As can be seen from the above explanation, Earth's water plays the role of
1) t he envi-ronment Itself, 2) environmental component, 3) habitat and 4)
resources . Although freshwater is acting as if it can be exploited indefinitely
by its s olar energy-driven circula-tion on the Earth, it is not by any means
an unlimit ed resources.
By 2025 when the world population is predicted to reach 8.3 billion, water dema
nd will rcach 6,400 km3/year due to population increase and overgrowth of indus
trial activity. Furthermore, 3,430 km3/year of water will be needed to maintain
the state of rivers and streams. This amounts to 70% of river flow that can be
used to maximum. For this rcason, sccuring of water resources and Improvement
of utilization efficiency is one of the top prlority issues face by humankinds
in the 21st Century along wlth the issues of energy and food.
