With sap-flow sensors, we determine the amount of water that flows through the stem.
In the sensor shown here, two metal needles are inserted into the active, water-conducting wood layer. The upper needle is heated constantly. The temperature difference between the two needles is measured continuously and allows us to calculate the transported water volume. From this we can estimate the water needs of trees and its transpiration through leaves and needles.
A researcher from the University of Auckland explains in a simple manner how sap-flow sensors work.