somewhat, although population increased. This encouraging trend has been the result of
water conservation efforts, especially in industry and agriculture. Industrial consumers of
water have become very clever in recycling water, including uses through several levels
requiring progressively lower water quality. Irrigation used to employ large numbers
of spray irrigators that spray the water up into the air, where much of it evaporates or
is carried away by wind. These have been largely replaced by systems that apply water
directly to soil. The ultimate in irrigation efficiency is trickle irrigation, which drips
the exact amount of water required directly onto plant roots. Exact computer control
of water usage has helped conserve water in the industrial sector and is even helping in
irrigation.
150-200100–15050–100<2525-50
50–100>200Figure 7.2. The six southwestern contiguous continental United States share characteristics of rapid
population growth and water deficiency leading to serious concerns regarding water supply.
The Hydrologic Cycle
One of nature’s major cycles is the hydrologic cycle (Figure 7.3). The oceans
constitute a vast reservoir of water with about 97% of Earth’s water. Most of the remaining
water is in the form of solid snow and ice, predominantly in Greenland and at both poles.
This leaves considerably less than 1% of Earth’s water as water vapor and clouds in the
atmosphere, as surface water in lakes, streams, and reservoirs, and as groundwater in
underground aquifers.
7.4. Bodies of Water and Life in Water
Thermal stratification of bodies of water into a warmer, less dense, oxygen-rich
upper epilimnion and a cooler, more dense, oxygen-poor hypolimnion (Figure 7.4)
strongly influences water chemistry and biology. The epiliminion often supports a high
164 Green Chemistry, 2nd ed