Water Cycles - Hydrogeology: Objectives, Methods, Applications - page 19

areas exploited by deeep wells

principal deep aquifers

from Margat, 1990

Figure 5 Global distribution of the principal deep aquifers.

Table 1 presents the estimated distribution of water across the planet. It
is only a guess, as the evaluation of groundwater reserves is very variable
depending on the author, going from 7 to 30 x 10 6 km 3 .

2.1 The water cycle

2.1.1 Driving forces and radiation budget

From the time it degasses from the planet's interior to the time it reaches
the hydrosphere, water is constantly passing from one reservoir to another,
caught up in an endless cycle powered by the sun and by geothermal
fl uxes. The Earth (including its oceans) and its atmosphere refl ect part of
the incoming solar radiation, while another part is absorbed by these two
media. The Earth then radiates this energy back into the atmosphere, which
radiates it both back towards the surface and out into space. The surface
receives an average of 94 W·m -2 . Figure 6 outlines the respective percentages
of emitted, refl ected, and absorbed radiation.
The Earth's radiation budget is balanced on a global scale; the planet
receives as much incoming radiation as it emits, ensuring that it is neither
an arid sphere nor a ball of ice. On a regional scale, however, the budget is
unbalanced. Polar regions, receiving sunlight at a shallow angle, experience
a defi cit, while the equator, where the incoming radiation is at a maximum,