Measuring Sap Flow: TDP compared to the Heat Ratio Method

TDP is a constantly heated sap flow principle that assumes there is no background temperature gradient between the probes and the plant (thermal insulation is required around the sensors and the stem). However, data shows that Natural Thermal Gradients (NTG) from the soil (which cannot be incorporated into the TDP measurement) are significant (1°C or greater) hence, the temperature gradient measured is not a true indication of the actual sap flow. The result is a substantial overestimation due to the background temperature gradient in the mornings and again underestimation in the evenings, none of which are of a constant magnitude to even begin to attempt compensating for.

Another cause of underestimation of sap velocity using TDP is the thickness of the sapwood. A standard TDP-30 sensor has a length of 30 mm consisting of a full length line heater and only a single thermocouple located at the halfway point (15 mm). The sensor design is adequate for use in boreal Pinus species with thick sapwood of at least 30 to 40 mm, but is inadequate for most other species. TDP sensors cannot be used to accurately measure sap flow of Eucalyptus species for example, because on average the sapwood thickness is approximately 20 to 25 mm thick. Therefore, the TDP sensor will almost always have the heater extend significant depths into heartwood or non-conducting xylem. This results in highly variable and underestimated sap velocity because the non-conducting xylem (heartwood) artificially increases the dT_max as it does not dissipate the heat.

With these two conflicting variables causing both positive and negative errors of unknown quantities, it is clear that the TDP principle has significant limitations in application.

The Heat Ratio Method (HRM) sensor is a modified heat pulse technique that provides a better accuracy of measurements, compared to the TDP. HRM consists of three needles: two measurement needles located equidistant above and below a central line heater. Because it is a pulsed technique using a short two to six second pulse of heat and a 100 second measurement window, the effect of NTGs are avoided as ambient temperature changes within the measurement interval are insignificant or non-existent. This has additional benefits in practical deployment as the sensors also require no thermal shielding to thermally isolate the measurement site against NTGs as with TDP.

The HRM needles are 35 mm in length and have two temperature measurement locations, 7.5 mm from the tip and 22.5 mm from the tip. This provides a measurement of radial velocity across the sapwood and quantifiably measures the velocity gradient and/or identifies when the needle extends into non-conducting xylem (so the inner measurement point can be discounted).

The HRM can measure very low flow (approx 1 cm hr^-1), zero flow, and reverse flow in a range of stem sizes, making it very robust and flexible while the TDP is unable to capture these important measurements. Missing those records will account, nearly 40% loss of sap flow data from the system.