evident from this figure that the longer the duration of precipitation, the less intense
will be the storm.
In addition to the variation of precipitation with time, there is a very definite
geographic variation in precipitation, particularly in considering a large area such as
a drainage basin. Numerous methods have been presented to determine a representative value for precipitation over a large drainage basin area. The first method for
determining a representative value would be to take the arithmetic mean of several
precipitation stations within the basin. Values can be based on monthly or annual
precipitation, but usually are not based on shorter periods of time. In the arithmetic
method, only values from stations that are located within the stated drainage basin
area are used. No results from adjacent stations are employed, even where a station
may be very close to, but outside of, the drainage basin under study.
This has been corrected significantly by utilization of the Thiessen method [4] for
determining precipitation over an area (Fig. 6.5). In this method stations located
close to the basin boundary may be considered, but only a weighted average as a
Fig. 6.5 Thiessen method
for average precipitation
over an area
242
D. B. Aulenbach et al.