When rain falls onto the earth, it just doesn’t sit there, it starts moving according to the laws of gravity. A portion of the precipitation seeps into the ground to replenish Earth’s groundwater. Most of it flows downhill as runoff. Runoff is extremely important in that not only does it keep rivers and lakes full of water, but it also changes the landscape by the action of erosion. Flowing water has tremendous power it can move boulders and carve out canyons; check out the Grand Canyon!
Runoff of course occurs during storms, and much more water flows in rivers (and as runoff) during storms. For example, in 2001 during a major storm at Peachtree Creek in Atlanta, Georgia, the amount of water that flowed in the river in one day was 7 percent of all the streamflow for the year.
Some definitions of runoff:.
1. That part of the precipitation, snow melt, or irrigation water that appears in uncontrolled (not regulated by a dam upstream) surface streams, rivers, drains or sewers. Runoff may be classified according to speed of appearance after rainfall or melting snow as direct runoff or base runoff, and according to source as surface runoff, storm interflow, or groundwater runoff.
2. The sum of total discharges described in (1), above, during a specified period of time.
3. The depth to which a watershed (drainage area) would be covered if all of the runoff for a given period of time were uniformly distributed over it.
Meteorological factors affecting runoff:
Type of precipitation (rain, snow, sleet, etc.)
* Rainfall intensity
* Rainfall amount
* Rainfall duration
* Distribution of rainfall over the watersheds
* Direction of storm movement
Antecedent precipitation and resulting soil moisture
*Other meteorological and climatic conditions that affect evapotranspiration, such as temperature, wind, relative humidity, and season.
Physical characteristics affecting runoff:
* Land use
* Soil type
* Drainage area
* Basin shape
* Direction of orientation
* Drainage network patterns
* Ponds, lakes, reservoirs, sinks, etc. in the basin, which prevent or alter runoff from continuing downstream
Runoff and water quality :
A significant portion of rainfall in forested watersheds is absorbed into soils (infiltration), is stored as groundwater, and is slowly discharged to streams through seeps and springs. Flooding is less significant in these more natural conditions because some of the runoff during a storm is absorbed into the ground, thus lessening the amount of runoff into a stream during the storm.
As watersheds are urbanized, much of the vegetation is replaced by impervious surfaces, thus reducing the area where infiltration to groundwater can occur. Thus, more stormwater runoff occurs—runoff that must be collected by extensive drainage systems that combine curbs, storm sewers (as shown in this picture), and ditches to carry stormwater runoff directly to streams. More simply, in a developed watershed, much more water arrives into a stream much more quickly, resulting in an increased likelihood of more frequent and more severe flooding.
What if the street you live on had only a curb built around it, with no stormwater intake such as the one pictured here. Any low points in your street would collect water when it rained. And if your street was surrounded by houses with yards sloping uphill, then all the runoff from those yards and driveways would collect in a lake at the bottom of the street.
A storm sewer intake such as the one in this picture is a common site on almost all streets. Rainfall runoff, and sometimes small kids’ toys left out in the rain, are collected by these drains and the water is delivered via the street curb or drainage ditch alongside the street to the storm-sewer drain to pipes that help to move runoff to nearby creeks and streams. ; storm sewers help to prevent flooding on neighborhood streets.
Drainage ditches to carry stormwater runoff to storage ponds are often built to hold runoff and collect excess sediment in order to keep it out of streams.
Runoff from agricultural land (and even our own yards) can carry excess nutrients, such as nitrogen and phosphorus into streams, lakes, and groundwater supplies. These excess nutrients have the potential to degrade water quality.
Why might stormwater runoff be a problem?
As it flows over the land surface, stormwater picks up potential pollutants that may include sediment, nutrients (from lawn fertilizers), bacteria (from animal and human waste), pesticides (from lawn and garden chemicals), metals (from rooftops and roadways), and petroleum by-products (from leaking vehicles). Pollution originating over a large land area without a single point of origin and generally carried by stormwater is considered non-point pollution. In contrast, point sources of pollution originate from a single point, such as a municipal or industrial discharge pipe. Polluted stormwater runoff can be harmful to plants, animals, and people.
Runoff can carry a lot of sediment
When storms hit and streamflows increase, the sediment moved into the river by runoff can end up being seen from hundreds of miles up by satellites. The right-side pictures shows the aftermath of Hurricane Irene in Florida in October 1999. Sediment-filled rivers are dumping tremendous amounts of suspended sediment into the Atlantic Ocean. The sediment being dumped into the oceans has an effect on the ecology of the oceans, both in a good and bad way. And, this is one of the ways that the oceans have become what they are: salty.
Florida, Oct. 14, 1999. When Hurricane Irene passed over Florida in 1999, the heavy rainfall over land caused extensive amounts of runoff that first entered Florida’s rivers which then dumped the runoff water, containing lots of sediment, into the Atlantic Ocean.
Florida, Dec. 16, 2002. The east coast of Florida is mostly clear of sediment from runoff. The shallow coastal waters to the west of Florida are very turbid (sediment-filled), perhaps from a storm that passed over a few days earlier.
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