Water Cycles - Hydrogeology: Objectives, Methods, Applications - page 28

Figure 9 Annual potential evapotranspiration across different regions of the globe (from Ahn
& Tateishi, 1994).
e) The hydrologic budget method
One can well imagine that, given the great number of parameters and the
diversity of natural environments, the experimental approach can be as
precise as calculations. Actual evapotranspiration can thus be determined by
creating a long term budget of all infl ows and outfl ows of water over well-
defi ned hydrologic or hydrogeologic units. If, over the entirety of the basin,
the amount of precipitation and the discharge of rivers are well-known
thanks to a network of pluviometers and the installation of streamgages:
AET = Q e - Q s ± ΔR
where Q e is total infl ow; Q s is total outfl ow; ΔR is the change in storage.
If these measurements can be acquired over several years, so as to
minimise the uncertainty tied to the renewal of groundwater reserves, then
the term ΔR becomes negligible as infi ltrated water is restored through
outfl ows, and AET = Q e - Q s