Why Study Continental Aquatic Systems? - Freshwater Ecology: Concepts and Environmental Applications - page 6

TABLE 1.2 General Ranges of Water Use with Varied Socioeconomic
Conditions on a Per Capita Basis a

Range or mean (m 3 year 1 capita 1 )

Society

Irrigated semiarid industrial countries 3000-7000

Irrigated semiarid developing countries 800-4000

Temperate industrial countries 170-1200

United States 2200

Switzerland 480

Jordan 200

Ghana 75

a Modified from Falkenmark (1992) and la Rivière (1989).

The maximum total water available for human use is the amount
that falls as precipitation on land each year minus the amount lost to
evaporation. As mentioned earlier, the maximum amount of water avail-
able in rivers is 22,000-35,000 km 3 per year. However, much is lost to
floods or flows occurring in areas far removed from human population
centers, leaving approximately 9000 km 3 per year for use (la Rivière,
1989). Humans cannot sustain use of water greater than this supply rate
unless additional supplies are withdrawn from groundwater at rates greater
than renewal, collected from melting ice caps, transported from remote
areas, or reclaimed (desalinized) from oceans. These processes are expen-
sive or impossible to sustain in many continental regions.
Predicting future water use is difficult but instructive for exploring pos-
sible future patterns and consequences of this use. Total annual offstream
withdrawals (uses that require removal of water from the river or aquifer,
not including hydroelectric power generation) in the United States in 1980
were 2766 m 3 per person and have decreased slightly since that time,
mostly due to a decrease in total industrial use (Fig. 1.3). If all the people
on Earth used water at the rate it is currently used in the United States (i.e.,
their standard of living and water use efficiency were the same as in the
United States), over half of all the water available through the hydrologi-
cal cycle would be used.
Globally, humans currently withdraw about 54% of runoff that is ge-
ographically and temporally accessible (Postel et al., 1996); if all people in
the world used water at the per capita rates used in the United States, all
the water available that is geographically and temporally accessible would
be used. On a local scale, water scarcity can be severe. Political instability
in Africa is predicted based on local population growth rates and limited
water supply (Falkenmark, 1992). Similar instabilities are likely to arise
from conflicts over water use in many parts of the world (Postel, 1996). In
the arid southwestern United States, uses can account for more than 40%
of the supply (Waggoner and Schefter, 1990). In such cases, degradation of
water quality has substantial economic consequences.
The population of the earth is currently over 6 billion people and
may double during the next 43 years (Cohen, 1995). Given the increase
in human population and resource use (Brown, 1995), demand for water
will only intensify (Postel, 1996). As the total population on Earth ex-