13 July, 2006
Urban Activities in Arid Environments Affect the Water Cycle
A new American study has recently shown, using a unique 108-year-old data record and
satellite data, that cities located in arid areas have a significant effect on rainfall patterns
around them. According to the results of the study, it appears that human activities such as
land use, aerosol generation, and irrigation in these arid urban environments have an
impact on the entire water cycle by changing the weather patterns in and near the cities.
During the past half-century, cities in arid areas have experienced an important growth due to new
methods of obtaining water and irrigation. While scientists have known for some time that the so-called
“heat-island” effect of large cities can affect their weather, the knowledge about this effect and other
processes in arid cities is very limited. It has become increasingly necessary to understand how
weather affects the water-cycle in cities because according to one United Nations estimate, 60 to 70
percent of the population will live in cities by 2025, and many of the fastest-growing areas for city
growth are in arid areas.
Recently, an American climatologist has studied the "heat-island" effect and rainfall patterns in arid
cities. To this end, he used a unique 108-year-old data record of rainfall and satellite-based rainfall
estimates for the arid cities of Riyadh in Saudi Arabia and Phoenix in United States.
One of the most interesting results of the study was the 12-14 percent increase in rainfall in the
northeast suburbs of Phoenix from the pre-urban (1895-1949) to post-urban (1950-2003) periods. A
previous study had suggested a possible anomaly but focused only on the post-urban period, leaving it
unclear if the change was related to urban topographic changes or human activity. Therefore, the
current study confirmed, for the first time, that a statistically significant change in rainfall took place in
certain areas of Phoenix from the late 1890s to present. It has been hypothesized that this increase in
the rainfall could be related to urban-topographic interactions and possibly irrigation moisture. Indeed,
the role of irrigation in changing the weather of cities in arid areas is one of the more interesting
findings, but more research in this regard is still needed.
A similar pattern was observed in Riyadh, Saudi Arabia, where the data show a significant increase in
rainfall around the city although the causes are less clear than in Phoenix. Its weather also has been
affected by the heat-island affect as large areas of pavement and concrete have increased the heat of
the city area, thus promoting the change in weather patterns around it.
These results suggest that human activities such as land use, aerosols generation and irrigation in
these arid urban environments are affecting the weather and the entire water cycle in arid regions.
Consequently, climate models for these areas should consider patterns of urban land use, aerosols and
irrigation in order to accurately predict those large-scale processes.
This study also illustrates how useful tool satellite data can be when observing the changing landscape
and climate in regions where traditional measurements are lacking or scarce.
Overall, the results of this study show how human-induced changes can affect the water cycle, in
particular rainfall patterns, even under arid or drought conditions. These findings may have implications
for water resource management, agricultural efficiency and urban planning. Weather and climate
models for these areas should deal with patterns of urban land use, aerosols emissions, and irrigation if
they are to help planners to understand and predict these large-scale processes.
Source 1: J.M. Shepherd (2006) "Evidence of urban-induced precipitation variability in arid climate regimes", Journal of Arid
Environments, In Press.
Source 2: Scripps Howard News Service 26-JUN-06
Contact: [email protected]
Theme(s): Water, urban environment
Opinions expressed in this News Alert do not necessarily reflect those of the European Commission
European Commission DG ENV
News Alert issue 30
July, 2006
1