Improved Sap Flow System Design for Heat Pulse Velocity and Thermal Dissipation Methodology

Abstract:

Ground-based ET measurements via methods such as flux tower, lysimeter, or sap flow remain relatively expensive, time consuming and cumbersome to employ. This has restricted direct ET measurements to a relatively small number of sites and made it difficult to assess a wide range of vegetative characteristics. Of all the flux measurement methods, sap flow probes offer potential for more widespread, widescale, and remote uses, but costs and power requirements currently limit these applications. This project assesses whether probes can be constructed at a lower cost and required skill level by using inexpensive temperature sensors and microcontroller-based data logging, while using less energy by manipulating the voltage, timing, and duration of heating required to derive accurate sap flow measurements. The probes are designed for use with all Heat Pulse Velocity (HPV; CHP, Tmax, HR, and Sapflow+) and Thermal Dissipation (TD) methods, allowing flexibility for researchers in the field. Battery lifespan was tested for single use Lithium and rechargeable NiMH AA batteries, resulting in a minimum of 14 and 7 days at a 30-minute sampling cycle, respectively. Empirical laboratory measurements of flow rate through cut branches were used to calibrate Granier’s equation and thermal diffusivity for the TD and HPV methods, respectively. Comparative field tests with the Dynamax TDP-30 were conducted using the Dual Method Approach, combining the Tmax and HRM to capture reverse to high flow rates. These inexpensive, easily constructed, and self-contained sensing units were used to capture a larger variation of physiological and meteorological drivers in plant water use, allowing for more representative estimates in both urban and forested settings.