Environmental Engineering Reference
In-Depth Information
To broaden the scientific foundation necessary for sustainable planning efforts, this
chapter builds on the basic concepts of watersheds introduced in Chapter 3 by begin-
ning with a description of the structure and function of watersheds. An overview of the
general impacts of urbanization follows, and the chapter concludes by investigating the
specific disruptions of matter and energy flows in urban watersheds incurred by these
human-generated and urban-related processes: (1) water infrastructure and its impacts
on ecosystems, (2) increased stormwater runoff, (3) accelerated erosion and sedimentation,
and (4) the creation of excess sensible heat. The information garnered will help steer the
pollution prevention measures necessary to begin the return to dynamic equilibrium in
our streams, aquifers, and watersheds.
12.2 Watershed Structure
To identify the components of watershed structure, it is instructive to return to the words
used by John Wesley Powell when he defined watersheds: “… that area of land, a bounded
hydrologic system, within which all living things are inextricably linked by their common
water course…”
Reduced to their basic parts, watersheds are composed of land, water, air, and living
things. They are, however, not simple things—but complex three-dimensional systems
(Figure 12.1).
12.2.1 The Atmosphere
Starting at the top of Figure 12.1, the atmosphere initiates the input of water to water-
sheds through precipitation. Once discharged from clouds, precipitation has several travel
options: (1) continue its direction downward into soil as infiltration, (2) follow the nearest
downward gradient on the surface and become runoff, or (3) just stop and park on the
surface of a leaf where it may drip to the ground and follow paths 1 and 2 or evaporate.
Several factors influence the routing:
• Rainfall intensity (measured in mm/h)
• Rainfall type (drizzle or large drops)
Precipitation
Atmosphere
Watershed
divide
River
mouth
Surface
Sub-surface
Groundwater
(aquifer)
FIGURE 12.1
Three dimensions of a watershed. (From Kentucky, State of Kentucky, Division of Water website. http://www.
water.ky.gov/watersheds (accessed June 3, 2010), 2010.)
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