Image Processing Reference
In-Depth Information
14.2.1
Importance of Urban Vegetation
The abundance and spatial distribution of vegetation has a strong influence on the
urban and suburban environmental conditions. Vegetation influences energy fluxes
by selective reflection and absorption of solar radiation.
Vegetation has a significant cooling effect because it absorbs
much of the incoming solar radiation and dissipates the
energy by transpiring water rather than converting the energy
to heat and reradiating it as built surfaces do (Goward et al.
1985
; Gallo et al.
1993
; Price
1990
; Carlson et al.
1994
;
Gillies et al.
1997
; Owen et al.
1998
). This results in differ-
ent fluxes of moisture and solar radiation, influences com-
fort levels, and ultimately result in energy savings from
cooling. In addition to providing shade and wind shelterbelts,
urban trees can lower the ambient temperature around a
building or in a park, improve the air quality as well as reduce
the formation of urban smog (Akbari
2002
). In cold climates, trees shield buildings
from cold winter wind and thereby save energy on heating energy. In warm
weather, well-planned landscaping of trees and shrubs can significantly reduce the
daily air-conditioning electricity consumption by as much as 50% (Parker
1981
).
Well-maintained urban vegetation in the form of trees, grasses, and flowers is visu-
ally pleasing, particularly in arid environments where native vegetation is limited by
harsh environmental conditions.
urban trees can
be thought of as
in situ filters as
they lower
temperature,
improve the air
quality, and
reduce the
amount of
urban smog
14.2.2
Urban Microclimates
Although urban areas currently occupy only ~2% of the Earth's habitable land area
(Small et al.
2005
), their energy fluxes have a considerable impact on microclimate
and ecosystem function at local and regional scales (Berry
1990
; Landsberg
1981
).
Urban nitrogen emissions (NO
x
) have been shown to have an adverse impact on
regional agricultural productivity (Chameides et al.
1994
). There is some evidence
to suggest that urban heat island effects are responsible for inducing atmospheric
convergence sufficient to influence thunderstorm formation and movements observed
near large urban areas (Bornstein and LeRoy
1990
; Bornstein and Lin
2000
).
Regional climate models also indicate a strong sensitivity to land cover variations
at scales of kilometers (Pielke et al.
2002
; Roy and Avissar
2000
; Li and Avissar
1994
) suggesting that urban land cover conversion may significantly modify the
physical environment beyond the city itself.
14.2.3
Public Health Issues
The land surface modifications associated with urban growth impact a variety of public
health issues. We focus here on those related to the thermal environment and air quality.
