Designing Sap Flow Measurement Projects

Sap flow (and the subsequent plant water use) measurement projects require careful planning and design, including:

determining the number of sap flow sensors needed
positioning of the sensors for sap flow measurement
positioning of the sap flow sensors for communication
the power supply solution.

Addressing this will optimise sap flow and plant water use measurement projects.

Determining how many sap flow meters are needed

Sap flow to measure plant or tree water use is a powerful tool. Combined with other simultaneous measurements, the resulting data can be foundational in the understanding of responses to climate change, new irrigation strategies, species selection for specific criteria, or measurement of water use for a catchment or forest. These objectives will determine the requirements when considered along with basic scientific rigour and statistical replication.

Planning stage

Trees are not homogeneous around their circumference, rather there will be differences in the position of the heartwood and sapwood relative to the proximity of other trees and to the available soil resources (moisture, nutrients etc). Through pre-research planning, the sapwood area is best identified using the HFD1 or HFD2, or a sap wood corer. The HFD1 or HFD2 allow measurements up to 100 mm into the trunk, and at 8 measurement points within that range. Installed at the cardinal points on the tree (north, south, east, and west) at the planning stage over a period of at least one week, the diurnal dynamics can be analysed. Furthermore, the use of the HFD1 or HFD2 will allow for the identification of thermsistor placement and needle lengths if a SFMx Sap Flow Sensor is used.

From the the planning exercise, determination of the diminishing return on the level of accuracy for 4, 3, 2, or 1 measurements per tree can be concluded, prior to an application of scientific benefit cost analysis to this level of accuracy per number of measurements per tree, versus the cost of the instruments. For example if the research requires an accuracy of greater than 5% of absolute water use of the tree, this may require at least 4 measurement points per tree, or it may only be a scoping study that needs to be indicative and an accuracy threshold of 50% might be adequate to provide such a feel for the situation, thus 1 instrument would be sufficient. The planning stage will determine this requirement through a little bit of extra work for one week at the front end of experimental design rather than regrets at the back end, when trying to publish.

Determining the need for custom sap flow needle lengths

Using the HFD1 or HFD2, non-destructive determination of the sap wood area can be undertaken. The use of the longer needles with the multiple measurement points allow the determination of the sap wood area, as well as the specific activity within the sap wood. Once this has been concluded, any specific requirements for needles can be identified and subsequently, ICT International will work to build needles to the meet the required dimensions.

As the sapwood area can vary around the tree circumference, the selection of the correct point(s) for measurements will have been identified in the planning stage.

Locating sap flow meters: measurements

Identification of the measurement points, along with the depth of the measurement, from the planning stage will allow for the most appropriate sensor positions to be selected. However, this does not answer the question of how many sensors (and in which plants/trees) are required.

Tree or plant selection

Applying basic scientific rigour and statistical design rules will lead to a minimum average of 3 trees per species, per site or per treatment providing quite robust data. The key to sap flow data is that it is a continuous high temporal resolution (10 min) logging of a plants integrated response to the abiotic environment. Thus, in some respects, each measurement is in some small way a replication in itself as the kinetic response is being repeatedly performed on the same piece of plant tissue. This is distinctly different to say photosynthesis measurements that are instantaneous and one off measurements of distinctly different pieces of plant material, hence greater statistical replication is required to filter out the ever changing sample set.

From this, and combined with any known geospatial variability, the exact number of trees and plants to be sampled can be calculated. ICT International is undertaking a project that will further refine this for future project planning specifically in relation to use of a catchment.

Position of the sensor for measurements

The experimental planning process will have allowed for the knowledge of where to optimally locate the sensor, this may be the cardinal point that is towards the suns daily path, or towards the readily available water source (irrigation or similar).

Locating sap flow meters: communications

This section should be read in conjunction with Designing IoT Soil, Plant, and Environment Measurements Network Solutions (opens in a new tab).

Communication options

ICT International SFM1 and SFMx Sap Flow Meters are self contained instruments, complete with internal logging, communication, battery, and charging circuit. This provides a versatile instrument; however, the physics of radio communications means that consideration must be made to obstacles (building, terrain, or vegetation) that may disturb the radio signals and prevent clear transmission of data.

Once the optimal position on the tree has been decided, then the instrument must be positioned such that the radio signal has the least obstructions to the receiver. This includes ensuring that the tree itself does not obstruct the radio signal, which will occur depending on the diameter of the tree and the frequency used.

Calculating signal wavelength in centimetres

To calculate the wavelength in centimetres, λ (Lambda), the speed of light is divided by the frequency (in hertz), and then by 10,000:

λ = c/f/10000

Where:

λ (Lambda) = Wavelength in centimetres
c = Speed of Light (299,792,458 m/s)
f = Frequency in hertz

Common frequencies used in ICT International instruments have been approximately calculated as:

LoRaWAN:
- 470MHz (China): 63.79 cm
- 865MHz (India): 34.66 cm
- 868MHz (EU): 34.54 cm
- 915MHz (Australia and USA): 32.76 cm
- 923MHz (Australia and USA): 32.48 cm
CAT-M1 or NB-IoT
- 700MHz: 42.83 cm
- 900MHz: 33.31cm
- 1700MHz: 17.63 cm
- 1900MHz: 15.78 cm

As can be seen, the different wavelengths can be blocked by the tree that the sensor is attached to. A tree with a diameter in excess of the calculated wavelength will block the signal in that direction. Solutions to this include mounting the antenna away from the instrument or tree, or when planning a LoRaWAN network, positioning the gateway facing towards the side of the tree or plant the Sap Flow Meter is mounted on.

Powering the sap flow meters

The SFMx Sap Flow Meter is powered by an internal 18650 Li-Ion battery, and contains an internal solar charge controller. With a power supply that is non-polarity sensitive, connected solar panels to the instrument are a reliable power source that are simple to install.

Alternatively, power can be supplied by an external 12V battery (details can be found on the Powering IoT soil, plant, and environmental systems page) or for a laboratory/greenhouse environment a 110/240 volt AC to 24 volt DC convertor.

Solar power

Installing solar panels under a tree canopy is a challenge. ICT International has developed solutions that provide reliable power for low light conditions, such as those found in plantations or forest environments. These solutions utilise two solar panels (with a minimum power rating of 20 watts and connected in parallel with blocking diodes) where each is angled to capture maximum sunlight for different parts of the day.