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Chapter 14
CHAPTER 14
Urban Ecosystems
URBAN ECOSYSTEMS ............................................................................................ 1
14.1
14.2
14.3
14.4
14.5
14.6
14.7
INTRODUCTION ........................................................................................................................ 2
URBAN CLIMATE ..................................................................................................................... 2
URBAN HYDROLOGY ............................................................................................................ 13
BIOGEOCHEMICAL CYCLES ................................................................................................. 14
BIODIVERSITY ........................................................................................................................ 16
THE ECOLOGICAL CITY ........................................................................................................ 18
TABLES .................................................................................................................................... 24
14.8
FIGURE LEGENDS ....................................................................................................................... 29
Ecological Climatology
14.1
Introduction
Large cities in the United States have a distinct morphology (Table 14.1). Single-family
residential is the dominant land use in all cities, ranging from 49% to 78% of total land area. Apartment
housing is generally modest (about 5%) except in Baltimore and Philadelphia, where row houses are
abundant. Industrial zones range from 10% to 22% of total area while commercial zones range from 7% to
17%. The height of buildings and the combined area of roofs and walls vary with these land uses. Building
heights are lowest in single family residences (one to two stories) and highest in apartment and commercial
buildings. Commercial buildings, which include high-rise office buildings and retail shopping centers, are
not as high as might be expected due to the small area devoted to central business districts. They do,
however, have a wide range from one-story retail centers to 40-story skyscrapers. The combined surface
area of roofs and walls ranges from a low of 72 km2 in Pittsburgh to a high of 213 km2 in Philadelphia.
Residential housing is the single biggest contributor to total surface area, accounting for 68-88% of total
wall and roof area. Expressed as a percent of the corresponding ground area, single family residential walls
and roofs cover about 40-50% of the corresponding ground area, though this number is as high as 74% in
Baltimore. Walls and roofs generally cover a similar or smaller fraction of ground area in industrial and
commercial zones. Whether residential or commercial, infill development or on the urban fringe, urban
land uses represent an alteration of the natural landscape. The vast tracts of impervious roads, sidewalks,
driveways, parking lots, roofs, and walls alter the cycles of energy, water, and chemicals between land and
atmosphere.
14.2
Urban climate
Cities are usually warmer than surrounding rural areas, especially at night – a phenomenon known
as the ‘urban heat island’ (Landsberg 1981; Atkinson 1985; Oke 1987, 1995). The urban heat island was
first recognized in London in 1820 with Luke Howard’s observation ‘night is 3.7 °F (2.1 °C) warmer and
day 0.34 °F (0.19 °C) cooler in the city than in the country’ (Landsberg 1981, p. 5). The development of
Columbia, Maryland, a planned community along the Baltimore-Washington, D.C. corridor, provides a
case study of how urban development alters local climate. In 1968, when the population of Columbia was
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Chapter 14 – Urban Ecosystems
only 1000 people, the maximum warming compared with the surrounding rural area was 1 °C (Figure
14.1). A small business center with office buildings and a large parking lot was a local heat island of up to
3 °C. By 1974, when the population had increased to 20 000 people, the geographic extent of the heat
island had increased. Most of the town was more than 2 °C warmer than the surrounding rural land. A
central commercial and residential district was 5-7 °C warmer. This trend in Columbia mirrors the regional
population growth and urban warming of the entire Baltimore-Washington corridor between 1950 and
1979 (Viterito 1989).
In general, there is a clear relationship between urban warming, defined as the difference in
temperature between a city and surrounding rural area, and population size. The maximum difference in
urban and rural temperatures at any time increases as a logarithmic function of population (Figure 14.2).
Large cities of 100 000 to 1 000 000 people can be 8-12 °C warmer than rural areas. In North America and
Europe, instantaneous temperature difference (∆T) and population (P) are related as
North America: ∆T = 5.21log10 ( P ) − 11.24
Europe: ∆T = 3.02 log10 ( P ) − 3.29
Large cities in North America are a few degrees warmer than comparable European cities. These
observations show that even a small town of only 1000 people causes a warming of about 2 °C – close to
what was observed at Columbia, Maryland.
The formation and intensity of an urban heat island depends on weather conditions. The urbanrural temperature difference, when it exists, is generally largest in evening, especially during clear, calm
conditions. It is smallest with cloudy and windy conditions (Kidder and Essenwanger 1995). Data collected
in and near St. Louis, Missouri, illustrate the diurnal cycle (Figure 14.3). On average, the urban site was
warmer than the rural site (25.5 °C versus 23.8 °C). This difference was small during the day and larger at
night, with a greatest difference of 3-4 °C between 2300 and 0400 hours. Atmospheric humidity, expressed
as a ratio of the mass of water to the mass of dry air, also had a strong diurnal cycle. The rural site was
more humid than the urban station in the central business district (14.5 g kg-1 versus 14.0 g kg-1). This
difference was strongest from 0700 to near midnight with a maximum difference of 1 g kg-1; thereafter, the
urban site was more humid. An additional factor affecting the urban heat island is wind (Figure 14.4).
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Ecological Climatology
There is a threshold wind speed above which the heat island is minimal. This is because strong winds
efficiently mix heat throughout the atmosphere. The wind speed that limits heat island development
increases with population size.
The relationships in the top panel of Figure 14.2 are the largest instantaneous temperature
difference, which usually occurs at night with clear sky and calm wind. The bottom panel shows the
relationship between mean daily temperature, daily minimum temperature, and daily temperature range
(maximum minus minimum temperature) averaged for all days in the year. Although the maximum urbanrural temperature difference at any time can be quite large, this temperature difference is much smaller
when averaged over the year (Kukla et al. 1986; Karl et al. 1988; .Jones et al. 1989, 1990; Gallo et al.
1993a,b; Johnson et al. 1994; Epperson et al. 1995). The mean daily temperature of a city in the United
States of one million people is about 1 °C warmer than its rural counterpart. Most of this warming occurs at
night, when the average daily minimum temperature is 1.8 °C warmer. Daytime maximum temperature is
relatively unchanged. As a result of warmer nights, the diurnal temperature range decreases by 2 °C. In
general, urban areas have a predominantly smaller diurnal temperature range than rural areas (Gallo et al.
1996, 1999). Similar effects occur in individual seasons, with larger temperature differences in summer and
autumn months than in winter and spring.
Within a city there is substantial variation in temperature related to topography, proximity to
lakes, rivers, and oceans, the density of development, the amount of vegetated cover, and type of building
materials. For example, a study of summer and autumn air temperature in Lawrence, Kansas, found land
use (e.g., residential, commercial, industrial, park) accounted for 17-25% of the variance in measured air
temperature (Henry et al. 1985; Henry and Dicks 1987). The type of surface material (e.g., asphalt,
concrete, brick, gravel, grass) accounted for a similar amount of temperature variance. High-resolution
satellite measurements confirm these general patterns, showing several degree differences in surface
temperature related to urban land use, with commercial-industrial areas having the warmest surfaces and
parks having cooler temperatures (Carlson et al. 1981; Vukovich 1983; Roth et al. 1989; Nichol 1996).
