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.