RAINWATER AS THE RESOURCE - Low Impact Development and Sustainable Stormwater Management - page 12

8

7

7.3 in.

6

5

4

3

3.3 in.

2

1

0

0 4 8 12

Time (hr)

16 20 24

Figure 1-9 The S curves of assumed rainfall intensity and distribution; 2- and 100-year-
frequency storms in southeastern Pennsylvania.

most critical aspect of site development. When this basic need is satisfied, we
consider the return of this water to the cycle, containing all of the pollutants
we have added during our use. The need for increasingly efficient pollutant
removal processes during the past century has resulted largely from the increase
in population and density in our land development, as we realized that the sewage
from one community is the water supply for downstream residents.
Stormwater has been regarded as a nuisance, to be drained away from our
developments as quickly as possible following a rainfall. In arid environments,
the value of rainfall to support our continued occupation of habitat is an unuti-
lized resource, especially when the available water supply is limited and runoff
is discharged to coastal waters. Nowhere is this lack of water resource manage-
ment more apparent than in southern California. In these coastal watersheds, land
developments draining to the Pacific coast and to inland waters and reservoirs
have generated significant increases in stormwater runoff volume, which in turn
has contributed to the discharge of pollutants into receiving waters, degraded
aquatic habitat, affected the recreational use of these waters, and interfered with
their use as water supply. Through implementation of LID practices, these pol-
lutant discharges can be reduced significantly, so that the quality of these coastal
and inland waters can be restored and sustained.
But the potential of LID goes well beyond reducing the volume of polluted
stormwater runoff. This rainfall can also be understood as a lost resource in the
semiarid environment of southern California, where increasing demand for fresh
water requires costly importation of water supplies to sustain ever-growing com-
munities. Interestingly, the quantity of water imported into southern California is
almost equal to the net loss of stormwater runoff to coastal waters (Table 1-5),
referred to as the salt sink .