These temperature differences are attributed to urban geometry – the size, shape, and orientation
of buildings and streets – and to the nature of urban surfaces – their albedo, heat capacity, thermal
conductivity, and wetness. These characteristics alter the radiation balance at the surface, the storage of
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Chapter 14 – Urban Ecosystems
heat in the urban fabric, and the partitioning of energy into latent and sensible heat (Landsberg 1981; Oke
1982, 1987, 1988a, 1995). In heavily polluted areas, the incoming solar radiation can be reduced by 1020% compared with rural regions due to the dirtier air. Even in less industrialized cities, solar radiation can
be reduced by 10%. This, however, is compensated by increased downward longwave radiation from the
gases and aerosols in the air. Radiative heating is augmented by anthropogenic heat sources from motor
vehicles, power plants, industrial processes, and heating systems. Anthropogenic heat sources can be
considerable and in some cases may be comparable in magnitude to net radiation. The net energy
impinging on the system must be balanced by the sensible and latent heat returned to the atmosphere and
heat stored in the urban system. Studies of the energy budget of cities demonstrate less latent heat flux and
greater sensible heat flux with urbanization (Grimmond and Oke 1995). Historical trends for 51 urbanizing
watersheds in eastern United States show decreased watershed evaporation and increased watershed
sensible heat with greater urban development (Dow and DeWalle 2000).
The urban energy budget has been studied in detail at Vancouver, British Columbia (Yap and Oke
1974; Oke 1979; Kalanda et al. 1980; Oke and McCaughey 1983; Cleugh and Oke 1986; Oke and Cleugh
1987; Grimmond 1992; Roth and Oke 1995). Figure 14.5 illustrates the diurnal cycle of surface energy
fluxes at rural and suburban locations measured on a typical summer day. During daytime, suburban-rural
differences in net radiation were generally small compared with greater sensible heat, decreased latent heat,
and more heat storage at the suburban site than the rural location. The largest differences were a decrease
in latent heat flux of 100 W m-2 and a corresponding increase in heat storage of some 100 W m-2. As a
result of decreased latent heat and increased sensible heat, the suburban site had a larger Bowen ratio (i.e.,
the ratio of sensible to latent heat) than the rural site. The maximum Bowen ratio at the rural location was
0.6 at midday whereas the midday Bowen ratio was 1.2 at the suburban site. Late afternoon values were in
excess of 1.8 at the suburban site. At night, turbulent fluxes were negligible and radiation was lost to the
atmosphere (i.e., net radiation was negative). The loss of radiation was compensated for by release of heat
stored during the day. The suburban site lost more radiation than the rural site (about 50 W m-2) and had
correspondingly larger release of stored heat.
Figure 14.6 shows the same data, but averaged over daylight hours. The rural site of managed
grassland had a higher albedo than the suburban site (0.20 versus 0.13). Because more solar radiation was
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Ecological Climatology
absorbed at the surface, the suburban site had a slightly higher net radiation flux than the rural site. Most of
the net radiation at the rural site (66%) was returned to the atmosphere as latent heat; 30% was returned as
sensible heat; only 4% was stored in the ground. In contrast, only 34% of the net radiation at the suburban
site was dissipated as latent heat; 44% was returned as sensible heat; and 22% was stored in the urban
fabric. Over the course of the day, the suburban site had 50% higher sensible heat flux and 46% lower
latent heat flux compared with the rural site. As a result, the rural site was somewhat cooler and pumped
more water into the atmosphere than the suburban site. The differences in energy fluxes were related to the
different characteristics and structure of the two landscapes. Twenty-five percent of the suburban site was
covered by buildings and structures and another 11% was paved. Built surfaces generally absorb more
solar radiation than vegetation. They are also impervious, covering the soil and preventing heat dissipation
from evapotranspiration.
In studying the urban climate and energy balance, it is necessary to distinguish the urban canopy
layer, which lies below rooftop level and comprises microclimates created by buildings, roads, and
vegetation, from the urban boundary layer above rooftop level that is an integration of the microclimates of
the urban canopy layer over a large area (Oke 1976b, 1995). The effect of urbanization differs in these two
layers. For example, shade from buildings in the urban canopy layer can create cooler local temperatures
than open areas. Vegetated lawns and parks within a city can have large latent heat fluxes (Oke 1979;
Suckling 1980). In addition, it is necessary to distinguish surface and air temperature (Voogt and Oke
1997). The urban-rural difference in surface temperature is generally greatest during the day whereas the
air temperature difference is greatest at night.
Within the urban canopy layer, street canyons illustrate the effect of urban form on microclimates.
The height of buildings and orientation of streets creates complex shading patterns over the course of a day
that affect air and surface temperatures (Arnfield 1990; Ruffieux et al. 1990; Nichol 1996). The geometry
of the canyon, especially the height of buildings relative to the width of the street, creates more opportunity
for radiative trapping within the canyon. This occurs when solar radiation reflected by a surface impinges
on other surfaces in the canyon, being partially absorbed and re-reflected. The net effect is more solar
radiation is absorbed than would be expected from the reflectivity of the surface material.
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Chapter 14 – Urban Ecosystems
Radiative trapping also occurs with longwave radiation due to changes in the sky view factor. Tall
buildings block some of the sky so that a point on the ground at the center of an urban canyon is exposed to
only a portion of the sky (Figure 14.7). The fraction of the sky seen is the sky view factor. For an infinitely
long street this is
ψ = cos( a )
where a is the angle defined by street width W and building height H as tan( a ) = 2 H / W . For a 16 m wide
street lined by 16 m tall buildings a = 63.4° and the sky view factor is ψ = 0.45. That is, the center of the
street is exposed to only 45% of the sky. As the ratio H/W increases, an increasing portion of the cold sky
is blocked by the warmer urban buildings. More of the longwave radiation emitted by the street surface is
absorbed by the surrounding buildings and less escapes to the atmosphere.
Studies of the energy balance of urban canyons confirm the importance of building height to street
width in determining the trapping of radiation within the canyon and warming (Nunez and Oke 1976,
1977; Oke 1987; Arnfield and Mills 1994a,b; Eliasson 1996; Arnfield and Grimmond 1998). In general,
sensible heat and storage are the dominant means by which net radiation is dissipated during the day. At
night, stored heat is released to counter the negative radiation balance. Surface and air temperature data
collected in and near an apartment complex on a clear, calm, summer afternoon illustrate the urban canyon
effect at a small scale (Figure 14.8). Late in the afternoon, the courtyard surface was 19 °C warmer than the
air in the courtyard. In contrast, an adjacent grass lawn and wooded area outside the courtyard were less
than 2 °C warmer than the air. As the Sun set, all three sites cooled, but even at 2115 hours the surface
temperature of the courtyard (30 °C) was several degrees warmer than that of the lawn (23 °C) or
woodland (25 °C). The air within the courtyard was about 1 °C warmer than the external air.
The enlargement of courtyards in Heidelberg, Germany, also illustrates the behavior of urban
canyons (Fezer 1990). With small courtyards, solar radiation did not reach the ground in the courtyards,
and longwave radiation was trapped within the courtyard. Consequently, air within the courtyard was cool
in the morning and mild at night compared with rooftop level. With courtyard enlargement, the sky view
factor increased. More solar radiation reached the ground during the day and less longwave radiation was
trapped. As a result, the air was warmer during the day and cooler at night than previously.
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Ecological Climatology
At the urban scale, there is a strong relationship between the urban-rural temperature difference
and sky view factor in which the magnitude of urban warming decreases as more of the sky is seen (Figure
14.7). The generality of this relationship is evident in the fact that European cities are generally cooler than
North American cities of the same size (Figure 14.2). But when related to view factor, all cities regardless
of location show the same relationship between urban warming and view factor. Thus, differences between
cities in Europe and the United States might be attributed to the denser and taller buildings in the United
States.
