Duration is the period of time that rain falls at a constant rate or intensity. It does not mean the total time, from beginning to end, of rainfall. During any rain fall event (storm) the rainfall intensity will not be constant. It may vary from quite high to very low; so it is necessary to think in terms of how long a specific rainfall intensity lasts (min or hr) at a certain rate (in/min or mL/min). The example of the rainfall intensity-duration for a rainfall event in Figure 16.5 has eight (8) different durations during a rainfall event that lasted for 94 min. The total amount of rain

FIGURE 16.5. Example of intensity-duration for a rainfall event.

Rainfall Intensity, Duration, and Recurrence 227

FIGURE 16.6. Typical rainfall intensity—duration curve.

Rainfall duration (hr)

(inches or millimeters) for a rainfall event is the sum of each rainfall intensity times the duration of that intensity.

The average rainfall intensity for a storm is between the highest and the lowest rainfall intensity. As a general rule, the high-intensity portion of a storm has a shorter duration than the low-intensity portions. A typical relationship between rainfall intensity and duration is illustrated in Figure 16.6.

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