Sap flow meters explained

25 August 2022


Sap Flow Sensors are an ideal instrument to help measure transpiration flow in stems, trunks and branches of trees.  Sap flow is the movement of fluid and is generally measured in the xylem tissue of plants.

So why measure sap flow?  Because measuring sap flow allows tree and plant water use to be calculated on a daily or seasonal basis and therefore provides important information for irrigation choices. A very good example of a Sap Flow Meter is ICT’s SFM1X available from HydroTerra (see image).  This is a complete package including sap flow sensors, datalogger, interface software, internal battery and IoT connectivity.

ICT SFM1X Sap Flow Meter

How does this sensor work?  The SFM1X uses a technique based on the Heat Ratio Method to determine sap flow (see graphic).  The SFM1x Sap Flow Meter consists of two temperature sensing needles arranged equidistance above and below a central heater. These needles are inserted into the water conducting tissue of the plant by drilling three small parallel holes. Heat is then pulsed every 10 minutes into the water conducting tissue of the plant. The heat is used as a tracer to directly measure the velocity of water movement in the plant stem. Find out more.



Sap flow sensor operating principle

Common applications include vineyards, orchards, macadamia crops, forestry, palm oil production, revegetation in mining projects and phytoremediation projects on old landfill sites. Even monitoring tree health in an urban environment is an application. An example application from a macadamia farm resulted in a 15 to 20 per cent reduction in irrigation and a significant saving for the grower.