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Sap Flow Sensors

The Greenspan Sapflow Sensors provide researchers, horticultural and forest managers with a simple, integrated system for low-cost, automated monitoring of tree water use.

Developed in partnership with CSIRO Australia, the Sapflow design incorporates technological advances to provide more accurate and reliable flux estimates.

Accurate & reliable tree scale water use measurements

The Greenspan Sapflow Sensor System offers a unique combination of advanced features:

Utilises heat pulse technique to determine Sapflow, which has proven more accurate and reliable on woody stems and trunks than the heat balance method used by other systems.
System is light and easy to carry and allows for installation of multiple sensors per tree with a choice of standard or miniaturised sensors.
Intelligent logger interface software (SAPCOM) provides real time sapflow output complete control over logging parameters.
A graphical Test Mode provides a visual check of system setup.
Self-calibrating feature ensures simplicity of use, yet provides accurate, repeatable sapflow records under demanding conditions.
Designed to withstand severe weather conditions over long periods with minimal maintenance.
SAPCAL is a fully integrated windows-style package providing a complete environment for analysing and plotting sapflow data.

Some background on Sapflow Technology

There is increasing awareness amongst plant physiologists that questions about tree or canopy water use are difficult to answer with leaf scale measurement. Physiologists are increasingly using techniques to measure processes at the appropriate scale, i.e. at least at tree level.

The Greenspan sapflow sensor is an accurate and reliable tool for tree scale water use, being both low cost per unit point measurement, and highly robust and portable. The original CSIRO prototype was specifically designed for larger scale measurement to calculate river red gum plantation water budgets.

Physics of the Compensation Method

The heat pulse, or compensation, method, which has been described as "the most elegant method to measure flow velocities" (Zimmerman 1983), was first used by Rein (1928) to measure the flow velocities of blood in animals and was later adapted by Huber (1932) to measure xylem sap velocities.

Xylem sapflow velocities (also referred to as sapflux densities ) are measured by injecting a small pulse of heat into the conducting wood of the tree. The sapflow velocity is determined from the rate of ascent of this pulse within the conducting wood of the tree. Sapflow velocity multiplied by the cross sectional conducting wood area gives the volume flow per unit time.

After taking a core sample to determine boundaries of the heartwood and the cambium, the probe is inserted to a depth which ensures the location of the thermistors within the sapflow boundaries.

Data Collection

At predetermined time periods, the probe takes measurements which are stored in the logger attached to the probe. Using a standard source of power, e.g. a 12V 5.7 Amp-hr gel cel, three months of data (on average depending on reading intervals) can be collected.

A portable computer is used to download the data from the logger to be analysed using the SAPCAL program either in the field, or at home base.

Wounding

From research carried out with the CSIRO Division of Water Resources, the tissue growth around or within the probe wound does not introduce significant error even in the fastest growing trees over a reasonable time period. A minimum time in the fastest of trees is usually six weeks.

However, because of the portability of the system (it weighs only 0.5 kilo), probes can be moved and replaced at a frequency determined by the researcher as necessary. The heat pulse technology does not introduce very high temperatures (30degC) to the plant, and then only for minute time periods. Therefore, it does not cause growth distortions, or fungal attacks in plants which can be a problem with other techniques. Because of the insertion of the probe, the system provides accurate data in real time.

Applications of Sapflow Technology

Since its introduction in April 1992 by Greenspan Technology, the Sapflow Sensor has proved to be a very effective hydrological engineering and research tool. It has seven main areas of application, and project examples include:

1. Agroforestry

Assessing the role of low density woodlands in controlling groundwater recharge. Water use of native trees on a salinity and waterlogging gradient. Broombrush: a potentially high wateruse crop.
Water use and productivity of fodder and eucalypts.

2. Effluent Reuse

Water use of an effluent irrigated blue gum woodlot.
Water use of Eucalyptus camaldulensis irrigated with saline drainage water.

3. Horticulture

Determination of optimal wateruse efficiency in high density Citrus plantings.
Transpiration and water relations of irrigated peach trees.
Transpirational water flow in mango.

4. Native Forests

Canopy transpiration and conductance across rainforest and eucalypt forest boundaries.
Moisture and energy budgets of a dry eucalypt forest.
Tree water use in a native poplar box community.

5. Plantation Forestry

Water use of E. globulus and E. nitens in irrigated and rainfed plantations.
Site adaptation in Eucalyptus camaldulensis.
The effect of management on water use by Pinus radiata.
The timing and movement of sugars in sugar maple plantings.

6. Riparian and Wetland Management

Development of a water budget for a riparian forest community.
Management of riparian vegetation in the Macquarie Marshes.
The hydrology of river red gum forests.
Water requirements of fringing wetland trees on the Swan coastal plain.
Water use characteristics of Melaleuca swampland.

7. Urban

Water use by isolated trees in urban areas and the effects on building foundations.

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Advanced Measurements and Controls Inc.