Geoscience Reference
In-Depth Information
Humid temperate
Semi-arid
Groundwater
recharge
Groundwater
recharge
Evaporation
Evaporation
Surface
runoff
Surface
runoff
Arid
Groundwater
recharge
Surface
runoff
Evaporation
Figure 1.7
Proportion of total precipitation that returns to evaporation, surface runoff or
groundwater recharge in three different climate zones.
Source
: UNESCO (2006)
in differing amounts depending on climate. In tem-
perate climates (i.e. non tropical or polar) around
one third of precipitation becomes evaporation,
one third surface runoff and the final third as
groundwater recharge. In arid and semi-arid regions
the proportion of evaporation is much greater, at the
expense of groundwater recharge.
With the advent of satellite monitoring of the
earth's surface in the past thirty years it is now
possible to gather information on the global dis-
tribution of these three processes and hence view
how the hydrological cycle varies around the world.
In Plates 1 and 2 there are two images of global
rainfall
distribution during 1995, one for January
and another for July.
The figure given above of 146 million litres of
fresh water per person per year is extremely mis-
leading, as the distribution of available water around
the globe varies enormously. The concept of avail-
able water considers not only the distribution of
rainfall but also population. Table 1.3 gives some
indication of those countries that could be consid-
ered water rich and water poor in terms of available
water. Even this is misleading as a country such
as Australia is so large that the high rainfall received
in the tropical north-west compensates for the
extreme lack of rainfall elsewhere; hence it is con-
sidered water rich. The use of rainfall alone is also
misleading as it does not consider the importation
of water passing across borders, through rivers and
groundwater movement.
Table 1.3 gives the amount of available water for
various countries, but this takes no account for the
amount of water abstracted for actual usage. Figure
