Stormwater Calculations - Mathematics and Statistics for the Environment - page 765

Atmospheric Water

Clouds

Clouds

Evapotranspiration
(from plants and
inland waters)

Precipitation

Evaporation
from the
ocean

Water
Distribution

Water
Processing

City

POTW

Surface
Water Supply

Wa stewater
Collection

River

City

Indirect
municipal reuse

Wa stewater
Treatment

Water
Processing

Water-based
Recreation
(Indirect Reuse)

City

Lake

Disposal

Estuary

OCEAN

FIGURE 25.2 Urban water cycle.

25.3.1 p reCipitation
Precipitation is a random event that cannot be predicted based on historical data. However, any
given precipitation event has several distinct and independent characteristics which can be quanti-
fied as follows:

• Duration —Length of time over which precipitation occurs (hours)
• Depth —Amount of precipitation occurring throughout the storm duration (inches)
• Frequency —Recurrence interval of events having the same duration and volume
• Intensity —Depth divided by the duration (inches per hour)

A specified amount of rainfall may occur from many different combinations of intensities and dura-
tions, as shown, for example, in Table 25.1. Note that the peak intensity of runoff associated with
each combination will vary widely. Moreover, storm events with the same intensity may have signif-
icantly different volumes and durations if the specified storm frequency (2-year, 10-year, 100-year)