RAINWATER AS THE RESOURCE - Low Impact Development and Sustainable Stormwater Management - page 7

Average
groundwater table
and stream base flow

Soil

Bedrock

Stream

Aquifer

Figure 1-5 Groundwater recharge feeds the local surface waters and sustains base flow.

initial point of infiltration. In coastal watersheds, the groundwater may discharge
directly to estuary systems, never reappearing on the land surface.
In some physiographic regions, a fraction of the infiltrating rain enters into
deeper aquifers and does not reappear at the surface, but may remain stored
for centuries. In active seismic regions, geothermal sources may actually bring
some of the deep water to the surface. This vertical flow of groundwater may
comprise a portion of surface systems, such as the Snake River tributary in the
Columbia River system, originating from the “hot spot” that forms the geysers
of Yellowstone (Figure 1-6). However, for most of the developed regions of the
United States, the simple model illustrated by Figure 1-2 is a valid representation
of this complex water balance.
While the full hydrologic cycle includes water movement on a global basis,
the consideration of stormwater management is limited to the freshwater portion
of the total resource, a fraction of the world's water (about 2.5%). Most of that
fresh water is currently contained in ice (although the future is quite uncertain)
and represents 77.2%, with an additional 22.6% contained in the subsurface
as groundwater, leaving only 0.32% in surface rivers and lakes, 0.18% as soil
moisture, and 0.04% in the atmosphere, for a total “available” water resource of
0.54% [8]. All of the following discussion is concerned with this sum of these
small portions, although it amounts to the trillions of gallons of water that sustain
the human biosphere.

1.2 THE WATER BALANCE BY REGION

Although the most obvious measure of the water resources available in a given
region of the country is the average annual rainfall received, this statistic can
be deceiving if we do not recognize the potential variability in this measure,