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Which Sap Flow Meter do I need?

There are a variety of instruments currently available to measure the sap flow of plants. ICT International has developed dedicated Sap Flow Meters intended for field deployment using two different methods for measuring sap flow, the heat ratio method (HRM – SFM1) and the heat field deformation method (HFD – HFD8)

Heat Ratio Method (HRM)

Temperature sensor needles are arranged equidistant from a central heater element, typically 5mm above and below. The heater is pulsed periodically with a discrete amount of heat (typically 20 Joules) at a set interval. The heat pulsed into the water conducting tissue is moved by the sap stream and used as a trace to directly measure the velocity of water movement.

Developed by the University of Western Australia and partner organisations, ICRAF and CSIRO, the HRM principle has been validated against gravimetric measurements of transpiration and used in published sap flow research since 1998. The Heat Ratio Method (HRM) is an improvement of the Compensation Heat Pulse Method (CHPM). Being a modified heat pulse technique, power consumption is very low, using approx 70 mAmp per day at a 10 minute temporal sampling interval under average transpiration rates.

Heat Field Deformation (HFD)

The HFD method has been in use since the 1990’s. Over the years many scientific studies have been published in leading international journals such as New Phytologist and Tree Physiology.

The HFD technique is a thermodynamic method based on measuring the dT of the sapwood both symmetrically (in the axial direction, above and below) and asymmetrically (in the tangential direction or to the side) around a line heater.

The heater is continuously heated at approx 50 mA and generates an elliptical heat field under zero flow conditions. Sap flow significantly deforms the heat field by elongating the ellipse. The symmetrical temperature difference (dTsym) allows bi-directional (acropetal and basipetal) and very low flow measurements, whereas asymmetrical temperature difference (dTas) is primarily responsible for the magnitude of medium and high sap flow rates.

By using the ratio of measured temperature differences and applying correction for each measurement points local conditions using the adjustable K-values, the common features of the medium (such as variable water content, natural temperature gradients, and wound effects) have negligible impact on sap flow calculations.

The value for parameter K is equal to the absolute value of dTs-a or dTas for a zero flow condition. Under flow conditions the parameter K can be extrapolated with accuracy using linear regression.


From Nadezhdina, 2002:

“Large systematic errors of -90 to 300% were found when it was assumed that sap flow was uniform over the sapwood depth. Therefore, we recommend that the radial sap flow pattern should be determined first using sensors with multiple measuring points along a stem radius followed by single-point measurements with sensors placed at a predetermined depth. Other significant errors occurred in the scaling procedure even when the sap flow radial pattern was known.
These included errors associated with uncertainties in the positioning of sensors beneath the cambium (up to 15% per 1 mm error in estimated xylem depth), and differences in environmental conditions when the radial profile applied for integration was determined over the short term (up to 47% error).
High temporal variation in the point-to-area correction factor along the xylem radius used for flow integration is also problematic. Compared with midday measurements, measurements of radial variation of sap flow in the morning and evening of sunny days minimised the influence of temporal variations on the point-to-area correction factor, which was especially pronounced in trees with a highly asymmetric sap flow radial pattern because of differences in functioning of the sapwood xylem layers.”

From Nadezhdina, 1998:

Two temperature differences, dTsym and dTas, are recorded in axial and tangential directions respectively around a linear heater in a certain stem section. They are then used to describe the deformation of the heat field by sap movement. Applying such sensor geometry, we can calculate the additional temperature gradient dTs-a =dTsymdTas. The value of dTsym alone provides precise measurements of zero or very low flow rates in both upward and downward directions, while the two other temperature differences, dTas and dTs-a, are primarily responsible for the magnitude of medium and high sap flow rates. A wide range of flow rates can be calculated using the ratio (K+dTs-a)/dTas, where parameter K characterises a certain temperature difference reflecting peculiarities of each measured point and its surroundings under zero flow conditions. The heat field, but not its deformation, is affected, for example, by air temperature or fluctuations in power supply. This effect is reflected by a shift of the mentioned ratio and hence can be corrected by a change in K. Thanks to this feature the surrounding medium has a negligible impact on sap flow calculations.

Independent testing trials, using different methods as reference, have been carried out in plants of different species and sizes. A linear relationship was found between measured flow rates and direct water losses performed with three small potted woody plants. Sap flow pattern measured by the HFD sensor in a lards Scots pine tree was well followed by the sap flow pattern derived from measures of stem diameter changes. The comparison of the HFD method with the known trunk tissue heat balance method was also positive. The important ability to measure very low, zero and vectorial (including reverse) flows by the HFD method was verified during numerous experiments with branch, root or stem severing.

One of the main features of the HFD method – measurements of flows of any direction simultaneously in a series of points along xylem radius at the same stem sector – makes this method a useful and convenient tool for detailed studies of tree hydraulic architecture and water redistribution within tree conducting xylem.


The SFM1 is based on the Heat Ratio Method (Burgess et.al. 2001). Only 2 radial measurement points are used to provide the greatest accuracy per unit cost.

