The easiest determination of Tg is when Tu = 0 resulting in Tg = T t where T t can be defined by sap flow measurements. However, the Tu=0 is likely only in dry seasons of arid or semi-arid locations with shallow aquifer. In all other cases, i.e. when
Tu ï 0, the partitioning of transpiration into Tu and Tg is needed. This can be done by combination of in-situ sap flow and stable isotope measurements  although this method is only successful if there is sufficient isotopic contrast between ground-water and unsaturated zone water .
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