This is a topic of ongoing ...

This is a topic of ongoing research and the results vary greatly among plant species and can be seasonal. If your tree species is common then a literature study or a search on the FAO website may give you the info you need.

There are several ways to measure the consumption. For individual trees there are instruments out there which measure the sap flow (not my expertise). 

If you look to determine the average "water loss" from larges areas then Eddy Covariance systems will help you greatly by determining the vertical flux of CO2 and H2O. Be warned, these setups are costly (about USD70,000) and require a fair amount of training to both operate and process the data.

Hope this helps