RAINWATER AS THE RESOURCE - Low Impact Development and Sustainable Stormwater Management - page 16

About 90% of the San Gabriel River's flow is captured for recharge into the ground-
water supply. On average, only about 20% of the upper Los Angeles River native
runoff is captured, due to a lack of sufficient spreading capacity and the prevalence
of clay soils.
The state Water Board has developed models of the current reality of water
balance in southern California, and these are a far cry from simplistic illus-
trations. Obviously, no single model adequately describes the existing cycle of
water in all of the region's watersheds and the importance of stormwater in the
cycle. Based on the current reality, LID must be integrated in the land devel-
opment and redevelopment process to use the limited rainfall received most
efficiently to sustain the daily water demand of 140 gallons per person, even
as imported supplies diminish. At the same time, we must restore those ele-
ments of the natural system that remain or can be sustained. Even this statistic
of per capita consumption must be reduced, and that reduction turns on ways
to sustain our landscapes with less water, using stored rainfall until we can
change the landscape paradigm to a more natural vegetative palette over the
next decade.
1.4 THE ALTERED WATER BALANCE AND HYDROLOGIC IMPACTS
Imperviousness
How do our current development practices affect the hydrologic cycle? The most
obvious alteration is the use of impervious materials to build our communities,
which for virtually all of the past 10 millennia consisted only of the rooftops of
our buildings, temples, and homes. Until the beginning of the twentieth century,
the land surface surrounding building structures was always comprised of com-
pacted soil or paving stones in one form or another (Figure 1-11), with only a
few major cities utilizing pavement surfaces that could be considered impervious.
This building pattern was totally altered a century ago, when the battle was
concluded over which energy system would be used to power the new horse-
less buggy. The internal combustion engine won out over the steam engine,
and gasoline became the fuel to power the rapidly expanding demand for trans-
portation vehicles. This production in the “cracking tower” of petroleum refining
resulted in an accumulation of “tar” in the bottom of the tower, similar in prop-
erty to natural occurring tar pits mined in a few locations around the world.
It was soon discovered that this residue of gasoline manufacturing could be
combined with the graduated stone roads of the previous century (macadam)
to form Tarmac surfaces, well suited for paving of our urban streets and road-
ways (Figure 1-12). As the demand for gasoline skyrocketed, the availability
of the newly defined “asphalt concrete” made the surfacing of roads affordable,
and so began a cycle of increased imperviousness that characterized the land
development process throughout the twentieth century.