Numerical Modelling of Sap Flow Meters

March 15, 2023

Sap flow is difficult to directly observe in trees, so measuring sap flow usually involves using heat as a tracer. To determine, how and when sap is flowing in maple trees, we are developing an external sap flow meter for use on saplings.  

The first sap flow meters were used in the 1950s, and involved inserting a probes into the trunk of a mature tree. A heat pulse was released, and the temperature measured above and below the pulse. The time required by the sap to transport heat to a particular location is used to calculate sap flow. 

But we need a different method for measuring sap flow in our research, because the horticultural row-crop approach uses saplings, with stem diameters of only a few centimetres. Inserting probes is too destructive. Instead, so we are using an external ring heat source, plus external thermistors to measure the heat pulse.

Close-up of heat pulse sensor attached to sapling stem for sensor calibration (outer insulation removed). Credit: Matt Rennie.

To support this work, Morgan Stuthridge, currently studying mechanical engineering at University of Canterbury, undertook a summer research project to develop a numerical model of the external sap flow meter.

The 2D model simplified the wood structure into a cylinder, and modelled the sap wood (not the bark) as a porous medium using COMSOL software for finite element analysis. This will be used to verify experimental data from our laboratory and field measurements.

This FEM model shows how heat propagates (upwards in this instance) from the sensors heating ring through the saplings stem after the heat pulse. This model helps us to  understand heat transfer of the sap flow sensor.

Plant Physiology