Asphalt parking lots, with their low albedo and low thermal conductivity, create their own
microclimate. Figure 14.9 shows surface energy fluxes for a parking lot and nearby field at noon on a
warm day. Both sites received the same incoming longwave radiation (300 W m-2), but the parking lot
received slightly more (20 W m-2) solar radiation than the field (perhaps due to small differences in slope).
Whereas the field reflected 209 W m-2 of solar radiation, for an albedo of 0.25, the asphalt surface reflected
only 48 W m-2, for an albedo of 0.06. Thus the radiative forcing on the parking lot was 181 W m-2 more
than that impinging on the field. For both surfaces, the vast majority of this energy was returned to the
atmosphere as longwave radiation. Because it was hotter, the parking lot emitted more longwave radiation
than the field. Of the 461 W m-2 net radiation on the field, 45% was returned to the atmosphere as latent
heat, 36% was stored in the ground, and 18% was dissipated as sensible heat. In contrast, the parking lot
had no latent heat flux. Sensible heat flux was reduced relative to the field, perhaps because the lower
surface roughness reduced the efficiency with which heat is carried away from the surface. Instead, 86% of
the net radiation at the surface was stored in the ground. As a result of the higher radiative forcing and
greater heat storage, the parking lot surface was 16 °C hotter than the field.
Similar microclimates are seen in a small airfield on a clear, calm, night (Figure 14.10). Lines of
equal air temperature and humidity are nearly vertical over concrete, with temperature decreasing and
humidity increasing with distance from the building. The cooling effect of grass is evident, with air
temperature warmer and humidity lower over the concrete surface than over the grass.
Numerous studies have demonstrated the importance of vegetated landscapes in ameliorating the
urban heat island. One is a study of nighttime air temperatures during summer in Washington, D.C (Figure
14.11). On the particular night studied, the average temperature obtained along a transect from the
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Chapter 14 – Urban Ecosystems
Northwest section of the city southeast through Rock Creek Park, the 16th Street business district, and
through the open parks of the Mall was 23 °C. However, temperature varied greatly depending on location.
A large wooded area of Rock Creek Park had air temperatures as low as 20 °C. The downtown business
district had air temperatures of about 25 °C while the nearby Mall area was about 1 °C cooler. Even during
the day, Rock Creek Park heated more slowly than the commercial districts and was usually 1-2 °C cooler
at midday.
A study of the climates of three parks in Göteborg, on the west coast of Sweden, documents both
the cooling effect of urban parks on air temperature and the extension of the park microclimate into the
surrounding built-up area (Upmanis et al. 1998). The parks ranged in size from 2 ha to 156 ha. The
maximum cooling compared with the surrounding area occurred at night and was 6 °C for the large park
compared with only about 2 °C for the smallest park. The cooling from the largest park extended 1000 m
from the park boundary whereas the influence of the smallest park was on the order of 30 m. Similar
results are seen in other cities. The nocturnal air temperature difference between urban parks and
surrounding built areas increases with park size (Figure 14.12). Similar results are seen in Tokyo, where
rice paddy fields in urban fringe areas cool air temperature on hot summer days by more than 2 °C
compared with urban air (Yokohari et al. 1997). This cooling effect increases as the area of paddy fields
increases.
Microclimatic effects can also be found at the scale of individual landscape elements (Table 14.2).
Over the course of a hot summer day, air temperature in a typical suburban Colorado residential landscape
ranged from a low of 26 °C in early morning to 34 °C in late afternoon. Surface temperature was measured
for various surfaces in the landscape with a hand-held infrared thermometer, which measures the
temperature at which an object radiates longwave radiation. Over the course of the day, surface
temperatures ranged from a low of 17 °C over flowers to a high of 62 °C over exposed dirt. A watered
grass lawn and flower bed were several degrees cooler than air throughout the day and had the smallest
range in temperature, warming on average by 12 °C over the day. In contrast, a dry native grass lawn was
warmer than air except early in the morning when shaded. Hard elements of the landscape were generally
several degrees to up to 28 °C warmer than air. This was particularly evident for exposed dirt, stone, and
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Ecological Climatology
mulch surfaces, where temperatures warmed on average by 24 °C over the day. Walls had a more complex
diurnal pattern related to incident solar radiation. The temperature of the north-facing wall, which received
only diffuse solar radiation, varied little over the day. The east-, south-, and west-facing walls, when
exposed to large amounts of solar radiation (often well over 500 W m-2), were up to 11-29 °C warmer than
air. With their low heat capacity, these walls quickly cooled to near air temperature when they became
shaded.
Figure 14.13 illustrates how the temperatures of individual landscape elements are integrated in
the residential landscape. Surface temperatures were measured at increasing distances from the house. The
field of view for the hand-held infrared thermometer increases with distance from the sensor. In this way,
as the sensor was held at greater distances from the house, background radiation from other elements of the
landscape (grass, trees, sky) became more important. Temperature was first measured adjacent to an eastfacing wall and then into an irrigated grass lawn and greenbelt extending away from the house.
Temperature was measured looking towards the house and away from the house towards the greenbelt. In
early morning, temperatures facing the greenbelt were cool (about 8 °C) and varied by 1 °C with distance
from the house. At each distance, temperatures looking toward the house were warmer, ranging from a
high of 14 °C near the house to a low of 9 °C far from the house. By noon, when the Sun had heated the
wall all morning, temperatures decreased from 49 °C adjacent to the house to 30 °C at 12 m to 25 °C
further into the greenbelt. In contrast, temperatures looking towards the greenbelt ranged from 21 °C to 24
°C. These measurements illustrate two points. First, the cooling influence of the greenbelt is clear, with
surface temperature of the house up to 28 °C warmer than that of the greenbelt. Second, temperatures
looking towards the house stabilized at about 12-15 m from the house, decreasing again with further
distance into the greenbelt. This suggests the thermal footprint of the house is about 12 m, with the
additional drop in temperature within the greenbelt associated with increasing view of the greenbelt.
The preceding studies show the warming effect of hard impervious surfaces such as asphalt and
concrete and the cooling effect of vegetated surfaces. These local landscapes affect climate citywide. The
urban-rural temperature difference for a city increases with greater impervious surface area and less
vegetated area (Landsberg 1979; Carlson and Arthur 2000). A study of the energy budget in three cities in
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Chapter 14 – Urban Ecosystems
the United States located in arid climates shows the Bowen ratio decreases as irrigated greenspace within
the cities increases (Figure 14.14). Impervious surface area is emerging as a key indicator for
environmental planning (Arnold and Gibbons 1996).
Numerical models of the energy balance of urban surfaces give insights to the physical causes of
the urban heat island. The energy balance of a city can be represented by a bulk formulation in which
(1 − r ) S ↓ + L ↓= L ↑ + H + λ E + G
where S↓ and L↓ are the incoming solar and longwave radiation, respectively, r is albedo, L↑ is emitted
longwave radiation, H is sensible heat, λE is latent heat, and G is heat storage. Like a leaf or vegetated
canopy, this equation can be solved for the surface temperature that balances the energy budget (Myrup
1969; Morgan et al. 1977; Carlson and Boland 1978; Tapper et al. 1981; Bornstein 1986; Ross and Oke
1988; Todhunter and Terjung 1988). Important surface properties influencing temperature are the albedo of
cities, their roughness, the thermal properties of buildings and paving materials, and the amount of
evaporating area (i.e., vegetation) in the city. These models apply to the urban boundary layer above
rooftop. Below rooftop, within the urban canopy, more detailed energy balance models are required to
account for the complex geometry of a street canyon, the different thermal properties of building materials,
evaporation by trees, and shading (Terjung and O’Rourke 1980a,b, 1981; O’Rourke and Terjung 1981).