Heat Ratio Method

Using two measurement points enables the radial profile to be characterised, ensuring an accurate measurement of total plant water use for the least cost, designed for ease of deployment.

The SFM1 probes consist of three 35mm long needles integrally connected to a 16-bit microprocessor. The top and bottom probes contain two sets of matched and calibrated high precision thermistors located at 7.5mm and 22.5mm from the tip of each probe. The third and centrally located needle is a line heater that runs the full length of the needle to deliver a uniform and exact pulse of heat through the sapwood.

The SFM1 is suitable for most species in most applications.


The HFD8-100 is based on the Heat Field Deformation method (Nadezhdina 1998). It is designed specifically for very high accuracy (10mm resolution) measurement of water movement in large trees. The 8 measurement points are designed to cover the entire water conducting xylem and construct a comprehensive radial profile of sap flux density.


The high temporal resolution of the HFD series of Sap Flow Meters (1 second sampling intervals) also enables the definitive mapping of the hydraulic architecture of plant water use. HFD Sap Flow Meters provide the unique ability to empirically measure the sap flux density radially across the sap wood of a tree.


The HFD8-50 serves the same function as the HFD8-100, however the HFD8-50 has shorter needles and a 5mm resolution. The HFD8-50 is ideal for thin ring porous tropical forestry species where the 5mm resolution can be the difference between accurate data and no data at all.


Empirically derived radial sap flow profiles developed using HFD technology can be used to enhance SFM1 Sap Flow Meter data sets that characterise the radial profiles with the standard 2 measurement point design.

Nadezhdina, N., Cermak, J., & Nadezhdin, V. (1998). Heat field deformation method for sap flow measurements. Paper presented at the Proceedings of the 4th international workshop on measuring sap flow in intact plants. Publishing House of Mendel University, Czech Republic.

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Bleby, T. M., Burgess, S. S., & Adams, M. A. (2004). A validation, comparison and error analysis of two heat-pulse methods for measuring sap flow in Eucalyptus marginata saplings. Functional Plant Biology, 31(6), 645-658. http://www.publish.csiro.au/paper/FP04013.htm

Buckley, T. N., Turnbull, T. L., Pfautsch, S., & Adams, M. A. (2011). Nocturnal water loss in mature subalpine Eucalyptus delegatensis tall open forests and adjacent E. pauciflora woodlands. Ecology and evolution, 1(3), 435-450. http://onlinelibrary.wiley.com/doi/10.1002/ece3.44/pdf

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Gharun, M., Turnbull, T. L., Pfautsch, S., & Adams, M. A. (2015). Stomatal structure and physiology do not explain differences in water use among montane eucalypts. Oecologia, 177(4), 1171-1181. http://link.springer.com/article/10.1007%2Fs00442-015-3252-3

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Pfautsch, S., Dodson, W., Madden, S., & Adams, M. A. (2015). Assessing the impact of large‐scale water table modifications on riparian trees: a case study from Australia. Ecohydrology, 8(4), 642-651.  PDF

Pfautsch, S., Keitel, C., Turnbull, T. L., Braimbridge, M. J., Wright, T. E., Simpson, R. R., … & Adams, M. A. (2011). Diurnal patterns of water use in Eucalyptus victrix indicate pronounced desiccation–rehydration cycles despite unlimited water supply. Tree physiology, 31, 1041-1051. doi:10.1093/treephys/tpr082 http://treephys.oxfordjournals.org/content/31/10/1041.full.pdf+html

Pfautsch, S., Peri, P. L., Macfarlane, C., van Ogtrop, F., & Adams, M. A. (2014). Relating water use to morphology and environment of Nothofagus from the world’s most southern forests. Trees, 28(1), 125-136.

Resco de Dios, V., Díaz‐Sierra, R., Goulden, M. L., Barton, C. V., Boer, M. M., Gessler, A., … & Tissue, D. T. (2013). Woody clockworks: circadian regulation of night‐time water use in Eucalyptus globulus. New Phytologist, 200(3), 743-752. http://onlinelibrary.wiley.com/doi/10.1111/nph.12382/abstract

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Van de Wal, B. A., Guyot, A., Lovelock, C. E., Lockington, D. A., & Steppe, K. (2015). Influence of temporospatial variation in sap flux density on estimates of whole-tree water use in Avicennia marina. Trees, 29(1), 215-222. http://link.springer.com/article/10.1007/s00468-014-1105-z

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Guyot, A., Ostergaard, K. T., Fan, J., Santini, N. S., & Lockington, D. A. (2015). Xylem hydraulic properties in subtropical coniferous trees influence radial patterns of sap flow: implications for whole tree transpiration estimates using sap flow sensors. Trees, 29(4), 961-972. http://link.springer.com/article/10.1007/s00468-014-1144-5

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Van de Wal, B. A., Guyot, A., Lovelock, C. E., Lockington, D. A., & Steppe, K. (2015). Influence of temporospatial variation in sap flux density on estimates of whole-tree water use in Avicennia marina. Trees, 29(1), 215-222. http://link.springer.com/article/10.1007/s00468-014-1105-z