Monthly and yearly streamflow patterns

Streamflow at Peachtree Creek varies by month

The U.S. Geological Survey (USGS) has been measuring daily mean (average) streamflow at Peachtree Creek since July 1958. With such a long period of record, it is possible to see a pattern of streamflow variation by month and season. The chart below shows the mean of monthly streamflow for each month. So, the January value of 174 cubic feet per second (ft³/s) was computed by averaging the 44 mean January streamflows (1959 to 2002). Atlantans would not be surprised that highest mean streamflows occurred in March. The chart also shows that while summer is indeed normally dry in Atlanta (except for 2003!), October has been the month when the lowest mean streamflows have been recorded.

Table of mean monthly streamflow statistics at Peachtree Creek (new window).

Seasonal streamflow patterns

To view the seasonal pattern of streamflow at Peachtree Creek is to look at a pie chart showing the percentage of yearly streamflow occuring in each month. Naturally, the streamflow patterns closely follow rainfall patterns. In Georgia, winter frontal storms result in moderate rainfall over multiple days; spring storms result in heavy rainfall; summer produces brief thunderstorms; and autumn is generally dry. This chart shows that 35.9 percent of the yearly streamflow occurred in the months of January to March, whereas only 17.2 percent of the yearly streamflow occurred during the typically drier months of August to October.

Yearly patterns: Dry years versus wet years

Streamflow at Peachtree Creek can vary greatly both in the short term and long term. The chart below compares the monthly mean streamflow (the average of all daily streamflows for each month) for 1999, a year of severe drought, with streamflow during 1975, which is considered a wet year.

Streamflows vary by month

The streamflow for 1975 show a more typical pattern of streamflow in Georgia, with relatively high streamflow occuring during the spring and lower streamflow during the autumn. The typical springtime streamflow pattern did not occur during 1999, with mean streamflow for March being only 80 cubic feet per second (ft³/s) as compared to 487 ft³/s during 1975. This makes the March 1975 streamflow about six times greater than streamflow in March of 1999.

Streamflow varies by year

The table below shows the annual mean streamflow for the years 1959-2001. Annual mean streamflow is the average of all daily streamflows for the year. The long-term annual mean streamflow for the period of record is about 136 ft³/s. Variability in annual mean streamflow is shown by contrasting the wet year of 1975, when annual mean streamflow was 213 ft³/s to the dry year of 1999, when mean streamflow was 86.9 ft³/s for the dry year of 1999. Thus, streamflow during 1975 was about 56 percent above the long-term average, and streamflow during 1999 was only about 63 percent of the long-term annual average.