Urban climates differ from their rural counterparts in ways other than just temperature. Within the
urban canyon, increased friction from trees, buildings, and other tall objects weaken surface winds, with
increased number of calm periods and reduced number of windy periods. Again, data collected during the
development of Columbia, Maryland, are illustrative, showing a reduction in the frequency of strong winds
and increase in the frequency of light winds between 1969 and 1974 (Landsberg 1979; Landsberg 1981,
pp. 128-129). In the urban boundary layer, however, the warm, rough urban surface can, under the right
conditions, create atmospheric circulations similar to sea breezes (Figure 14.15). Rising air over the warm
urban center is replaced by surface winds from the surrounding countryside. These country breezes are
most obvious when regional winds are calm or weak.
Urbanization can alter regional precipitation. The rising motions over urban centers draw cool,
moist low-level air into the city from surrounding rural areas. This, combined with increased amounts of
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Ecological Climatology
aerosols in the polluted urban air that act as cloud condensation nuclei, can lead to increased rainfall and
increased frequency of severe weather. For example, a study of St. Louis, Missouri, found summer rainfall
on the leeward side of the city was greater than in other sectors (Figure 14.16). Moreover, storms resulting
in over 25 mm of rain were 50% more frequent over urban areas than suburban or rural areas. Despite
methodological difficulties in establishing causality, it is likely the St. Louis precipitation anomaly is
caused by the city itself. Indeed, summer rainfall increases of 10-20% and increased thunderstorm activity
have been observed in numerous cities (Table 14.3). Conversely, industrial pollution can also inhibit
rainfall. The small pollution particles emitted into air can inhibit water droplets from coalescing into larger
droplets to create rain. Consequently, precipitation from certain types of clouds can be substantially
reduced downwind from large urban and industrial areas (Rosenfeld 2000).
Perhaps the most striking evidence of human influences on precipitation comes from reports of a
weekly cycle in which precipitation is greater at the end of the workweek than at the beginning (Ashworth
1929; Lawrence 1971; Simmonds and Keay 1997; Cerveny and Balling 1998). For example, near-coastal
ocean areas along the Atlantic seaboard of the United States receive more rain on weekends than on
weekdays (Figure 14.17). Total rainfall in the region increases from 537 mm on Monday to 657 mm on
Saturday. This corresponds to the observed weekly cycle of air pollution in which lowest concentrations of
carbon monoxide and ozone, two common urban pollutants, are observed early in the week. Weekend
storms may be enhanced by pollution, which provides condensation nuclei around which rain drops form.
A weekly cycle in which the heat island effect is a few tenths of a degree smaller on weekends than on
weekdays has also been observed for air temperature during winter (Mitchell 1961; Landsberg 1981, pp.
107-108; Gordon 1994; Simmonds and Keay 1997). Since it is unlikely that natural processes vary on a
seven-day cycle, the weekly cycles of precipitation and temperature in urban regions are a strong indicator
of human influences, primarily in the form of pollutants and heat from fuel combustion, on weather and
climate.
Phoenix, Arizona, provides a case study for the multiple changes in climate associated with
urbanization. Analysis of long-term weather records through the mid-1980s show: large warming, with
nighttime summer temperatures increasing by 4 °C between 1948 and 1984; a decline in dewpoint
temperature and relative humidity; significant increases in early morning wind speed associated with
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Chapter 14 – Urban Ecosystems
thermal circulations; increased duration of smoke and haze; more frequent late afternoon and evening
storms; and increased evaporative demand as a result of the warmer, drier, windier urban environment
(Balling and Brazel 1986, 1987a,b,c; Balling and Cerveny 1987; Brazel and Balling 1986; Brazel et al.
1988). These changes are apparently related to the rapid urbanization of the surrounding irrigated
farmland. In contrast, Tucson, just to the southeast of Phoenix, is surrounded by dry farmland or natural
desert and showed no such changes in climate associated with urbanization (Balling and Brazel 1987c).
14.3
Urban hydrology
The impact of humans on the hydrologic cycle is paradoxically a story of too little and too much
water. Water is required to grow food, to manufacture many goods, and to generate power. In addition, we
use water domestically to prepare foods, for cleaning, and for disposing wastes. When summed over all the
people in the United States, annual water withdrawals amount to 408 billion gallons or 1620 gallons per
person (Figure 14.18). Almost one-half of this water is used for thermoelectric purposes; one-third is used
for irrigation. The rest is used for domestic, industrial, and other uses. However, some of the withdrawn
water is returned to the system and made available for reuse; only a portion is actually evaporated,
transpired, or incorporated in plants, animals, or products and therefore unavailable for reuse. This
consumptive use of water amounts to 94 billion gallons annually or 370 gallons per person. Globally, 54%
of accessible runoff is withdrawn annually for human uses (Table 1.4).
Figure 14.19 illustrates the seasonal pattern of water use in the city of Boulder, located in the
semi-arid climate of eastern Colorado. The background rate of about 12 million gallons per day increases
to 35-40 million gallons per day in summer months as water is used to irrigate lawns and gardens, wash
cars, and in other outdoor activities. The effect of weather on water use is also evident. Water use dropped
to under 20 million gallons per day in early August. This was a period of cool, wet weather during which
lawns and gardens did not have to be watered. By the end of August, when hot, dry weather returned,
water use quickly climbed to over 35 million gallons per day. In a semi-arid climate with only about 380
mm of rain per year, the seasonality of water use and the effect of weather on water use might be extreme
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Ecological Climatology
compared with more humid parts of the country. Nevertheless, the same general conclusions of increased
water use in warm months and decreased water use in rainy periods applies universally.
Urban landscapes alter the local hydrologic cycle by generating excessive surface runoff
compared with rural landscapes (Hall 1984; Lazaro 1990). Grimmond et al. (1986) and Grimmond and
Oke (1986, 1991) are an example of an urban hydrology model. More rainfall is transformed into runoff by
roofs, streets, sidewalks, parking lots, and other impervious surfaces in an urban watershed compared with
forest or grazed pasture (Figure 14.20). The hard impervious surfaces and compacted soils hinder
infiltration into the underlying soil, increasing the volume of runoff compared with rural landscapes. In
addition, gutters, sewers, and man-made drainage systems quickly convey this water to river channels. The
net effect is that urbanization increases the peak discharge rate, the speed of the runoff, and the total
volume of runoff (Figure 14.21). A study of historical streamflow for 39 urban watersheds in the United
States found increases in annual streamflow in proportion to cumulative changes in population density
(DeWalle et al. 2000).
The effect of urbanization is greatest for small, frequent storms. Larger floods brought about by
intense or prolonged rainfall are less affected because even in rural settings the soil moisture deficit is
satisfied, the saturated zone expands, and runoff is rapidly conveyed to stream channels. This is illustrated
by Figure 14.22, which shows the size of urban floods relative to rural floods decreases from more than
twenty times rural floods for storms with recurrence intervals less than one year (i.e., small, frequent
storms) to less than twice the size with recurrence intervals greater than ten years. A doubling of storm
runoff, such as occurs with a 100-year storm as the impervious area increases from 10% to 45%, can have
significant economic and social consequences.
