STORMWATER HYDROLOGY AND QUALITY - Low Impact Development and Sustainable Stormwater Management - page 34

sources: base flow from the groundwater discharge and surface runoff immedi-
ately flowing precipitation. In most undeveloped perennial streams and rivers,
the base flow comprises about 60% of the annual discharge and occurs about
90% of the time; that is, when it has not rained in a day or two, all of the stream
discharge is comprised of base flow.
2.2 REGIONAL HYDROLOGY
The landform, or physiography, creates the boundaries between watersheds and
controls the movement and direction of surface waters and rainfall runoff. The
amount of runoff that occurs within a basin in any given year, however, is the
result of regional hydrology and climate. Most of the United States experiences
seasonal differences because of location, and within any given climate zone
fairly large variations are possible from year to year. In northern watersheds,
the available water can be stored in the form of snow for many months before
it is released to the surface stream network, resulting in dry conditions in a
watershed even when rainfall (or snowfall) has occurred. While the hydrologic
cycle remains a constant over time, the amount of infiltration, evapotranspiration,
surface runoff, and base flow can vary greatly with relatively equal rainfalls.
Historically, water was presumed to be a resource to be exploited, and a
watershed was viewed as capable of “yielding” the total incident rainfall. A
few decades ago, some studies proposed that vegetation be totally removed or
greatly reduced in heavily wooded basins in North Carolina to eliminate evap-
otranspiration (usually more than half of the annual rainfall) and allow greater
runoff capture in lakes and reservoirs for human activities such as cultivation
and consumption. With an increased awareness of the importance of woodlands
and forests in sustaining not only the hydrologic cycle but also the atmospheric
balance of CO 2 , these types of schemes have faded from general consciousness,
although in some parts of the world they are still given consideration.
In any region of the country, the cycles of extreme weather, which from a water
perspective translate into flood and drought, are exacerbated by human activity.
This is especially true in those basins or regions that have experienced exten-
sive development and the construction of impervious surfaces, because every
drop of rainfall becomes immediate polluted runoff. In many highly developed
watersheds, the total discharge of surface waters may remain the same, but the
distribution between immediate runoff and base flow changes dramatically, as
does the ecology of the receiving streams and rivers.
In Chapter 1 we offered some statistics on rainfall variability in different
regions of the United States, but perhaps the most telling statistic is the difference
between rainfall and evapotransporation for very different parts of the United
States. This is the volume of rainfall that is available within a given watershed
for all uses, both natural and human. It also illustrates the seasonal variability of
the two major elements of the hydrologic cycle and the resulting surface flows
that support or limit land development.