What is a Watershed?
A watershed is an area of land where all the water including rain, snowmelt and groundwater flow into the same stream, lake, river or bay. Just as cities, counties or states have boundaries, so does a watershed. The boundary of a watershed is not tied to any political boundaries, but to the topography of the land. A ridgeline, or high area, separates one watershed from another. Precipitation falling inside this line is delivered to small streams or tributaries which join to form rivers. A watershed with its small tributary streams and larger rivers can be compared to a leaf with its sub-veins and main vein.
EPA Waters Quality Video explaining what a watershed is and why it's important.
A Closer Look...
The total quantity of water on the earth remains essentially constant. Water moves about, changing form (vapor, liquid, solid) and location as part of the hydrologic cycle. Water is delivered to the earth as precipitation (rain or snow) and then seeps into the ground (infiltration) or travels over the ground (runoff). Some of the water moving over land or in streams and lakes is lost to the atmosphere through evaporation. In addition, plants extract water from the ground and release it to the atmosphere as water vapor known as transpiration. Losses to evaporation and transpiration are referred to collectively as evapotranspiration. Water that has seeped into the soil moves as ground water flow. Water which runs off to streams and moves as streamflow.
Water vapor in the atmosphere condenses and is delivered to the earth as dew, rain and snow. Condensed water droplets, or ice particles in the atmosphere, are visible to us as clouds. The movement of water from the atmosphere to earth is called precipitation.
Some of the precipitation that falls on land seeps into the ground where it is stored in aquifers and is transported to streams and lakes by subsurface flow. The amount of infiltration is influenced by the permeability and moisture content of the soil, the presence of vegetation and the volume and intensity of precipitation. The amount of water in an aquifer is indicated by the height of the water table which is the upper boundary of the aquifer.
Water vapor in the atmosphere condenses and is delivered to the earth as dew, rain and snow. Condensed water droplets, or ice particles in the atmosphere, are visible to us as clouds. The movement of water from the atmosphere to earth is called precipitation.
Some of the precipitation that falls on land seeps into the ground where it is stored in aquifers and is transported to streams and lakes by subsurface flow. The amount of infiltration is influenced by the permeability and moisture content of the soil, the presence of vegetation and the volume and intensity of precipitation. The amount of water in an aquifer is indicated by the height of the water table which is the upper boundary of the aquifer.
Water which seeps into the ground passes through a zone of aeration where the open spaces between soil particles are filled with both air and water. We cannot pump water from this zone. Below the zone of aeration lies the zone of saturation where all of the spaces between soil particles are filled with water. The water in this zone is called groundwater and is available for pumping. An aquifer is made up of soil, rock and water in the zone of saturation. The dividing line between the two zones is called the water table. Ground water moves through the subsurface supplying streams and lakes.
Water that does not infiltrate the soil runs off across the surface of the watershed. Runoff begins as a sheet of water, and then turns into small trickles and then streams. Runoff may contribute water directly to lakes, or it may flow first to streams (streamflow) and then to lakes or the ocean.
Runoff can erode the land surface leading to pollution of lakes and rivers. If precipitation is intense and/or the soil is saturated, flooding can occur. Watershed managers try to reduce erosion and flooding, by using good land use practices and by building control structures.
Evaporation is the process by which water is converted from its liquid form to its vapor form and thus transferred from land and water masses to the atmosphere. Evaporation from the oceans accounts for 80% of the water delivered as precipitation, with the balance occurring on land, inland waters and plant surfaces.
Water that does not infiltrate the soil runs off across the surface of the watershed. Runoff begins as a sheet of water, and then turns into small trickles and then streams. Runoff may contribute water directly to lakes, or it may flow first to streams (streamflow) and then to lakes or the ocean.
Runoff can erode the land surface leading to pollution of lakes and rivers. If precipitation is intense and/or the soil is saturated, flooding can occur. Watershed managers try to reduce erosion and flooding, by using good land use practices and by building control structures.
Evaporation is the process by which water is converted from its liquid form to its vapor form and thus transferred from land and water masses to the atmosphere. Evaporation from the oceans accounts for 80% of the water delivered as precipitation, with the balance occurring on land, inland waters and plant surfaces.
The rate of evaporation depends on:
· Wind speed: the higher the wind speed the more evaporation.
· Temperature: the higher the temperature, the more evaporation.
· Humidity: the lower the humidity, the more evaporation.
Runoff that is not extracted from land surfaces by evaporation and transpiration is delivered to streams, lakes and oceans. Changes in streamflow over time can be illustrated using a hydrograph.
The amount of urbanization of a watershed has a dramatic effect on the amount of precipitation which is delivered to streams or lakes, and that which enters the soil or returns to the atmosphere. Urban watersheds typically have fewer plants for less transpiration and more impervious surfaces which cause less infiltration. This leads to increased runoff and higher streamflow and possible flooding during heavy rain.
In a watershed, we all live downstream. This means that the quality of water available to us is determined by our upstream neighbors. The water provided from the glaciers on Mt. Shasta supply drinking water for 23 million Californians. At the same time, up to one billion gallons of partially treated sewage is dumped into Sacramento-San Joaquin Delta and its associated waterways each day. Thus it is vital that we serve as environmental stewards of our watersheds, protecting the quality of that water for those who live downstream. And remember, everyone lives downstream.
· Wind speed: the higher the wind speed the more evaporation.
· Temperature: the higher the temperature, the more evaporation.
· Humidity: the lower the humidity, the more evaporation.
Runoff that is not extracted from land surfaces by evaporation and transpiration is delivered to streams, lakes and oceans. Changes in streamflow over time can be illustrated using a hydrograph.
The amount of urbanization of a watershed has a dramatic effect on the amount of precipitation which is delivered to streams or lakes, and that which enters the soil or returns to the atmosphere. Urban watersheds typically have fewer plants for less transpiration and more impervious surfaces which cause less infiltration. This leads to increased runoff and higher streamflow and possible flooding during heavy rain.
In a watershed, we all live downstream. This means that the quality of water available to us is determined by our upstream neighbors. The water provided from the glaciers on Mt. Shasta supply drinking water for 23 million Californians. At the same time, up to one billion gallons of partially treated sewage is dumped into Sacramento-San Joaquin Delta and its associated waterways each day. Thus it is vital that we serve as environmental stewards of our watersheds, protecting the quality of that water for those who live downstream. And remember, everyone lives downstream.
References:
Conservation Technology Information Center. 2010. "What is a Watershed?". http://www.conservationinformation.com/?action=learningcenter_kyw_whatisawatershed.
Shrestha, Ravi. 2004. “The Watershed Concept”. Michigan Environmental Education Curriculum. http://techalive.mtu.edu/meec_index.htm.
United States Environmental Protection Agency. July 22, 2009. “What is a Watershed?”. http://www.epa.gov/owow/watershed/whatis.html
Conservation Technology Information Center. 2010. "What is a Watershed?". http://www.conservationinformation.com/?action=learningcenter_kyw_whatisawatershed.
Shrestha, Ravi. 2004. “The Watershed Concept”. Michigan Environmental Education Curriculum. http://techalive.mtu.edu/meec_index.htm.
United States Environmental Protection Agency. July 22, 2009. “What is a Watershed?”. http://www.epa.gov/owow/watershed/whatis.html
