Environmental Engineering Reference
In-Depth Information
DID YoU KNoW?
A person must consume 2.5 quarts of water per day from all sources
(drinking and eating) to maintain health.
The Water Cycle
The natural
water cycle
or
hydrologic cycle
(the journey water takes during its
constant, inevitable motion) is the means by which water in all three forms—
solid, liquid, and vapor—circulates through the biosphere. Water lost from
the Earth's surface to the atmosphere, either by evaporation from the sur-
face of lakes, rivers, and oceans or through the transpiration of plants, forms
clouds that condense to deposit moisture on the land and sea. A drop of
water may travel thousands of miles between the time it evaporates and the
time it falls to Earth again as rain, sleet, or snow. The water that collects on
land flows to the ocean in streams and rivers or seeps into the Earth, join-
ing groundwater. Even groundwater eventually flows toward the ocean for
recycling (see
Figure 2.2
).
Note:
Only about 2% of the water absorbed into plant roots is used in
photosynthesis. Nearly all of it travels through the plant to the leaves,
where transpiration to the atmosphere begins the cycle again.
Key Concept:
The hydrologic cycle describes water's circulation through
the environment. Evaporation, transpiration, runoff, and precipitation
describe specific water movements.
DID YoU KNoW?
If it rains 1 inch, the amount of water that falls on a quarter-acre lot
is about 7000 U.S. gallons (26,500 liters), or nearly 30 tons (27 metric
tons) of water.
When humans intervene in the natural water cycle, they generate artificial
water cycles or
urban water cycles
(local subsystems of the water cycle—an
integrated water cycle) (see
Figure 2.3
). Although many communities with-
draw groundwater for public supply, the majority rely on surface sources.
After treatment, water is distributed to households and industries. Water
that is wasted (wastewater) is collected in a sewer system and transported
