where V is the axial flux of sap (the sap flow
[m3sap s-1] per
unit of cross sectional area of sapwood
[m-2sapwood]; V thus has
units of m3sapm-2sapwood s-1,
which is commonly expressed with the units centimetres per hour (cm
h-1)), Q is the total heat released per unit
length of the heater (J m-1), ρ is the sapwood
bulk density (including water and wood) (kg m-3),c is the sapwood heat capacity (J kg-1K-1) andk is the thermal
diffusivity of the sapwood-water matrix (m2s-1). To avoid confusion arising from the historical
use of the symbol ”V ” to describe sap flux, we will follow
previous authors in referring to V as ”sap velocity” (Edwards,
Becker, & Èermák, 1996). Applying Eqn. 1 to two probe
locations, i and j , and rearranging provides an estimate
of V :