Bob Paddock wrote: > >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 > cities. > 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?
While this seems reasonable at first glance it grossly overestimates the amount affected. First when it rains, the majority of water runs off, instead of soaking into the ground. Second, much of what soaks into the ground evaporates in the following 24 to 48 hours, and pavement not only stops the water from soaking in, but also stops the evaporation. Lastly, at least around here, when new pavement is installed, retention ponds are put in to hold the runoff, and these retention ponds offset the difference, and in some cases possibly actually increase the amount of water that soaks into the ground over time. Marshall