RAINWATER AS THE RESOURCE - Low Impact Development and Sustainable Stormwater Management - page 2

Transpiration

Shallow
infiltration

Evaporation

Deep
infiltration

Groundwater discharge to lakes,
streams, & oceans

Figure 1-1 The hydrologic cycle.

The hydrologic cycle or water balance serves as a model for understanding
the concept of sustainability of our water resources (Figure 1-1). The challenge
of sustainability is to draw upon elements of this cycle to serve our needs
without significantly disrupting the balance. With careful land use planning and
water resource management, every available drop of rain can be used and reused
to meet our needs without destroying the quality or affecting the character of
natural streams and rivers. Many of our uses, such as drinking supply, can be
largely recycled with the proper waste system design, and many other uses can
be reduced in quantity if they are largely “consumptive” uses, such as irrigation
of artificial landscapes. Consumptive demands of cultivation can also be reduced
by methods such as drip irrigation, and energy systems can be designed that
do not consume fresh water in the cooling process. All modern water supplies
require energy, and most energy systems affect water. Similar to the land-water
dynamic, the energy-water interrelationship requires that any system changes
consider both resources.
The principle of water balance is best understood in the context of a measurable
land area—watershed, drainage basin, or land parcel—that quantifies the water
cycle. The rain that falls on the land surface over a period of time defines the
magnitude of the resource and the quantity required to sustain the cycle. The
potential demands on this balance imposed by our land development process can
then be applied to this model as an initial step in understanding how the cycle
should guide our activity on the land.
Perhaps the easiest way to understand the concept of the water balance is
to consider a small unit area (Figure 1-2), an acre or hectare, and measure the
movement of rainfall through this tiny portion of the planet. The flow begins (or
continues) with rainfall, shown in the figure as the annual average for a temperate
climate, the mid-Atlantic region of eastern North America. Whereas the annual
amount of rainfall varies greatly from place to place across the United States
(Table 1-1) and can also experience significant seasonal differences (Table 1-2),
the hydrologic cycle remains a constant.