RAINWATER AS THE RESOURCE - Low Impact Development and Sustainable Stormwater Management - page 4

Table 1-2 Seasonal Variation of Rainfall in Regional Watersheds

Rainfall (in.)

Major River

Region Basin and City Spring Summer Fall Winter Total

Northeast Delaware River 11 11.3 10.6 9.7 42.5

Philadelphia, PA 26% 26% 25% 23%

Northwest Willamette River 13.8 3.5 15.7 27.2 60.2

Portland, OR 23% 6% 26% 45%

Southeast St. Johns River 10 10 19.1 12.2 51.2

Jacksonville, FL 19% 19% 37% 25%

Southwest Santa Anna River 4 0.4 2.7 7.5 14.6

Los Angeles, CA 27% 4% 15% 51%

The development of plants on the planet surface long preceded the mammals
from which we evolved, and plants have fulfilled their part in the hydrologic cycle
for several billion years. On those land surfaces that evolve a natural vegetative
cover, especially woodlands, the trees and grasslands utilize the input of rainfall
to live by photosynthesis [2], drawing the infiltrating moisture from the soil (or
directly from the atmosphere) and transforming the water into oxygen and organic
matter, a process described by the reaction

( 6H 2 O + 6CO 2 ) + ( sunlight, 48 mol ) = 6O 2 + C 6 H 12 O 6

This simple miracle of plant life is carried out by the role of chlorophyll in the
vegetation. In addition to producing the oxygen by which all species live, this
process maintains the critical balance of CO 2 in the atmosphere for the benefit
of all animal life forms, including the human species. While the air we breathe
is comprised primarily of 78% nitrogen with slightly less than 21% oxygen, the
role of minor gases (argon, 0.93%; CO 2 , 0.038%) and water (1%) is critical in
maintaining the temperature at a relatively constant level over time. The rapid
increase in CO 2 over the past century has played an important and causal role
in global warming, specifically as the result of burning fossil fuels [3, 4]. Thus,
the importance of sustaining surface vegetation, especially trees, during the land
development process cannot be overstated (Figure 1-4), as it compensates for
this human impact [5, 6]. It should be noted that terrestrial vegetation provides
only a portion of the photosynthetic production, with marine plankton actually
generating more of the balance on a global basis.
On a naturally vegetated land surface, about half of the rain that falls is
returned to the atmosphere by the evapotranspiration process. The balance of
infiltrating rainfall, not utilized by the vegetation or evaporated from the surface
by sunlight and air currents, infiltrates or percolates slowly (or quickly) into the
soil mantle. A portion of this rain drains deep into the soil and weathered rock
surface, eventually reaching the zone of saturation, described as the water table
(Figure 1-5), and becomes groundwater.