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How much water flows during a storm?

Much of the total streamflow occurs in a short period of time

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.

Pie charts below show that a large amount of the total streamflow occuring during a year can occur in just a few days

If you have ever wondered how many gallons of water falls during a storm, use our interactive rainfall calculator. to find out!

Case study: The storm of December 24, 2002

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).

Chart showing rainfall every 15 minutes and continuous streamflow during December 24, 2002.

A lot more water flows during a storm than during base-flow conditions

Picture showing a yard alongside Peachtree Creek, Atlanta, Georgia during a dry period and also during a flood.

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.

Comparison of streamflow before and during the flood of Dec. 24, 2002
 Instantaneous streamflow 
TimeStream stage, in feetCubic feet per secondGallons per secondStreamflow, in gallons, during 15-minute interval
   Data are rounded

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.

Peachtree Creek is typical of urban streams

The chart of the Dec. 24th storm illustrates a number of typical patterns significant to how small urban streams react to heavy rainfall.

Can you guess how much water falls during a storm?
Visit our Activity Center and find out.

Sources and more information

Related topics:

Impervious surfaces  Rivers and sediment  Rain  100-year floods
Stormflows   Streamflow patterns  Measuring streamflow   Floods Q&A
Impervious surfaces and flooding   A water monitoring site   High-water marks

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