Nonpoint Education for Municipal Officials
The Effects of Urbanization
on the Water Cycle
Jeff Zimpfer and
Megan Toaspern,
Extension Specialists
University of Hawai‘i
Sea Grant College Program
“Helping
communities
protect natural
resources while
accommodating
growth.”
A
s our islands’ populations
increase, we notice many visible
changes, including additional
housing, road networks and retail and
resort developments. One potential
impact of such development is pollution
of our vital water resources. To
understand how our water supply can
become polluted and/or reduced, it is
important to revisit and understand
the oldest solar-powered “recycling”
system - the water cycle, also called the
hydrologic cycle.
In the Hawaiian hydrologic cycle, water
is transported between the ocean, the
atmosphere and our islands. The major
processes moving water are evaporation,
transpiration, condensation, and
precipitation. Evaporation occurs
when heat from the sun turns liquid
water on the earth’s surface into water
vapor, which enters the atmosphere.
Transpiration occurs when water
vapor enters the atmosphere as it is
released from plant leaves. Collectively,
these two processes are known as
evapotranspiration. The hydrologic cycle
continually renews and refreshes our
finite water supplies.
On our islands, the geophysical
environment is shaped by the
interaction of volcanic geology with
rainfall patterns influenced by the
northwesterly trade winds. Mountains
lead to the development of wet
windward and sunny leeward areas on
all the islands. As warm, moisture-laden
trade winds contact the mountainous
islands, the air is forced upwards and
cools. As a result of cooling, the water
vapor condenses and orographic rainfall
occurs on the windward uplands. As
cool air descends on the leeward sides,
the air warms and absorbs moisture,
resulting in drier “rain shadows.”
Hydrologic cycle is the natural recycling
process powered by the sun that causes
water to evaporate into the atmosphere,
condense and return to earth as
precipitation.
Evapotranspiration refers to water lost
through evaporation from the soil and
surface, water bodies, and water that
evaporates from plant leaves.
Geophysics is the study of the physical
characteristics and properties of the earth.
Orographic rainfall refers to precipitation
that results when moist air is lifted over the
mountains. It is a common climate pattern
on islands in the tropics and subtropics.
Fact Sheet
The Effects of Urbanization on the Water Cycle | 1
Hawaiÿi Nature Guides
Figure 1. The hydrologic cycle in Hawaiÿi.
The oceans contain approximately 97% of
the earth’s water. Of the fresh water supply
on earth, 78% is held in polar ice caps
and snow, leaving only a minute fraction
available for use by people and other
organisms. Of the available fresh water,
98% is in the form of groundwater, while
the remaining two percent is in the form
of surface water. Because our freshwater
supply is so limited, it is vitally important
that we protect this precious natural
resource.
On Oÿahu, groundwater is an important
source of fresh water, accounting for 85%
to 90% of water used for municipal,
industrial, agricultural, and military
purposes. Groundwater accounts for 15%
of the municipal water used on Kauaÿi. It
accounts for 50% and 75% respectively of
the municipal water used on the islands of
Maui and Hawaiÿi.
On an average day on the island of Oÿahu
alone, two billion gallons of rain falls.
This rain soaks into mountain slopes and
slowly percolates through the volcanic
2 | The Effects of Urbanization on the Water Cycle
rock ending up as groundwater. It takes on
average 25 years from the time rain falls on
the mountains for it to reach the aquifers,
be pumped up to the surface through wells
and pipelines, and finally into your tap.
Once water percolates into the volcanic
rock, if not used, it eventually returns to
the ocean. An important factor in retaining
the islands’ freshwater is the natural
occurrence of a caprock, a thick layer of
sediment and coral limestone that has
formed along our islands’ coastal margins.
In Hawaiÿi, as in other oceanic islands,
caprocks contribute to the development of
the freshwater lens that floats on the denser
saltwater. The area where the water layers
meet is brackish, or somewhat salty.
How Urbanization Affects our Water
Cycle
Hawaiÿi’s fresh water resources are limited
while our demand for water is increasing
as the state’s population continues to grow.
