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 :