water is a renewable resource,
but its availability
is variable and limited.
Nearly every country in the world
experiences water shortages during
certain times of the year (Gleick
1993a), and more than 80 countries
now suffer from serious water shortages
(Falkenmark and Lindh 1993).
Factors such as rainfall, temperature,
evaporation, and runoff determine
water availability. Clean water
resources per capita are declining
rapidly as the needs of the growing
population increase (Pimentel et al.
1994). Population growth not only
reduces water availability per person
but stresses the entire environmental
system. As the world population
increased from 3.8 billion to 5.4
billion during the last two decades,
water use worldwide increased threefold
(Postel 1992). In addition, factors
such as pollution, erosion, runoff,
and salinization associated with
irrigation, plus the overall inefficient
use of water, contribute to the
decline in water resources (Pimentel
et al. 1994).