Heat Pulse Velocity (HPV)- this method measures sap flow by determining the velocity of a short pulse of heat carried by convection in the transpirational stream. HPV methods, which were developed by Huber and colleagues in the 1930’s, were the first sap flow techniques to utilize heat as a tracer of sap movement. A theoretical framework for HPV methods was further developed in the 1950’s by Marshall (1958), and numerical corrections to account for departures from idealized heat transport theory were developed several decades later by Swanson and Whitfield (1981) and Green et al. (2003). Briefly, HPV methods use needle-like, line heater and temperature sensor probes inserted into conducting sapwood. Temperature probes often contain one or two temperature sensors (thermocouples or thermistors) for point measurements at different depths into the sapwood. A short pulse of heat (1-6 sec) is released into the sap stream and sapwood temperature is monitored at points upstream and downstream from the heater.