“Protecting water quality through community planning”
Fact Sheet-04-43
Nonpoint Education for Municipal Officials
By Susan Donaldson,
Water Quality Education
Specialist, University of
Nevada Cooperative
Extension
The Effects of Urbanization
on the Water Cycle
A
As our communities grow, we
ter vapor leaves plants in a process
water. The oceans contain most
notice many visible changes, includ-
called transpiration. Collectively,
of the earth’s water, or about 97.2
ing housing developments, road
these two processes are called
percent. Of the fresh water supply
networks, expansion of services,
evapotranspiration.
on earth, 78 percent is tied up in
and more. These changes impact
The water vapor in the atmo-
polar ice caps and snow, leaving
our precious water resources, with
sphere cools to form clouds (con-
only a minute fraction available for
pollution of water resources being
densation). Through precipitation in
use by people. Of the available
one potential impact. To under-
the form of rain or snow, the water
fresh water, 98 percent is in the
stand how our water supplies can
returns to earth. Snow accumulates
form of groundwater, while the re-
surfaces, including
become polluted, it’s important to
in the mountains, providing storage
maining 2 percent is in the form of
understand the oldest solar-pow-
in the form of a snow pack that will
surface water. Because our usable
rooftops and
ered “recycling” system: the water
slowly melt and release water in
water supply is so limited, it’s vitally
cycle, also called the hydrologic
the spring and summer. Some of
important to protect water quality.
cycle.
the rain runs off the land, into rivers
Within the context of the
or lakes. While it’s hard to believe,
hydrologic cycle, there is no “new”
ports water between earth’s land,
rivers contain only about 0.0001
water ever produced on the earth.
that soaks into the
atmosphere, and oceans. The
percent and fresh water lakes only
The water we use today has been
major processes moving water
about 0.009 percent of all water
in existence for billions of years,
ground, or infiltrates.
are evaporation, transpiration,
on earth!
and may have once provided a
“The increase in
impervious or hard
pavement, decreases
the amount of water
This increases the
amount of surface
runoff.”
The hydrologic cycle trans-
condensation, and precipitation.
Rain also soaks into the
drink for a dinosaur. The hydro-
Evaporation occurs when the sun’s
ground, or infiltrates, and replen-
logic cycle continually renews and
energy turns liquid water on the
ishes the water stored in the soil.
refreshes our finite water supplies.
earth’s surface into water vapor,
We call the subsurface water supply
which enters the atmosphere. Wa-
in the saturated zone below the
COOPERATIVE EXTENSION
Bringing the University to You
tributes flow to rivers and oceans.
How does
urbanization affect
the water cycle?
Aquifers are geologic formations
When we develop within a wa-
containing groundwater in sufficient
tershed, we affect the hydrologic
quantities to provide water to
cycle. The increase in impervious
extraction wells.
or hard surfaces, including rooftops
water table groundwater. It con-
Groundwater accounts for
about 0.61 percent of the earth’s
and pavement (roads, driveways,
and parking lots), decreases the
PAGE 2
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THE EFFECTS OF URBANIZATION ON THE WATER CYCLE
For more
information:
amount of water that soaks into the
Contact the NEMO Project,
impervious surfaces collect and ac-
c/o Susan Donaldson, Ph.D.
cumulate pollutants, such as those
University of Nevada
leaked from vehicles, or deposited
Cooperative Extension
from the atmosphere through rain
5305 Mill St.
or snowmelt. The runoff water
Reno, NV 89502
carries pollutants directly into water
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ground, or infiltrates. This increases
the amount of surface runoff. The
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Tel: (775) 784-4848
tion, peak flows of storm water
FAX: (775) 784-4881
runoff are larger and arrive earlier,
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increasing the magnitude of urban
Internet:
www.unce.unr.edu
floods.
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However, affects on the water
cycle are not limited to surface
water. Paving may alter the location
Funding
provided by:
of recharge, or replenishment, of
Nevada Division of
infiltration is decreased sufficiently,
Environmental Protection,
groundwater levels may decline,
Nonpoint Source Pollution
affecting stream flows during
Management branch.
dry weather periods. Lowered
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groundwater supplies, restricting it
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to the remaining unpaved areas. If
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subsequent well failures.
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groundwater levels can result in
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While the effects of urbaniza-
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tion on the water cycle can be
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major, if wise choices are made
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during the development process,
the impacts can be minimized and
our future water supply protected.
This fact sheet is one in a series
Schueler, T.R. and H.K. Holland.
that discusses the choices
2000. The Practice of Watershed
that can be made
during the
development
process to
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employs only United States citizens
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work in the United States.
protect our
water
resources.
References:
”While the effects of
urbanization on the water cycle can be
major, if wise choices are made during the
development process, the impacts can be
minimized and our future
water supply protected.”
Protection. Center for Watershed
Protection, Ellicott City, MD.
U.S. EPA and Purdue University.
1997. Human Impact on the Water Cycle, in Ground Water Primer.
http://www.epa.gov/seahome/
groundwater/src/cycle.htm.
U.S. Geological Survey. 1984.
The Hydrologic Cycle. U.S. Dept.
of the Interior. GPO 1984-421618/109.