The double-ratio method (DRM): a robust three-probe sap flow technology
that can accurately measure high, low and reverse flows
Abstract
Sap velocity measurements are useful in fields ranging from plant water
relations to hydrology at a range of scales. Heat-pulse based techniques
are among the most common methods to measure sap velocity, but most lack
the ability to measure velocities across a wide range, including very
high, very low and negative velocities (reverse flow). We propose a new
method, the double-ratio method (DRM), which is robust across an
unprecedented range of sap velocities and can provide real-time
estimates of the thermal diffusivity of wood. The DRM employs one
temperature sensor proximal and two distal to the heat pulse probe and
facilitates several theoretical, heat-based approaches to quantifying
sap velocity. We tested the DRM using whole-tree lysimetry in Eucalyptus
cypellocarpa and found strong agreement across a wide range of
velocities.