Water Resources of Orlando Florida

Duration_Plots Based on Mean Daily Flow

02231000 St Marys River near Macclenny, FL

02232000 St Johns River near Melbourne, FL

02232400 St Johns River near Cocoa, FL

02232500 St Johns River near Christmas, FL

02234000 St Johns River ab Lake Harney near Geneva, FL

02235000 Wekiva River near Sanford, FL

02266300 Reedy Creek near Vineland, FL

02312000 Withlacoochee River at Trilby, FL

02313000 Withlacoochee River near Holder, FL

Explanation of Duration Graphs

Duration graphs charaterize the state of the hydrology of a stream with respect to time. The plotted percentiles are best explained by an example. Suppose 40 years of daily value flow data exist for a station and the 5-percentile flow is 25 cubic feet per second (cfs) for a single day of the year, say January 3. This means that 5 percent of all flows that occurred on the day January 3 of each of the 40 years of data were equal to or less than 25 cfs. To quantify the state of the hydrology for the current year, the current daily mean flows are compared to these percentiles.

In the duration graphs, the percentiles are smoothed to minimize considerable day-to-day variation. However, the daily minimums and maximums for the period of record are not smoothed.

To interpret the graphs, please consider the following:

1. Although not completely correct in a statistical sense, the non-exceedance of the 5-percentile and the exceedance of the 95-percentile flows may be considered to have a recurrence interval of approximately 20 years.

Likewise, the 10- and 90-percentile flows may be considered to have a recurrence interval of approximately 10 years, and the 25- and 75-percentile flows may be considered to have recurrence intervals of approximately 4 years.

2. In the graphs, it is assumed that "normal flows" lie between the 25- and 75-percentiles, based on the assumption that agriculture, industry, and municipalities can operate normally under high and low flows having a recurrence interval of 4 years.

3. The 7-day minimum flow with a recurrence interval of 10 years is represented on the graph as the 7Q10 line.

4. A line representing 20 percent of the average flow at a station is also included as an indicator of drought severity.