14.4
Biogeochemical cycles
Urban soils differ markedly from productive soil (Craul 1999). Urban soils often have high levels
of contaminants. Insecticides, fungicides, and herbicides are used to control insects, diseases, and weeds.
Salts are used to de-ice roads. Heavy metal compounds (i.e., containing elements such as arsenic,
cadmium, chromium, copper, lead, mercury, nickel, and zinc) fall from the polluted air during rainstorms
14
Chapter 14 – Urban Ecosystems
and settle out from the sky. Construction rubble – bricks, concrete, asphalt, nails, and other debris – slowly
weather over time. Excessive nutrients beyond that which can be utilized by plants often leach into surface
and groundwater supplies. In contrast to point pollution, which originates with a well-defined source such
as effluent from a factory or sewer system, this non-point pollution originates from many sources spread
over large areas and is considered to be a greater environmental threat because it is hard to identify and
regulate (Carpenter et al. 1998).
Urban soils are compacted during construction and by vehicular traffic. As a result, drainage is
poor, aeration is restricted, and roots develop in compact, shallow profiles. Even the weight of people
walking over the ground, when summed over time, can compact the underlying soil (Vimmerstedt et al.
1982; Runion et al. 1993). A study of several campsites in Ohio found reduced infiltration, increased
surface runoff, and increased erosion rates as a result of compaction and loss of ground cover (Table 14.4).
Soil in the low-use area had a bulk density of 1.28 g cm-3 whereas that in the high-use area was 12% more
dense. Runoff increased from 18% of precipitation in the low-use zone to 74% in the high-use zone.
Erosion rates increased by a factor of 25, from 0.06 grams of soil per liter of water to 1.49 grams per liter
due to loss of ground cover.
Reduced vegetation and litter not only increases surface runoff but also accelerates soil loss
during erosion. This can be particularly evident during construction. A mid-1960s study in the BaltimoreWashington, D.C. corridor found erosion rates of 100-300 metric ton km-2 yr-1 in rural watersheds but 35050 000 ton km-2 yr-1 in watersheds with construction of housing and roads (Figure 14.23). These rates for
construction sites may no longer be valid due to erosion-control practices. Moreover, the wet climate with
precipitation on the order of 1100 mm per year accelerates erosion compared with other regions. Still, the
stark increase in sediment yield with construction is indicative of the high potential for soil loss during
urban development.
Another significant change in soil fertility is an interruption of the natural cycling of litter and
nutrients between plants and soil. A large tree with a leaf area index of 5 m2 m-2 might contain 400 grams
of leaves (dry weight) per square meter of ground (i.e., 125 cm2 of leaves has a mass of one gram).
Approximately one-half of this mass (200 g m-2) is carbon and 8 g m-2 is nitrogen. This carbon and
nitrogen is deposited annually to the soil in litterfall, which slowly decays over time and releases
15
Ecological Climatology
mineralized nitrogen that is available for plant use. The urban landscape alters this natural cycling of
nutrients. Trees growing in small pits along streets or sidewalks or in plazas do not have access to recycled
nutrients. In parks and residential yards, leaves are collected and carried offsite. The result is that urban
soils have low organic matter, which contributes to their low porosity, and low nutrient availability – an
environment that is favorable for neither plants nor soil microorganisms.
A study of coarse woody debris along the shoreline of 16 lakes in Wisconsin and Michigan
quantifies the impact of human activities on biogeochemical cycles (Figure 14.24). Coarse woody debris is
a crucial input from forested watersheds to streams and lakes (Figure 10.28). However, the density of
coarse woody debris was found to decline with increasing density of lakeshore cabins. Within developed
lakes, forested sites had a higher density of coarse woody debris than cabin sites (379 versus 57 logs per
km of shoreline). This likely reflects the removal of debris by people and thinning of the natural forest
vegetation.
14.5
Biodiversity
Urbanization has profound consequences on biodiversity. In the United States, most threatened
and endangered species are clustered in distinct geographic regions such as Hawaii, the arid Southwest
including southern California and Arizona, much of Florida, and coastal areas (Dobson et al. 1997b;
Flather et al. 1998). Similar hotspots of biodiversity are found throughout the planet (Myers et al. 2000).
Altered or destroyed habitat is widely regarded as the single biggest threat to biodiversity (Wilcove et al.
1998). As an example, a study of the effect of residential development on bird communities in the Ozarks
region of central Missouri found forest interior migrant birds were more common on undeveloped land
than residential areas while nest predator or parasite birds were most common on residential land (Nilon et
al. 1995). The spread of non-native species is another significant threat to biodiversity (Wilcove et al.
1998). Introduction of non-native species to new habitats represents a significant change in the structure
and composition of ecosystems (Bright 1998; Cox 1999; Mooney and Hobbs 2000). For example, domestic
pets can have a substantial impact on wildlife through predation (Churcher and Lawton 1987). Non-native
plants are introduced intentionally through, for example, the planting of species for aesthetics or the
16
Chapter 14 – Urban Ecosystems
introduction of plants for erosion control. Introduction can also be inadvertent through the dispersion of
seeds, larvae, or diseased plant material from one region to another through commerce and travel.
Studies in Argentina illustrate the effect of humans on introduced species. A 1977 survey of the
flora around an isolated cabin in the Patagonian Mountains found that even in this remote, old-growth
forest more than one-third of the plants within 6 m of the cabin were non-native (Rapoport 1993). By 1990,
this increased to 100%. In addition, the area in which introduced species were found increased from 10 m
around the cabin in 1977 to 17 m in 1990. The introduction of non-native plants is readily apparent at the
residential scale of development, as shown by a study of plant biodiversity in and near the city of Bariloche
(Figure 14.25). Throughout the city, the percent of species that were introduced increased with higher
housing density. Variation at a given density appeared to be related to neighborhood economic status. A
detailed survey of plant species around houses on the edge of the city illustrates the geography of
introduced species for single-family residences (Figure 14.25). On average 41% of the species were
introduced, but this ranged from 9% to 100% with highest percent introduced species near the houses and
lowest introduction along an ephemeral stream. These studies clearly demonstrate that the inhabitants of
Bariloche generally do not prefer native Patagonian plants and replace them with exotic introduced species.
Urban forests have received much study because of their social, recreational, ecological, and
aesthetic benefits to the urban landscape (Moll and Ebenreck 1989; Grey 1996). Urban forests clearly
reflect human values, preferences, and choices. This is seen in a study of the age structure and species
composition of urban forests in 22 cities throughout the United States (McPherson and Rowntree 1989).
There is evidence that homeowners select trees in residential landscapes to recreate their childhood
environment. A study in the San Francisco Bay area found that residents from Rocky Mountain states,
where the natural vegetation is primarily spruce, fir, and pine, preferentially used alpine conifers in their
landscapes while residents from New England preferred maples and birches (Worthen 1975). A
comparison of natural and urban forests in Menlo Park, California, near San Francisco, and South Lake
Tahoe, California, demonstrates the importance of human choices (Table 14.5). The natural landscape of
Menlo Park is oak forest and oak savanna. Relict natural landscapes are markedly different from vegetation
in the city. In natural ecosystems, species diversity was low (3 to 5 species); savanna vegetation had a
lower tree density and sparser tree cover than forest. In contrast, urban forests were much more diverse
17
Ecological Climatology
(130 to 145 tree species). The density of trees and their coverage was higher than that of savanna, but much
less than that of forest. These trends in crown cover and density clearly reflect a balance between a desire
for trees and open space for houses, streets, and yards. In the South Lake Tahoe area, only five tree species
were introduced. This illustrates a higher aesthetic value for the pre-settlement forest, as might be expected
in a mountain recreational setting, but even here tree density was reduced by 50% and forest cover was
decreased.
