Heat Field Deformation Sapflow

The Heat Field Deformation (HFD) technique is a radically new method for measuring sapflow. It is ideally suited to sapflow research projects that require the measurement of extended radial sapflow profiles to accurately map hydraulic architecture of trees. Similar to the HRM sap flow sensor the HFD sensor can measure high sapflow rates as well as low to zero and reverse sapflow. Hence as both sensors can measure in the same range the HFD sensor provides an extension of the HRM method making both sensors highly complimentary to each other in most sapflow measurement applications.

Developed by Dr. Nadezda Nadezdina Mendel University, Czech Republic the HFD technique has been used in published sapflow research since 1998 to study many previously unanswered plant physiological questions. Nadezhdina N., Ferreira M. I., Silva R., Pacheco CA. 2008 Seasonal variation of water uptake of a Quercus suber tree in Central Portugal. Plant and Soil, 305: 105-119.

Applications

Radial sap velocity profiles
Mapping Hydraulic Architecture of Trees
High, meduim, low and zero sapflow rates
Reverse sapflow rates
Hydraulic redistribution both within roots and stems
Arid ecosystems and drought
Thick Sapwood Species
Forestry
Horticulture
Automatic Irrigation control

Features

Plug and Play operation
Microprocessor controlled
Works with Sap Flow Tool software
Measurements are independent of:

Wounding
Wood water content
Natural thermal gradients

Integrated voltage regulation
User variable energy input options
Correction of any changes in local settings through adjustable K-values for each individual measurement point
Low power consumption
2 milli-degree temperature resolution

Principle of measurement

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. Sapflow significantly deforms the heat field by elongating the ellipse as shown in the photo of a thermal image of a HFD measurement. 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.

Sensor design

The HFD sensor consists of three measurement needles, and a heater needle integrally connected to a 24-Bit digital microprocessor based smart interface. The sensor is compatible with the SL5 Smart Logger only and, reports dT Symmetric (dTsym) and dT Asymmetric (dTas) values in ^oC for each radial measurement point within the needle set.

Each HFD sensor includes a high precision voltage regulator to control the voltage supplied to the heater between 0 and 12V in 0.01V increments. This voltage can be set independently of any other sensor on the system providing complete experimental design flexibility. The voltage can easily be adjusted via the sensor setup menu either through direct connection to the system or remotely via wireless modem.

Needle configurations

When beginning Sapflow research on a species for the first time it is imperative to learn and understand the structure and function of the species before commencing long term whole tree water use studies. The standard HFD needle set configuration consists one needle set with 8 radial measurement points — for Deep Sapwood - Initial structure and function measurements. Other needle configurations are available on request.

Data output

The HFD sensor has a maximum capacity of 8 radial sensing points along each needle, or 24 measured inputs in total across all three measurement needles. Via the sensor set up menu the user can set the number of measurement points from 1 to 8 dependant upon the needle set being used.

Because the HFD principle uses ratio's of differential temperatures to calculate sapflow there are only 16 outputs or addresses used by the Smart Logger. These data are the differential temperature between the Upper and Lower, corresponding sensing points (dTsym) and the differential temperature between the Side and Lower, corresponding sensing points (dTas).

The data is logged in ^oC and is downloaded in ASCII format as a Comma Separated Values (CSV) file and is imported into the HFD-TOOL software. This software automatically identifies the correct number of sensing points per needle set and calculates the value for parameter K before processing the raw dT values to Sapflow data.