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Natural Resources Extension Program
Watersheds - Water Quality
Modified from Living on the Land
2001: Stewardship for Small Acreages
Project Leaders: Susan Donaldson, University of Nevada,
Cooperative Extension
and Sherman Swanson, University of Nevada, Reno
"When
we develop within a watershed, we often alter
the hydrologic cycle." |
Most of the world’s water is contained in oceans
or seas or is tied up in icecaps and glaciers. Less
than 0.65% of all the water on earth today is liquid
fresh water. For this reason, it is critically important
that we protect our limited water resources. Source:
US Geological Survey.
 The
hydrologic cycle replenishes the water in watersheds.
A watershed is the area of land that catches rain and
snow and drains or seeps into a marsh, stream, river,
lake or ground water. Homes, farms, ranches, forests,
small towns, big cities and more can make up watersheds.
Some watersheds cross county, state, and even international
borders.
We all live in a watershed, and our actions may affect
everyone in the watershed. Watersheds function to capture,
store, and safely release water. For example, as snow
melts on mountain peaks in the spring, much of the water
soaks into the ground, replenishing soil moisture and
ground water. This water will be a source of flow to
local streams and rivers during dry seasons. The soils
and vegetation in the watershed are essential to proper
watershed functioning.
Watersheds come in all shapes and sizes. Some are millions
of square miles, others are just a few acres. Just as
creeks drain into rivers, watersheds are nearly always
part of a larger watershed.
How does urbanization affect watersheds? When we develop
within a watershed, we often alter the hydrologic cycle.
The increase in impervious surfaces, including roofs
and pavement, increases the amount of runoff and decreases
infiltration. The runoff water carries pollutants directly
into water bodies.
The effects of urbanization are broad, and include
such things as a proliferation of lawns that require
more water than native vegetation, reduced or eliminated
riparian areas that no longer filter runoff water as
well as they used to, and less evapotranspiration (less
vegetation). Combustion from the burning of gasoline
has become a significant source of atmospheric moisture
in the 20th century.
Our actions in a watershed help determine the quality
of water flowing from the watershed. The boundaries
of a watershed are determined by topography. The highest
points surrounding a stream or river, called divides,
are the boundaries of a watershed. If a drop of water
falls on one side of a divide, it will drain into that
watershed’s river or lake. If it lands on the
other side, it will drain into the river or lake of
the adjacent watershed.
The cycling of Earth’s water is one of the oldest
‘recycling’ schemes in nature. Water evaporates
from oceans, lakes and rivers when heated by the sun.
Plants also release water vapor into the air via transpiration.
The water vapor condenses into clouds, and water returns
to the land by way of rain or snowfall, called precipitation.
Some of the water runs off the land surface to fill
lakes, streams, rivers, and oceans, called “surface
water.” Surface water is easily contaminated because
it is open to inputs of wastes and other pollutants.
Some of the precipitation soaks into the ground (infiltrates)
to replenish the ground water supply. The level at which
the underground geologic formation is saturated with
water is called the water table. Some ground water seeps
back into streams and lakes to maintain flows.
An aquifer is a formation that produces enough water
to be useful for well water supplies. Ground water produces
about 50 percent of drinking water nationwide. It is
more protected from our activities than surface water,
but shallow aquifers are still at risk of contamination.
The following concepts are important in understanding
watersheds:
- Capture: Rain naturally enters
the earth through soil, roots, and animal tunnels.
When buildings, pavement and other hard surfaces cover
the ground, instead of soaking in, water runs off
into road ditches and into the nearest stream, resulting
in increased erosion and flooding.
- Store: After rain enters the soil,
it fills the spaces between rocks and soil particles
and becomes ground water. Shallow ground water provides
water for plant roots and the breakdown of pollutants.
Deeper ground water supplies wells.
- Release: Ground water is slowly
released to springs, wetlands, and streams. If water
was not captured and stored, it cannot be released.
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