RAINWATER AS THE RESOURCE - Low Impact Development and Sustainable Stormwater Management - page 10

Figure 1-7 Hurricane Katrina strikes the U.S. Gulf coast.
the intensity of that precipitation have much to do with the resulting impact,
the hydrologic response of any given watershed is also a function of land cover
conditions, especially vegetation, and season, with frozen ground producing some
of the most severe runoff conditions during early spring in mountainous regions.
If we were to measure all of the rainfalls at a given location over a century, we
would find that the vast majority were of very small magnitude (Figure 1-8). The
pie chart in the figure shows rainfall distribution for southeastern Pennsylvania,
with a total annual rainfall of 44 in./yr. The relative distribution is the same
for most other regions, with most of the storms less than 3 in. in total rainfall,
and offers insight as to the defining statistic for a stormwater volume reduction
management strategy.
While the traditional focus of concern has been the extremes of rainfall
or drought, the major portion of our precipitation actually occurs in smaller,
more frequent events. In fact, in almost every major physiographic or climato-
logic region, the 2-year-frequency rainfall serves as the defining statistic for the
stormwater management designs that are outlined in this topic. This rainfall and