Water Science Pictures An example of a watershed (drainage basin)
You're expecting to see a picture of a watershed -- so why am I showing you a picture of my in-law's pool in winter? This covered pool is really a good example of a watershed.
The blue cover represents the watershed, the area in which precipitation that falls flows "down-gradient" towards the lowest part of the basin, the center of the pool cover in this case. Likewise, in nature, water flows towards a valley, river, or lake. The lake in the center of the pool forms for the same reason that a lake will form on the landscape -- it is the lowest area around and the water comes in, through a river, seepage into the ground, or by evaporation, faster than it can get out. In most valleys, the land slopes downhill somewhere, in which case, a river will form.
Do you see why this pool cover is not exactly the same as a real drainage basin?
What is a watershed?
A watershed is an area of land that drains all the streams and rainfall to a common outlet such as the outflow of a reservoir, mouth of a bay, or any point along a stream channel. The word watershed is sometimes used interchangeably with drainage basin or catchment. Ridges and hills that separate two watersheds are called the drainage divide. The water resources of a watershed include surface water--lakes, streams, reservoirs, and wetlands--and all the underlying groundwater.
A watershed is a precipitation collector
Most of the precipitation that falls within the Peachtree Creek watershed upstream of Northside Drive collects in the creek and eventually flows by the Peachtree Creek gaging site. Many factors, some listed below, determine how much of the streamflow will flow by the monitoring site. Imagine that the whole basin is covered with a big (and strong) plastic sheet. Then if it rained one inch, all of that rain would fall on the plastic, run downslope into gulleys and small creeks and then drain into Peachtree Creek. Ignoring evaporation and any other losses, then all of the approximately 1,512,000,000 gallons of water that fell (you can use our interactive rainfall calculator to find out how many gallons of water fall during a storm) as rainfall would eventually flow by the Peachtree Creek monitoring site.
Not all precipitation that falls in a watershed flows out
To picture a watershed as a plastic-covered area of land that collects precipitation is overly simplistic and not at all like a real-world watershed. A career could be built on trying to model a watershed water budget (correlating water coming into a watershed to water leaving a watershed). There are many factors that determine how much water flows in a stream (these factors are universal in nature and not particular to a single stream):
Precipitation: The greatest factor controlling streamflow, by far, is the amount of precipitiation that falls in the watershed as rain or snow. However, not all precipitation that falls in a watershed flows out, and a stream will often continue to flow where there is no direct runoff from recent precipitation.
Infiltration: When rain falls on dry ground, some of the water soaks in, or infiltrates the soil. Some water that infiltrates will remain in the shallow soil layer, where it will gradually move downhill, through the soil, and eventually enters the stream by seepage into the stream bank. Some of the water may infiltrate much deeper, recharging ground-water aquifers. Water may travel long distances or remain in storage for long periods before returning to the surface. The amount of water that will soak in over time depends on several characteristics of the watershed:
Soil characteristics: Clayey and rockey soils of Georgia's Piedmont region (including the Atlanta area) absorb less water at a slower rate than sandy soils, such as in Georgia's Coastal Plain. Soils absorbing less water results in more runoff overland into streams.
Soil saturation: Like a wet sponge, soil already saturated from previous rainfall can't absorb much more ... thus more rainfall will become surface runoff.
Land cover: Some land covers have a great impact on infiltration and rainfall runoff. Impervious surfaces, such as parking lots, roads, and developments, act as a "fast lane" for rainfall - right into storm drains that drain directly into streams. Flooding becomes more prevalent as the area of impervious surfaces increase.
Slope of the land: Water falling on steeply-sloped land runs off more quickly than water falling on flat land.
Evaporation: Water from rainfall returns to the atmosphere largely through evaporation. The amount of evaporation depends on temperature, solar radiation, wind, atmospheric pressure, and other factors.
Transpiration: The root systems of plants absorb water from the surrounding soil in various amounts. Most of this water moves through the plant and escapes into the atmosphere through the leaves. Transpiration is controlled by the same factors as evaporation, and by the characteristics and density of the vegetation. Vegetation slows runoff and allows water to seep into the ground.
Storage: Reservoirs store water and increase the amount of water that evaporates and infiltrates. The storage and release of water in reservoirs can have a significant effect on the streamflow patterns of the river below the dam.
Water use by people: Uses of a stream might range from a few homeowners and businesses pumping small amounts of water to irrigate their lawns to large amounts of water withdrawals for irrigation, industries, mining, and to supply populations with drinking water.