Examination of urban forest cover in relation to population size and ecoregions also illustrates the
effect of human values on urban forest structure and composition (Figure 14.26). In a study of eight cities
in the northern hardwood forest region of the United States with populations ranging from 2000 to 200 000
people, tree cover as a percent of total area ranged from 18% to 38% (Halverson and Rowntree 1986). Tree
cover decreased as population increased due to the larger area devoted to transportation, commercial, and
industrial uses. Another study related tree cover in 58 cities in the United States to the surrounding natural
vegetation (Nowak et al. 1996). Tree cover as a percent of total land area ranged from 0.4% to 55%. Tree
cover was highest in cities located in forested ecoregions (31%), lower in cities in grassland ecoregions
(19%), and lowest in desert regions (10%). Within a city, land use was the primary determinant of tree
cover. Each land use had a characteristic structure and function that influenced tree cover. Tree cover for
park and vacant urban land was highest in forest ecoregions and lowest in desert ecoregions, reflecting the
decreased availability of water. Differences in tree cover were not as large for residential land use (though
forested regions had higher cover than other regions), reflecting a desire by homeowners for trees
regardless of environment.
14.6
The ecological city
Urban landscapes are quite different from rural landscapes. Consider the environmental changes
as a parcel of land is developed. More solar radiation is absorbed at the surface due to decreased albedo.
Less longwave radiation is lost from the surface as tall, narrow urban canyons reduce the sky view factor
and limit the loss of radiation to the sky. Heat is generated from buildings and traffic. This energy is stored
during the day in the urban fabric – the steel, bricks, wood, asphalt, and concrete that form a city – and
18
Chapter 14 – Urban Ecosystems
slowly released at night. The hard, impervious urban fabric further alters the energy balance by limiting
evapotranspiration. Air is more polluted, which might alter rainfall. As population increases, the use and
consumption of water rises. Rivers are dammed, reservoirs constructed, deep aquifers are tapped, and the
natural flow of water across the landscape is altered to provide a reliable supply of water. A greater portion
of rainfall becomes runoff as the hard urban surface prevents infiltration and conveys surface water quickly
to rivers. Constructed drainage systems of gutters, pipes, and treatment plants replace the networks of
gullies, ditches, creeks, and streams that carry water across the landscape. Biogeochemical cycles are
altered. Grass is cut, lawns are irrigated, fallen leaves are collected, and fertilizers, pesticides, and
herbicides are applied in search of the perfect lawn. Native vegetation is removed and replaced with
monolithic tracts of grass and scattered relic or introduced trees.
Urban and suburban ecosystems are quite unlike natural ecosystems. In 1973, H.E. Landsberg, a
pioneer in the study of urban meteorology, published a paper entitled ‘The meteorologically utopian city’
(Landsberg 1973). In it he argued for greater planning of cities with respect to urban climates. Today, there
is renewed scientific interest in the urban ecosystem and the application of this knowledge to landscape
planning and design.
There is a rich and robust ecological design movement within the landscape planning and design
professions (Steiner et al. 1988). Frederick Law Olmsted, considered the father of landscape architecture
and planning in the United States, designed public parks in the 1800s as pastoral retreats within the city to
escape the squalid urban environment and to reduce flooding. These concepts are embodied in his designs
for Central Park in New York City and the Back Bay Fens of Boston (Spirn 1984). The need for pure air
and water, improved drainage, and bright sunshine guided the early 1900s City Beautiful movement (Hall
1988). This movement saw urban parks as the means to improve health, safety, and welfare. The
subsequent Garden City movement combined the social and economic benefits of cities with the clean air,
clean water, gardens, and open spaces of the country. The regional planning movement of this era
emphasized the relationships among people, climate, vegetation, geology, and soil.
The ecological movement continues today, advocating design based on and integrated with a
site’s ecology – its climate, hydrology, soils, vegetation, and wildlife (McHarg 1969; Spirn 1984, 1988;
Hough 1984, 1995; Koh 1988; Olin 1988; Lyle 1994; P.H. Lewis 1996; Van der Ryn and Cowan 1996;
19
Ecological Climatology
Thompson and Steiner 1997). It sees nature as the best solution to modern environmental problems,
especially by using nature’s patterns to dictate land uses. Identity and form arise from natural processes and
features of the land. The ecological design movement has contributed to a growing recognition that the
built environment is part of nature rather than separate from nature. Ian McHarg, with his seminal
publication Design with Nature, is credited with defining the modern ecological design movement
(McHarg 1969). He strove to understand landscapes as the sum of overlying physical, biological, cultural,
and historic values. He emphasized in broad terms Earth as a system, with the biosphere as a central
regulator of the planetary health through flows of energy, water, nutrients, and biomass. Anne Spirn and
Michael Hough followed McHarg with their books The Granite Garden (Spirn 1984) and City Form and
Natural Process (Hough 1984, 1995). Both searched for a new design aesthetic that stemmed from
ecological function rather than the traditional design principles of form, composition, color, and texture.
They saw nature and natural processes as a source of urban form and as solutions to urban problems. Spirn
and Hough recognized the services ecosystems provide, emphasizing nutrient and water cycles,
biodiversity, habitat quality, and resource depletion and the need to make these processes more visible to
people. Sim Van der Ryn and Stuart Cowan again emphasized ecological functions and services as a
source of form and aesthetics in their book Ecological Design (Van der Ryn and Cowan 1996). These
themes have since been incorporated into landscape planning and design texts (Simonds 1998).
There have also been topical treatments of the urban environment by designers and architects – its
climate (Aronin 1953; Olgyay 1963; Givoni 1976, 1998; Robinette 1983; Lowry 1988; Brown and
Gillespie 1995), hydrology (Robinette 1984; Ferguson 1994, 1998), and soils (Craul 1999) and how to
incorporate these into urban planning and design. Perhaps because climate affects human health and
comfort in many ways, consideration of climate in the built environment has received much attention.
Jeffrey Aronin, in his 1953 book Climate and Architecture, outlined the central tenets by which climate
influences and is influenced by building and urban design (Aronin 1953). He discussed microclimates and
the need to accommodate these in the design process – first by site selection and second by the design
itself. Aronin’s ideas closely matched those of Victor Olgyay, who with his 1963 book Design With
Climate recognized the fundamental role of climate in building and urban design (Olgyay 1963). Both of
these authors were concerned with using climate to improve living conditions and recognized that air
20
Chapter 14 – Urban Ecosystems
temperature, radiation, and wind act in concert to affect thermal comfort. Cooling breezes and shade are
needed during hot, overheated periods; the warmth of sunlight and protection from winds are needed
during cold, underheated periods. This interest in microclimatic design continues today (Givoni 1976,
1998; Robinette 1983; Lowry 1988; Keeble et al. 1990/91; Crowther 1992; Moffat and Schiler 1994;
Brown and Gillespie 1995).
