Environmental Engineering Reference
In-Depth Information
FIGURE 15.2
Site and situation.
Situation
Atmosphere
Site
Interactions
Inputs
(Matter and
energy)
Outputs
Air, soil, water, slope,
vegetation, proposed
uses, energy conversions
Surface
Subsurface
FIGURE 15.3
Inflows and interactions at a parcel-scale site.
encompassing watershed (the dashed-line outer boundary) exhibit a site and situation rela-
tionship. The bidirectional arrow indicates there are flows of matter and energy to and
from the site.
What are some of these flows? From the vertical perspective, the atmosphere may deliver
precipitation from the watershed area and beyond onto the parcel. At the surface level,
eroded material washed down a hillslope in the headwaters region of the stream may be
deposited in a sink located within the parcel. Below the surface, contaminated ground-
water from outside the parcel may migrate to surface water within the parcel. Thus, the
site (e.g., parcel, city, and watershed) is an intersection of matter and energy flows, where
the different processes initiating and sustaining the flows interact (Figure 15.3). With land
parcels, the major share of the flows originates from off-site, and the different widths of the
arrows indicate the relative flow contributions of situation and site.
Although sites have readily identifiable geographic scales (e.g., a bounded city or a parcel
of property), situational geographic scales often vary. For instance, the precipitation deliv-
ered to a site may have been acidified hundreds of kilometers away, while the eroded soil
deposited on the site originated from the adjacent parcel. Accounting for these different
situational scales allows the investigator to identify and characterize the materials and
energy reaching the site and is essential to understanding the interactions on the current
landscape.
15.3.5 On-Site Interactions and Thresholds
The list of landscape elements shown interacting at the site in Figure 15.3 is generalized. At
a finer level of distinction, water includes precipitation, interflow, baseflow, surface runoff,
streamflow, soil moisture, water vapor, and interception storage. What changes will occur
to all of these water forms and flows when a new structure is built on a hillslope? Now
the interactions must include a consideration of soil type (composition and texture), soil
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