measure sap flow for many decades.
In most cases, analysis and interpretation of the temperature signals has remained largely unchanged since the pioneering work of Marshall (1958) and the theoretical calibrations of Swanson and Whitfield (1983). Two recent advances to heat-pulse have been developed to improve measurements at low flows; the Average Gradient method (CAG) of Villalobos and Testi (2008), which modified the compensation method (CHP) to improve sensitivity at low flows (i.e. < 10 cm h-1), and the Heat Ratio Method (HRM) of Burgess et al. (2001) which employs temperature ratios measured at equal distances either side of the heater source.
Preliminary results from CHP, CAG and HRM are compared against lysimeter data from potted trees to demonstrate the performance of these methods across a range of flows.
We also propose and test two new methods to resolve low sap flow.
The first method we will call the Symmetrical Gradient method (SGM). It consists of averaging the temperature-difference signal (ΔT) of two probes that are equidistant from the heater.
The second method we will call the Maximum Derivative Method (MDM). The analysis of MDM is based on the maximum rate of change of the temperature difference curve (i.e. the derivative, ΔTmax). We have applied this derivative approach to temperature data collected using both symmetrical and assymetrical probe arrangements.
Our measurements suggest these two indicators (ΔT and ΔTmax) are proportional to the sap flow across a wide range of flows.
Data suggests our new calibrated maximum derivative CMD method (CMD), is a simple and practical means to extend CHP over the low-flow regime.
In most cases, analysis and interpretation of the temperature signals has remained largely unchanged since the pioneering work of Marshall (1958) and the theoretical calibrations of Swanson and Whitfield (1983). Two recent advances to heat-pulse have been developed to improve measurements at low flows; the Average Gradient method (CAG) of Villalobos and Testi (2008), which modified the compensation method (CHP) to improve sensitivity at low flows (i.e. < 10 cm h-1), and the Heat Ratio Method (HRM) of Burgess et al. (2001) which employs temperature ratios measured at equal distances either side of the heater source.
Preliminary results from CHP, CAG and HRM are compared against lysimeter data from potted trees to demonstrate the performance of these methods across a range of flows.
We also propose and test two new methods to resolve low sap flow.
The first method we will call the Symmetrical Gradient method (SGM). It consists of averaging the temperature-difference signal (ΔT) of two probes that are equidistant from the heater.
The second method we will call the Maximum Derivative Method (MDM). The analysis of MDM is based on the maximum rate of change of the temperature difference curve (i.e. the derivative, ΔTmax). We have applied this derivative approach to temperature data collected using both symmetrical and assymetrical probe arrangements.
Our measurements suggest these two indicators (ΔT and ΔTmax) are proportional to the sap flow across a wide range of flows.
Data suggests our new calibrated maximum derivative CMD method (CMD), is a simple and practical means to extend CHP over the low-flow regime.
Publication
Authors
S.R. Green, R. Romero
Keywords
compensation heat-pulse, heat-ratio, average-gradient, maximum-derivative method, sap flow
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