As architects and planners such as McHarg, Olgyay, and others were advocating design based on
natural processes such as climate and ecology, scientists were beginning to examine the impact of natural
and designed landscapes on microclimates. Over fifty years ago, Joseph Kittredge’s Forest Influences
described the manner in which forests regulate temperature, humidity, wind, runoff and erosion (Kittredge
1948). Rudolf Geiger’s The Climate Near the Ground, though over 35 years old, is still a classic reference
on microclimates, the manner in which landscape elements create their own microclimates, and human
influences on microclimates (Geiger 1965). The 1960s and 1970s saw a substantial scientific effort to
characterize and understand differences in urban and rural climates, culminating in a major study of the
climate of St. Louis, Missouri (Changnon 1981a). The development of Columbia, Maryland, from rural
farmland to suburban city beginning in the late 1960s provided an opportunity to examine changes in
climate with urban development (Landsberg and Maisel 1972; Landsberg 1979). The impact of the built
environment on climate led to concern among a few atmospheric scientists about urban planning and the
lack of communication between architects, planners, and environmental scientists (Landsberg 1970b, 1973;
Oke 1976a, 1984, 1988b; Changnon 1979, 1981b). The World Meteorological Organization held several
symposia that addressed not only climate as it determines land use practices but also the impact of land use
practices, especially urban development, on climate (WMO 1970a,b, 1976). Interest in the impacts of
urban growth and development on climate continues today, but with a focus on developing countries and
tropical climates (WMO 1986, 1994, 1996, 1997a).
Ecologists have also been interested in land use, land planning, and urban environments. With the
advent of landscape ecology as a formal specialty in ecology, there are efforts to apply the theory and
principles of landscape organization to landscape planning (Forman and Godron 1986; Forman 1995;
Dramstad et al. 1996). Others have attempted to provide a framework within which ecological principles
and methods of study can be applied to urban settings (McDonnell and Pickett 1990; McDonnell et al.
21
Ecological Climatology
1993; Platt et al. 1994; Flores et al. 1998; Zipperer et al. 2000; Dale et al. 2000). The U.S. National
Science Foundation recognized urban settings as unique ecosystems, adding the cities of Phoenix, Arizona,
and Baltimore, Maryland, as research sites in its Long-Term Ecological Research program (Parlange
1998).
One example of the merging of ecology and landscape design is the American residential
landscape. In Redesigning the American Lawn, F.H. Bormann, a prominent ecologist, teamed with
landscape architects to study how residential lawns can be designed to minimize their detrimental impact
on the environment but still meet required aesthetic, social, and recreational needs (Bormann et al. 1993).
These authors documented the artificial nature of the residential lawn and the need for equipment,
pesticides, herbicides, and irrigation to maintain it. They advocated redesigning lawns in light of an
ecosystem approach, focusing on biogeochemical cycles, the hydrologic cycle, and other ecosystem
functions, to minimize environmental impacts and offer aesthetically pleasing, ecologically responsible
alternatives.
Science, especially ecology, provides a useful framework to address land use, but cannot provide
all the answers. Ecology is both a science based on facts and an ideology based on values. Incorporation of
ecological concepts into planning is often more in tune with environmentalism than ecological science
(Daniels 1988; Flores et al. 1998). Moreover, the role and significance of ecology in planning and design
divides these professions (Thompson and Steiner 1997; Mozingo 1997). This divide arises because of the
schism between those who see landscape architecture as an art and those who see it as environmental
stewardship. Much of the debate concerns whether ecological design is an aesthetic or merely an ethic.
Ecology as a design aesthetic and source of form is clearly at the heart of the ecological movement, but the
ecological design movement has been criticized for considering theory, criteria, and methodologies at the
expense of aesthetic implications and other design criteria (Thompson and Steiner 1997; Mozingo 1997).
Indeed, Calthorpe (1993) acknowledged this was a shortcoming of the Van der Ryn and Calthorpe (1986)
ecological design guidelines. Improving the quality of the urban ecosystem and minimizing human impacts
on nature requires a balance between the natural and cultural components of landscapes and between the
facts of science and the values of planning. Balancing these is a core part of site planning (Lynch and Hack
22
Chapter 14 – Urban Ecosystems
1984; Berger and Sinton 1985; Steiner 1991; P.H. Lewis 1996) and is essential to the application of science
to landscape and urban planning (Oke 1984, 1988b; Platt et al. 1994; Dramstad et al. 1996).
23
Ecological Climatology
14.7
Tables
Table 14.1. Morphology of 10 large cities in the United States by land use
Area
Wall and roof area
Mean height
(percent of zone)
(number of stories)
(km2)
R-SF (%)
R-A (%)
IZ (%)
CZ (%)
R-SF
R-A
IZ
CZ
R-SF
R-A
IZ
CZ
Total
Pittsburgh
157.59
68
6
17
10
41
114
30
66
2
7
3
7
Boston
199.84
56
7
21
17
51
98
19
38
2
6
2
4
Sacramento
207.58
72
2
10
16
43
11
13
30
1
2
2
4
Cincinnati
225.97
62
6
22
10
42
33
43
34
2
2
2
4
Baltimore
227.86
49
21
14
16
74
130
39
32
2
5
3
4
Philadelphia
323.35
53
20
13
14
55
119
51
48
2
7
3
4
Denver
345.98
78
3
12
7
42
43
22
29
1
4
2
3
Seattle
390.82
73
1
15
10
32
105
43
31
2
3
2
3
Atlanta
440.28
74
5
12
8
39
40
27
38
1
2
2
5
Houston
493.01
67
3
21
10
42
42
25
63
2
2
2
5
Note: Total land area is divided into single-family residential (R-SF), residential apartment (R-A),
industrial (IZ), and commercial (CZ) zones. Wall and roof area is the combined surface area expressed as a
percent of land area in the land use zone. Data from Ellefsen (1990/91). His 17 urban zones were reduced
to 4 zones: residential, single family (A3, Dc3, Do3); residential, apartment (A2, Dc2, Do2); industrial
(A4, Dc4, Do4); commercial (A1, A5, Dc1, Dc5, Dc8, Do1, Do5, Do6).
24
Chapter 14 – Urban Ecosystems
Table 14.2. Surface temperature (°C) measured in a suburban Colorado residential landscape on a hot
summer day
Time of day
0715
0945
1330
1530
26.4
31.5
33.2
33.7
Flagstone patio
31.4
44.3
49.4
44.6
Exposed soil
30.6
53.4
61.8
41.5
Rock mulch
28.3
52.4
58.4
48.1
Watered grass
25.4
31.6
29.7
23.3
Watered flower bed
17.3
28.1
32.1
27.6
Dry grass
21.5
34.6
38.7
36.3
North
33.4
33.1
31.7
31.8
East
55.7
53.7
32.9
31.4
South
24.0
37.5
45.0
37.4
West
23.4
27.7
44.0
61.2
Air
Hardscape
Vegetation
Walls
Note: Shading shows temperatures less than air temperature.
Source: Data from Bonan (2000).
25
Ecological Climatology
Table 14.3. Maximum increases in summer rainfall and thunderstorm activity for 9 cities in the United
States expressed as a percent of rural value
Rainfall
Thunderstorms
Cleveland
27
38
Detroit
25
-
Chicago
17
42
St. Louis
15
25
New Orleans
10
27
Washington, D.C.
9
36
Houston
9
10
Tulsa
0
0
Indianapolis
0
0
Source: Data from Changnon (1981a, p. 4). See also Changnon (2001).
