Environmental Engineering Reference
In-Depth Information
Earth's blood, water, is pumped, not by a heart, but by the hydrologic
cycle—the water cycle. A titanic force of nature, the water cycle is beyond
our control—a fact that we ignore until weather patterns shift and inundated
rivers suddenly flow where they will and not within human-engineered
banks, floodwalls, dikes, or levees. In the water cycle, water evaporates from
the oceans; falls as rain, hail, sleet, or snow; and strikes the Earth again …
thus, the cycle continues.
In cities, in summer, rain strikes hot cement and asphalt and evaporates
or runs into storm drains, swiftly rejoining the cycle. In fields, rain brings
essential moisture to crops and, sinking deeper into the Earth, ends up as
groundwater. If water strikes a forested area, the forest canopy breaks the
force of the falling drops. The forest floor, carpeted in twigs, leaves, moss,
and dead and decaying vegetation, keeps the soil from splashing away as
the water returns to the depths of the Earth or runs over the land to join a
stream.
Whenever water strikes the Earth, it flows along four pathways that carry
water through the cycle just as our veins, arteries, and capillaries carry our
blood to our cells. Water may evaporate directly back into the air. It may
flow overland into a stream as runoff. It may soak into the ground and be
taken up by plants for evapotranspiration. Or, water may seep down to
become groundwater. Whatever pathway it takes, one fact is certain: Water
is dynamic, vital, constantly on the move. And, like human blood, which
sustains our lives, Earth's blood, to sustain us as well, must continue to flow
(Spellman, 2007).
Introduction
Take a moment to perform an action most people never think about doing.
Hold a glass of water (like the one shown in
Figure 2.1
) and think about the
substance within the glass—about the substance you are getting ready to
drink. The water held by a drinking glass—the water we are about to drink,
the water we do drink—is not one of those items people usually spend much
thought on, unless they are tasked with providing that drinking water.
Let's think about water now. Earlier we stated that water is special, strange,
and different. We find water fascinating—a subject worthy of endless inter-
est because of its unique behavior, endless utility, and ultimate and intimate
connection with our existence. We agree with Tom Robbins (1976, pp. 1-2),
whose description of water follows:
Stylishly composed in any situation—solid, gas, or liquid—speaking
in penetrating dialects understood by all things—animal, vegetable or
mineral—water travels intrepidly through four dimensions,
sustaining
