Agriculture is a major water hog. The Great Plains of the United States get enough rain to support wheat farming, but many farmers use irrigation in order to assure a satisfactory crop, even in drought years. So, because of this huge demand for irrigated water, the Great Plains has been tapping into the Ogallala aquifer (circled above) which contains an estimated 2 billion acre-feet of water (1). This water is fossil water, which has been accumulating for more than 1 million years. Because this water is not self sustaining, we have been pulling from it at a much faster rate than it can refill naturally. The Great Plains are now at risk of running dry.
As you can see above, Portland, like the Great Plains, also has a large aquifer residing below ground. However, there are a few differences between how Portland gets its water, compared to the Great Plains. Portland does not use this aquifer for the much of its water supply. Since 1895, the city of Portland has diverted water from the Bull Run Watershed, about 26 miles from the city (2). This water comes into the city unfiltered yet still satisfies regulations for safe drinking water.
The benefit to using the watershed is that the water source is more sustainable than an aquifer. Many large aquifers accumulate water over long periods of time, while watersheds are replenished by rivers and precipitation.
1. Stephen S. Birdsall. Regional Landscapes of the United States and Canada
2. http://www.portlandonline.com/water/index.cfm?c=29332&a=327613
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