Urban impacts on climate - The Global Climate System: Patterns, Processes, and Teleconnections - page 238

7.6 Moisture and precipitation
Urban environments create major alterations to the water balance. The solid
impervious surfaces in the city encourage water runoff and prevent infiltration
into the subsurface. Infiltration is an important feature of the rural environment.
According to Bonan ( 2002 ), on average in cities, 83% of precipitation runs off
the urban surfaces (mainly into sewers and gutters, depending on the develop-
ment level of the city), and only 17% reaches the subsurface. The amount of
evaporation was not considered. In heavy rain situations, peak discharges are
increased, runoff speed is enhanced, and the volume of runoff is much higher
than in a rural environment. Flooding can be more prevalent if the sewage and
storm water runoff system cannot handle the excess water. The solid urban
surfaces also ensure that evaporation in the city remains considerably lower
than in the rural environment, and the lack of trees and parks creates reductions
in transpiration as well (see Figure 7.7 ).
As a result of the urban surface plus the existence of the UHI, there are
differences between urban and rural atmospheric moisture levels (absolute
humidity). Humidity variations in the city are caused by varying evaporation
rates, varying levels of condensation, advection of air from outside the city,
extra turbulence and anthropogenic moisture release from different activities
such as combustion or watering the lawn (Oke 1997 ). Holmer and Eliasson
( 1999 ) describe an urban moisture excess (UME) in city atmospheres at night,
and a moisture deficit during the daytime, for temperate climate cities. The
UME also varies by season; for example winter UME is usually smaller than
summer UME, but colder overall climate and artificial moisture sources may
cause exceptions. Excess heat in the urban atmosphere means that urban air
can hold more moisture as water vapor at night, and condensation processes
such a dewfall may be less likely in areas with high Ys and low vegetation
amounts.
The greatest difference between urban and rural atmospheric moisture levels
occurs during the afternoon and early evening. In St Louis, USA, for example, a
rural site averaged 14.5 g kg 1 and the urban site 14 g kg 1 , but from 0700 to
midnight, the urban-rural difference exceeded 1 g kg 1 (Karl et al. 1988 in
Bonan 2002 ). In Mexico City, the urban center tends to be more humid at
night (by about 6-8 g kg 1 ); the rural area more humid during the day (by
about 1-3 g kg 1 ) (Jauregui and Tejeda 1997 ). Stronger winds reduce urban-
rural humidity differences. Several other cities exhibited urban-rural moisture
differences of between 1 and 3 g kg 1 (Holmer and Eliasson 1999 ), but with
considerable individual and diurnal variation.
Holmer and Eliasson ( 1999 ) suggest that there can be a linear relationship
between UME and UHI development, with the former lagging behind the latter
by 1 to 5 hours. Ideally a UME should enhance UHI because a warmer, more
humid atmosphere should add 20-40Wm 2 to sky-emitted longwave radiation.