Environmental Engineering Reference
In-Depth Information
Figure 5.6
Mean monthly precipitation at Amarillo, Texas, in
a subtropical summer rainfall zone.
Cherrapunji in the Khasi hills of Assam, India, for example, mean rainfall during August
is over 1600 mm; from November to February it is almost zero. In 1974 24,550 mm were
recorded, the vast majority falling in the May-September period, and on 16 June 1995
1563 mm fell in one day. Much of the precipitation is the result of a funnelling of moist
air from the Bay of Bengal up the slopes of the Khasi hills by upper-tropospheric
easterlies, not simple convection. In the monsoon areas of Africa and northern Australia
seasonal differences are also great, so that hydrological conditions vary considerably
throughout the year. During the dry season there is practically no surface run-off, but in
the wet season run-off is extensive. Vegetation, geomorphological processes and human
activities all respond to these changes.
RAINFALL FREQUENCY
In view of the important consequences of extreme variations in rainfall, it is useful to
have some measure of the reliability of precipitation. This may be expressed in a number
of different ways. One of the most common is to plot graphs of what are called rainfall
recurrence intervals. Using data from a long time period, say fifty years, it is possible to
estimate the frequency with which storms of a particular amount or intensity are
exceeded. In general, small storms occur most commonly and very heavy storms only
rarely. Thus a graph like that in Figure 5.7 is obtained. From this it is possible to tell how
frequently a storm giving, for example, 50 mm or less in a day will occur, or how many
years it will be on average between storms of 100 mm or more per day. Such information
can be very useful in planning bridges or drains, when the aim is
