Environmental Engineering Reference
In-Depth Information
watersheds—stormwater and erosion—are connected by the flows of water, and land sub-
sidence related to groundwater withdrawal has become an issue in some urban areas.
15.3.3 The Dimensions of Landscape
The formative processes of landscapes proceed within a three-tiered geometry (Marsh
2010). In the vertical direction are the atmosphere, surface, and lithosphere. The atmo-
sphere contributes water from precipitation and wind as a result of pressure differentials.
Wind velocities near the ground are dampened by friction, but between altitudes of 7.5
and 15 m (25-50 ft) they increase significantly. This increase can influence the erosion and
particle transport capacity, as is seen with smokestacks.
The surface component of the vertical profile ranges from the top of tree canopies to the
bottoms of root systems. Here, flows of water, soil, and rock shape and reshape the features
of the landscape. Vegetation stabilizes the soil, intercepts precipitation, provides habitat
for organisms, and produces oxygen for the atmosphere and organic matter returned to
the soil. At the lowest level of the vertical extent, below the plant roots to the bedrock, are
the aquifers which, in humid regions, accept inflows from precipitation and discharge to
surface waters.
Horizontally, landscapes exhibit a length and width. This extent determines their geo-
graphic scale , which is defined through the observation and/or measurement of events
and processes. Table 15.2 shows the different scales associated with a small sample of
landscape events and processes.
The ordering of scales in Table 15.2 represents a hierarchy through which different pro-
cesses operate (perform work). For example, wind operates at all scales from the global to
the micro. We may also see the effects of plate tectonics at the local or microscale in the
form of a hillslope, or shattered or folded rock sequences.
15.3.4 Site and Situation
For the individual land parcels within watersheds, most of the matter and energy exchanges
originate “off-site.” Site refers to a fixed place, and situation represents the outlying places
related to the site (Figure 15.2). For example, a land parcel (the shaded rectangle) and its
TABLE 15.2
Common Associative Scales for Selected Landscape Processes
Scale
Association
Examples
Global
Entire earth
Plate tectonics, global warming
Continental
Entire continents, significant areas of multiple
continents, such as Eurasia
Orogeny (mountain building), glaciations
Regional
Small parts of continents, several nations, one or
more states within countries, large watersheds
Climate and weather, environmental
hazards (earthquakes, floods, hurricanes)
Local
Urban metropolitan areas, neighborhoods, several
small farms, small watersheds, subwatersheds
Photochemical smog, stormwater runoff,
groundwater, and surface water pollution
Microscale
Areas under 0.4 ha in size
Microclimates, edge effects a
a Edge effects are located at the margins of physical entities, such as forests. Different processes occur at the edge
(usually a very small area) because this location creates different relationships with other surrounding pro-
cesses. For instance, the trees located at the forest edge receive more sunlight and usually exhibit larger
canopies.
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