As a general rule, prior to development,
10% of the precipitation falling on a given
University of Nevada Cooperative Extension, NEMO Program
University of Nevada Cooperative Extension, NEMO Program
Figure 2. Before urbanization, much of the rain falling on our islands
infiltrates into the soil.
Figure 3. Following urbanization, less of the rainwater infiltrates into the
soil which results in increased runoff.
area is lost from the islands as surface
runoff with 50% infiltrating to become
groundwater. The remaining 40% is lost to
evapotranspiration. In contrast, following
development, 55% of precipitation is
lost via surface runoff and just 15%
infiltrates to become groundwater with
the remainder lost to evapotranspiration.
Thus, the transition from an undeveloped
to a developed area results in more than a
five-fold increase in water lost via surface
runoff and greater than three-fold decrease
in infiltration.
These changes occur because development
results in an increase in impervious or hard
surfaces including rooftops and pavement
(roads, driveways, and parking lots) which
decreases the amount of water that is able
to soak into the ground, or infiltrate.
The Effects of Urbanization on the Water Cycle | 3
For more information:
www.BuildingGreenCommunities.org
Jeff Zimpfer
UH-Hilo, PACRC
200 West Kawili St.
Hilo, HI 96720-4091
(808) 936-9769
[email protected]
Jeff Zimpfer
Megan Toaspern
P.O. Box 791545
Paia, HI 96779
(808) 573-5357
[email protected]
Figure 4. With increased urbanization comes more impervious surfaces, which intensify storm
water runoff and pollutant concentration.
Water that does not infiltrate adds to the
amount of surface runoff, which in turn
goes directly to the ocean and therefore
is no longer available for use on our
islands. These impervious surfaces collect
and accumulate pollutants, such as those
leaked from vehicles or deposited from the
atmosphere through rain. Consequently,
runoff water carries pollutants directly into
water bodies in which they drain such as
streams, rivers, lakes and oceans. Because
there is less pervious or porous areas for
infiltration, peak flows of stormwater runoff
are larger and arrive earlier, increasing the
magnitude of urban flooding.
However, effects on the water cycle are not
limited to surface water. Paving may alter
the location of recharge, or replenishment,
of groundwater supplies restricting it to the
remaining unpaved areas. If infiltration is
decreased sufficiently, groundwater levels
may decline, affecting stream flows during
dry weather periods. Lowered groundwater
levels can result in subsequent well failures
and salt water intrusion into wells, an
exceedingly difficult problem to address.
While the effects of urbanization on
the water cycle can be significant,
prudent choices can be made during the
development process to minimize associated
impacts and ensure our future water supply
is protected.
References
Schueler, T.R. and H.K. Holland. 2000.
The Practice of Watershed 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. Geological Survey. 2000.
Groundwater in Hawai‘i. FS 126-00
U.S. Dept. of the Interior. GPO 1984421618/109.
4 | The Effects of Urbanization on the Water Cycle
This project has been
jointly funded by the U.S.
Environmental Protection
Agency (“Agency”) under
Section 319(h) of the Clean
Water Act, and the Hawai‘i State
Department of Health Clean
Water Branch. Although the
information in this document
has been funded wholly or in
part by a Federal Grant to the
Hawai‘i Department of Health,
it may not necessarily reflect the
views of the Agency and the
Hawai‘i Department of Health
and no official endorsement
should be inferred.
The Effects of Urbanization on
the Water Cycle is supported
by a grant from the National
Oceanic and Atmospheric
Administration, project M/C-1,
sponsored by the University
of Hawai‘i Sea Grant College
Program/SOEST, under
Institutional Grant No.
NA05OAR4171048 from the
NOAA Office of Sea Grant,
Department of Commerce.
The views expressed herein are
those of the authors only.
UNIHI-SEAGRANT-AB-06-04
The University of Hawai‘i Sea
Grant College Program is a
charter member of the National
NEMO Network.
© The University of
Connecticut. Adapted with
permission of the University
of Connecticut Cooperative
Extension System and Susan
Donaldson, University of
Nevada Cooperative Extension,
NEMO Program.