WATERSHEDS - Water Quality Engineering in Natural Systems - page 202

6

WATERSHEDS

6.1 INTRODUCTION Other goals of watershed management include flood

protection, erosion control, groundwater protection,
and habitat preservation.
Effective watershed management involves many sci-
entific disciplines, including geology, biology, chemistry,
and hydrology; however, socioeconomic and political
considerations are frequently deciding factors. Public
awareness of the relationship between polluting activi-
ties within a watershed and the quality of the receiving
water body can contribute significantly to water-quality
protection.
The two categories of land uses that most affect the
quality of ground and surface waters are urban and
agricultural land uses. This chapter covers pollution
sources commonly associated with urban and agricul-
tural lands, the fate and transport of pollutants originat-
ing on these lands, and stormwater control measures
(SCMs) that can be used to limit pollution originating
from these lands.

A watershed is an area that captures atmospheric pre-
cipitation (rain and snow) and drains any resulting
surface runoff to a surface water body, such as a river,
lake, or estuary. In the case of rivers and streams, a
watershed is typically associated with the entire length
of the stream, and the fraction of the watershed that
contributes flow to any location along the stream
increases as you move downstream. The portion of a
watershed contributing surface runoff to a given stream
segment is called a subwatershed . Features of a typical
watershed are illustrated in Figure 6.1. Large water-
sheds are sometimes called basins (e.g., the Colorado
River Basin or the Susquehanna River Basin). Concep-
tually, all of the Earth's land surface can be divided into
distinct watersheds, with a water body or stream associ-
ated with each watershed. Most importantly, since con-
taminant input to a water body is generated from
activities within its watershed, the control of polluting
activities and surface runoff pathways within a water-
shed is a fundamental component of water-quality
control. This approach is commonly referred to as water-
quality based watershed management , and this approach
works best when there is a single governmental entity
with jurisdiction over the entire watershed.
The primary goal of water-quality based watershed
management is to control activities within a watershed
and thereby limit the input of contaminants into the
receiving water body. Watershed-scale risk assessment is
concerned with the quantitative stochastic relationship
between contaminant sources and surface-water quality.

6.2 URBAN WATERSHEDS

Urban development is associated with population
increases, development of transportation networks and
other urban infrastructure, and the increased presence
of pollution sources associated with traffic density and
commercial and industrial development. Urbanization
invariably increases the percentage of impervious area
in a watershed, and imperviousness and drainage
infrastructure are by far the most important features
to which pollution loads can be related. Impervious