From todays http://www.keelynet.com/ item:
"There's a change in rain around desert cities
Urban areas with high concentrations of buildings, roads and other artificial
surface soak up heat,
lead to warmer surrounding temperatures, and create "urban heat-islands."
This increased heat may promote rising air and alter the weather around
Human activities such as land use, additional aerosols and irrigation in these
arid urban environments also
affect the entire water cycle as well...."
One Inch of water on one acer of land, is 18,000 gallons of water.
When you pave over one acer of land, that is 18,000 gallons of water
that is no longer soaking into the ground,
is it any wonder the water cycle changes when you pave thousands of acres?