Geoscience Reference
In-Depth Information
hydrology, but other areas where urbanisation may
have an impact are point source and diffuse pollu-
tion affecting water quality, river channelisation to
control flooding, increased snow melt from urban
areas and river flow changes from sewage treatment.
Urban runoff change
The changes in climate are relatively minor com-
pared to the impact that impermeable surfaces in the
urban environment have on runoff hydrology. Roofs,
pavement, roads, parking lots and other imperme-
able surfaces have extremely low infiltration charac-
teristics, consequently Hortonian overland flow
readily occurs. These surfaces are frequently linked
to gutters and stormwater drains to remove the
runoff rapidly. The result of this is far greater runoff
and the time to peak discharge being reduced.
Cherkauer (1975) compared two small catchments
in Wisconsin, USA. The rural catchment had 94 per
cent undeveloped land while the urban catchment
had 65 per cent urban coverage. During a large storm
in October 1974 (22 mm of rain in five hours)
the peak discharge from the urban catchment area
was over 250 times that of the rural catchment
(Cherkauer, 1975). The storm hydrograph from this
event was considerably more flashy for the urban
catchment (i.e. it had a shorter, sharper peak on
the hydrograph).
Rose and Peters (2001) analysed a long period
of streamflow data (1958-96) to detect differences
between urbanised and rural catchments near
Atlanta, Georgia, USA. The stormflow peaks for
large storms were between 30 and 100 per cent
larger in the urbanised catchment, with a consid-
erably shorter recession limb of the hydrograph. In
contrast to the stormflows, low flows were 25-35
per cent less in the urban catchment, suggesting a
lower rainfall infiltration rate. Overall there was no
detectable difference in the annual runoff coefficient
(runoff as percentage of precipitation) between
urban and rural catchments.
Figure 8.12 shows some data from a steadily
urbanising catchment (13km
2
) in Auckland, New
Zealand. There has been a drop in the percentage
of baseflow leaving the catchment (the baseflow
index - BFI) which could be attributed to declining
infiltration to groundwater and therefore less water
released during the low flow periods. Care needs to
be taken in interpreting a diagram like Figure 8.12
because it is also possible that the decline in BFI was
Urban climate change
In Table 8.5 some of the climatic changes due to
urbanisation are expressed as a ratio between the
urban and rural environments. This suggests that
within a city there is a 15 per cent reduction in the
amount of solar radiation reaching a horizontal
surface, a factor that will influence the evaporation
rate. Studies have also found that the precipitation
levels in an urban environment are higher by as
much as 10 per cent. Atkinson (1979) detected an
increase in summer thunderstorms over London
which was attributed to extra convection and con-
densation nuclei being available. Other factors
greatly affected by urbanisation are winter fog
(doubled) and winter ultraviolet radiation (reduced
by 30 per cent).
Table 8.5
Difference in climatic variables
between urban and rural environments
Climatic variable
Ratio
of city:
environs
Solar radiation on horizontal surfaces
0.85
UV radiation: summer
0.95
UV radiation: winter
0.70
Annual mean relative humidity
0.94
Annual mean wind speed
0.75
Speed of extreme wind gusts
0.85
Frequency of calms
1.15
Frequency and amount of cloudiness
1.10
Frequency of fog: summer
1.30
Frequency of fog: winter
2.00
Annual precipitation
1.10
Days with less than 5 mm precipitation
1.10
Source
: From Lowry (1967)
