Water Cycles - Hydrogeology: Objectives, Methods, Applications - page 24

2.2.3 Hydrogeologic basins
These systems correspond to the underground sections, containing water,
of a part of, the entirety of, or several hydrologic basins. They are made up
of one or more aquifers.
2.2.4 Budgets
In order to study hydrologic systems, measurements are taken and budgets
calculated, quantifying infl ows and outfl ows, in order to evaluate the
volume of water circulating in different reservoirs in the water cycle.
P = Q + ET + ΔR
where P is precipitation; Q is total discharge, including runoff and
groundwater outfl ows; ET is evapotranspiration; ΔR is the change in storage
(groundwater, ice, snow) during the study period. Values are given in mm
or in m 3 .
This budget is calculated over a hydrologic year, a period of 12 months
chosen so as to minimise ΔR, the change in storage.
The frequency and duration of measurements are very important points.
Due to climactic variations, data going back at least 10 years is essential in
order to defi ne a representative hydrologic year. The ideal would be to have
access to continuous measurements, something that technological progress
is now making fi nancially accessible.
During applied hydrogeologic studies, the importance of collecting
suffi cient data is often the point it is the most diffi cult to persuade project
directors to take into account, as the problems involved in applied
hydrogeology (increased pressure on resources, the fi ght against pollution)
generally require a more rapid response.
2.3 Evapotranspiration
Of all the different parts of the water cycle, evapotranspiration is the most
diffi cult to evaluate. Unlike rain or runoff, which can be satisfactorily, if
not precisely, quantifi ed, it is currently very diffi cult to directly measure
the fl ux of water vapor to the atmosphere.
Evapotranspiration depends on a number of factors:
• soil, air, and water temperature;
• air moisture;
• state and quantity of soil moisture;
• solar radiation;