The USGS Water Science School
As is typical of small urban streams, Peachtree Creek in Atlanta, Ga., USA, rises quickly and exhibits a large increase in streamflow when a major rainstorm hits the watershed. Thus, during a storm, many times more water can flow in a few hours as flows in a few days of base flow (generally periods of minimal/near minimal streamflow when no precipitation has fallen for a while). The pie charts below show that a large amount of the total streamflow occuring during a year can occur in just a few days. In 2001, for example, the 10 highest days of daily mean streamflow accounted for 36 percent of the total streamflow for the year.
If you have ever wondered how many gallons of water falls during a storm, use our interactive rainfall calculator. to find out!
On Dec. 24, 2002, about two inches of rainfall fell in the Peachtree Creek watershed. This provides a good example to describe streamflow characteristics during a storm since the rain fell for only a few hours on that day and Peachtree Creek was at base-flow conditions before the rain started.
The chart below shows rainfall, in inches, during each 15-minute increment on Dec. 24th and the continuous measure of streamflow, in cubic feet per second (ft3/s).
This pictures is a good example of how much more water is flows during high-water event than during baseflow conditions. You might be surprised at just how much more water is flowing. As the table of streamflows during the storm of Dec. 24, 2002 shows, streamflow during a flood can be well over 100 times more than during base flow.
|Time||Stream stage, in feet||Cubic feet per second||Gallons per second||Streamflow, in gallons, during 15-minute interval|
The instantaneous streamflow at 10:00 was about 154 times greater than it was at midnight. Almost 50,000 gallons of water per second was flowing during the peak streamflow period.
It is possible to estimate the total amount of water that flowed during Dec. 24, 2002, and compare it to a day when the streamflows are at baseflow conditions. Using the rating curve for Dec. 24th, on a day when stream stage stays at 2.81 feet, an estimated 27,800,000 gallons of water will flow by the Peachtree Creek gaging station. Using mean streamflows for each 15-minute period during the storm of Dec. 24, an estimated 4,290,000,000 gallons flowed by. That would be about 154 times more water than during a day of base flow.
The chart of the Dec. 24th storm illustrates a number of typical patterns significant to how small urban streams react to heavy rainfall.
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