Geoscience Reference
In-Depth Information
Estimates cited in Gleick (1993) show a range in
total from 1.36 to 1.45 thousand million (or US
billion) cubic kilometres of water. The vast majority
of this is contained in the oceans and seas. If you
were to count groundwater less than 1 km in depth
as 'available' and discount snow and ice, then the
total percentage of water available for human con-
sumption is around 0.27 per cent. Although this
sounds very little it works out at about 146 million
litres of water per person per day (assuming a world
population of 7 billion); hence the ease with which
Stumm (1986) was able to state that there is enough
to satisfy all human needs.
Figure 1.6 shows the movement of water around
the earth-atmosphere system and is a representation
of the global hydrological cycle. The cycle consists
of
evaporation
of liquid water into water vapour
that is moved around the atmosphere. At some stage
the water vapour condenses into a liquid (or solid)
again and falls to the surface as
precipitation
. The
oceans evaporate more water than they receive as
precipitation, while the opposite is true over the
continents. The difference between precipitation
and evaporation in the terrestrial zone is
runoff
,
water moving over or under the surface towards
the oceans, which completes the hydrological cycle.
As can be seen in Figure 1.6 the vast majority of
evaporation and precipitation occurs over the oceans.
Ironically this means that the terrestrial zone, which
is of greatest concern to hydrologists, is actually
rather insignificant in global terms.
The three parts shown in Figure 1.6 (evaporation,
precipitation and runoff) are the fundamental pro-
cesses of concern in hydrology. The figures given
in the diagram are global totals but they vary enor-
mously around the globe. This is illustrated in
Figure 1.7 which shows how total precipitation is
partitioned towards different hydrological processes
47
119
P
72
505
458
E
E
45
Q
Q
G
2
Ocean
Figure 1.6
The global hydrological cycle. The numbers represent estimates on the total amount of water
(thousands of km
3
) in each process per annum.
E
= evaporation;
P
= precipitation;
Q
G
= subsurface runoff;
Q
= surface runoff.
Source
: Redrawn from Shiklomanov (1993)