26
Chapter 14 – Urban Ecosystems
Table 14.4. Bulk density, infiltration, runoff, and erosion in relation to low, moderate, and high use zones
around campsites in Ohio
Use
Low
Moderate
High
1.28
1.32
1.44
Infiltration (mm per 100 mm)
82
50
26
Runoff (mm per 100 mm)
18
50
74
0.06
0.72
1.49
Bulk density (g cm-3)
Erosion (g liter-1)
Note: Data are for well-drained soils sampled in autumn. Infiltration and runoff are for 100 mm of water
applied over one hour. Erosion is based on 250 ml samples of runoff taken after 15 minutes.
Source: Data from Vimmerstedt et al. (1982).
27
Ecological Climatology
Table 14.5. Diversity, density, and crown cover for natural and urban forests in oak savanna and oak
forest areas of Menlo Park and pine forests of South Lake Tahoe
Menlo Park
South Lake Tahoe
Oak savanna
Oak forest
pine forest
Natural
Urban
Natural
Urban
Natural
Urban
Number of tree species
3
130
5
145
1
6
Density (per hectare)
5
35
279
42
761
373
Crown cover (%)
14
25
92
34
57
19
Source: Data from McBride and Jacobs (1986).
28
Chapter 14 – Urban Ecosystems
14.8 Figure Legends
Figure 14.1. Evening air temperature for Columbia, Maryland. Temperatures are the departure from a rural
location. Top: 1968 when the population was 1000. Bottom: 1974 when the population was 20 000.
Adapted from Landsberg and Maisel (1972) and Landsberg (1979).
Figure 14.2. Effect of urbanization on air temperature. Top: Maximum temperature difference between
urban and rural areas in relation to population size. Data are for 18 cities in Canada and the United States
and 11 cities in Europe. Adapted from Oke (1981). Bottom: Statistical relationship between population size
and daily mean temperature, daily minimum temperature, and daily temperature range (maximum minus
minimum temperature). Temperatures are for an average day of the year. Data from Karl et al. (1988).
Figure 14.3. Diurnal cycle of air temperature (top) and humidity (bottom) for urban and rural sites in St.
Louis. Data are for an average summer day from 1972 to 1975. Numbers in parentheses are daily means.
Adapted from Semonin (1981). Changnon (1981a) describes the St. Louis study.
Figure 14.4. Influence of wind on heat island development. Top: Generalized reduction in heat island with
increasing wind speed. Bottom: Limiting wind speed for heat island formation in relation to population.
Data from Landsberg (1981, p. 117).
Figure 14.5. Diurnal cycles of net radiation, sensible heat, latent heat, heat storage, and Bowen ratio for
Vancouver, B.C., on an average summer day. Top: Rural site. Middle: Suburban site. Bottom: Suburbanrural difference. Data from Cleugh and Oke (1986).
Figure 14.6. As in Figure 14.5, but averaged over daylight periods. Arrows are proportional to the size of
the flux. Data from Cleugh and Oke (1986).
Figure 14.7. Influence of urban geometry on the heat island. Top: Drawing of an urban canyon consisting
of a 16 m wide street lined by 16 m tall buildings and associated view factors. Bottom: Maximum
29
Ecological Climatology
temperature difference between urban and rural areas in relation to sky view factor. Data are for 29 cities in
Canada, the United States, and Europe (Oke 1981).
Figure 14.8. Surface and air temperatures measured in a small courtyard and adjacent grass lawn and
woods on a clear, calm, summer afternoon. Top: Plan view of courtyard and adjacent grass lawn and
woods. Bottom left: Surface temperatures. Bottom right: Air temperatures. Data from Landsberg (1970a)
and Landsberg (1981, pp. 73, 85, 86).
Figure 14.9. Surface energy fluxes measured at noon in a field (left) and nearby parking lot (right) in
Columbia, Maryland. Arrows are proportional to the size of the flux. Data from Landsberg and Maisel
(1972).
Figure 14.10. Climate of a small airfield at night as a function of height above ground and distance from a
building. Note the change from concrete to a grassy surface at about 100 m from the building. Top:
Relative humidity. Bottom: Air temperature. Adapted from Geiger (1965, pp. 247-248).
Figure 14.11. Air temperatures measured on a summer night in Washington, D.C. along a transect from the
northwest residential area southeast through Rock Creek Park into the 16th Street business district and the
grass parks of the Mall. Adapted from Landsberg (1981, p. 233).
Figure 14.12. Influence of park size on the urban-park temperature difference. Data are for nine temperate
parks, four tropical parks, and one arid city. Adapted from Upmanis et al. (1998).
Figure 14.13. Surface temperature measurements in a suburban Colorado residential landscape on a hot
summer day as a function of distance from house and viewing direction. Temperature measurements are at
0600 (left) and 1200 (right) local time. House: Looking towards the house. Greenbelt: Looking in the
opposite direction, towards an adjacent grassy greenbelt.
Figure 14.14. Influence of percent of irrigated greenspace on the Bowen ratio for suburban sites in three
cities in the United States. Adapted from Grimmond and Oke (1995).
Figure 14.15. Generalized urban heat island atmospheric circulation.
30
Chapter 14 – Urban Ecosystems
Figure 14.16. Average summer rainfall near St. Louis expressed as a percent of urban rainfall. Adapted
from Changnon (1981a, p. 6).
Figure 14.17. Weekly cycle of air pollution (carbon monoxide CO and ozone O3) and precipitation along
the Atlantic coast of the United States. Data from Cerveny and Balling (1998).
Figure 14.18. Annual water use in the United States. One gallon equals 3.785 liters. Top: Total and per
capita withdrawal and consumption since 1940. Data from U.S. Bureau of the Census (1997, p. 233).
Bottom: 1990 annual use by sector. Data from U.S. Bureau of the Census (1993, p. 223).
Figure 14.19. Boulder, Colorado, daily water use for 1997. The bottom panel shows daily water use and
rainfall for July and August. One gallon equals 3.785 liters. One inch equals 25.4 mm.
Figure 14.20. Infiltration and surface runoff in response to 100 mm of rainfall over 24 hours for forest,
pasture, residential, and commercial lands using the Soil Conservation Service method (Chapter 5).
Figure 14.21. Idealized storm hydrographs for urban and rural settings.
Figure 14.22. Effect of urbanization on floods. Top: Ratio of discharge after urbanization to discharge
before urbanization in relation to impervious area and flood recurrence interval. Bottom: Ratio of discharge
after urbanization to discharge before urbanization in relation to flood recurrence interval for 20%
impervious area. Adapted from Hollis (1975).
Figure 14.23. Sediment yield in relation to drainage basin area for 10 rural watersheds and 12 watersheds
undergoing urban development in the Baltimore-Washington, D.C. corridor circa 1965. Adapted from
Wolman and Schick (1967).
Figure 14.24. Effect of residential development on coarse woody debris in 16 northern temperate lakes in
Wisconsin and Michigan. Data are per km of shoreline. Data from Christensen et al. (1996).
Figure 14.25. Non-native plant species in the city of Bariloche, located in the Rio Negro Province of
Argentina (1991 population 81 000). Top: Percent of non-native plant species in relation to housing
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Ecological Climatology
density. Bottom: Percent of non-native plant species around individual homes on the city edge. Adapted
from Rapoport (1993).
Figure 14.26. Tree cover in cities in the United States. Top: Percent of total land covered by trees in
relation to population for eight cities in the northern hardwood forest region. Data from Halverson and
Rowntree (1986). Middle: Percent of total land covered by trees for 58 cities in forest, grassland, and
desert ecoregions. Data from Nowak et al. (1996). Bottom: Percent of land covered by trees in relation to
urban land use and ecoregions for 58 cities. Data from Nowak et al. (1996).
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