A sap flow sensor using the Stem Heat Balance Method is a non-destructive tool to quantify water flow in plant body. Plant water uptake/transpiration amount (depending on the installation position and plant size). It helps in scheduling the timing of irrigation and the water amount.
By dividing the sap flow rate obtained as the transpiration volume by the VPD, you can find out the gas exchange capacity (Leaf/Shoot/Canopy Conductance). At what time did the stomata close? Which varieties are more resistant to soil drought and productive?
It's not just about measuring water uptake, it's also about whether a tree is vigorous or not. Then, actual plant water uptake is the only way to know at soil types where soil moisture sensors cannot install. You can get to know the plant hydraulic status in detail by just one way of using it!