Water Cycles - Hydrogeology: Objectives, Methods, Applications - page 22

of these precipitations is estimated to be 577,000 km 3 . They feed ice and
snow deposits in cold regions. Part of the precipitation can be intercepted
by vegetation and returns to the atmosphere through evaporation or
sublimation. In hot regions, rain not intercepted by vegetation turns into
runoff, which accumulates into rivers. It can also be be taken up by living
organisms and constitute biological water. Part of the total precipitation seeps
into the ground and makes up infi ltrated water, domain of hydrogeology.
Indeed, once water reaches a permeable surface, infi ltration can occur.
Infi ltration is partial and diffuse in most cases, or quasi-nonexistent in
impermeable rock (clay), but it can be total and concentrated in certain karst
systems, where rivers seep into underground cavities (see chap. A8).
Hydrogeology is primarily concerned with the subterranean part of
the water cycle, between infi ltration and reemergence. This segment of the
cycle can be very rapid, but it can also be very slow, as in the case of the
Saharan fossil aquifers, within which the passage of water can take several
thousands of years.

Table 2 Residence Time in Different Reservoirs.

Natural environment Residence time

atmosphere a few hours to a few days

continental glaciers a few hundred to a few thousand years

soil surface a few days to a few months

aquifers a few weeks to several hundreds of thousands of years

Aquifers and glaciers therefore play a long-term regulating role for
water passing through continents.
With the exception of infiltrated water, which is protected from
incoming solar radiation, the other classes of water are subject to phase
changes due to:

• sublimation of ice and snow;
• evaporation of surface and ocean water;
• use and transpiration of water by living organisms, particularly
plants.

These three processes are grouped under the term evapotranspiration
(ET). They can affect a layer of soil a few meters to a few tens of meters
deep, depending on the climate. For open surface water or in areas with
no vegetation cover, evaporation and evapotranspiration are lumped as
one term.
Over a certain area:

P = Q + I + ET + ΔR