Even more clearly than soil, water is the lifeblood of the biosphere. Life is
only possible because of the solar-driven circulation of water through the
hydrological cycle from the ocean to the atmosphere, from the atmosphere to
the land, and back to the ocean. Water is a renewable resource, but as you
can see by reviewing Figure 1.2, most water circulates from the ocean to the
atmosphere and back. A much smaller fraction falls as precipitation over
land, and of that, much reevaporates or runs off back to the ocean so that an
even smaller fraction is available for human agricultural, industrial, and
household use. Usable water is very unevenly distributed over the earth's
surface, so that getting enough water has often been a source of political con-
flict. Because water is renewed within the global cycle, we tend to treat it as
a renewable, free, and unpricedcommon good. However, it is not the
volume of water that determines how much is available for use over time,
but its renewal or "recharge" rate for groundwater, lakes, and rivers
(Falkenrnarkand Widstrand, 1992: 4-5). Worldwide, surface water and
groundwater each supply about half of the needed fresh water, but the
recharge rate for groundwater is very slow, about 1 percent per year (Miller,
1998: 491--493).