Water Cycles - Hydrogeology: Objectives, Methods, Applications - page 23

where P is precipitation; Q is discharge; I is infiltration; ET is
evapotranspiration, and ΔR is the change in storage. Values are generally
given in mm.
Effective precipitation (EP) is the amount of water actually available
to feed runoff and infi ltration.
EP = P - ET
2.2 Hydrologic systems and budgets
The water cycle can be considered on a global scale, but also on the scale
of the structural or geographic units forming hydrologic sysems. These
systems can be classifi ed into three domains.
2.2.1 Hydrologic basins
These correspond to the drainage basin of a river and its tributaries,
generally defi ned by topographic highs, with the exception of certain cases
such as karst zones (see chap. A8). On the basin scale, the budget is balanced
over the long term. The volume of effective precipitation is theoretically
equal to the total outfl ow.
PE = Q T
where Q T : total discharge, in mm
However, Q T is not easy to quantify, as part of the water circulates below
ground, and can leave the system unseen, for example, by fl owing into a
different basin or directly into the sea, in coastal areas.
2.2.2 Aquifers
Aquifers (from the Latin aqua fero: to carry water) are permeable geologic
structures containing water, such as, for example, river alluvium, fi ssured
granite, a limestone plateau, etc. The water held in an aquifer, originating
from infi ltrated precipitation, circulates below ground and feeds one or more
springs, or fl ows unseen into a receiving body, such as a sea, lake, river, or
other aquifer. An aquifer is characterised by its geometry, its areal extent
and its depth, and by the intrinsic characteristics of the rock from which it is
formed: lithology, porosity, permeability, fracturation, homogeneity, etc.
Aquifers can store infi ltrated water in varying quantities and for varying
amounts of time; their residence times can range from a few hours to several
hundreds of thousands of years.