Even in good years, farmers in the Midwest supplement rainfall with irrigation from the Ogallala Aquifer. The map below shows irrigated areas in blue; the darker the color, the heavier the irrigation. That big, dark patch in the middle, to the left of the little icon, is irrigated by both surface water and water from the Ogallala and other aquifers in the High Plains system.
The Ogallala spreads across 174,000 square miles, providing drinking water and irrigation to a huge swath of the United States, replenished slowly by rainfall in the region. It's a critically important resource, which is why it's been a big part of the Keystone XL fight -- if it's polluted by tar-sands oil, the damage could be catastrophic.
The Ogallala and other aquifers around the globe are also threatened by overuse. According to research published this week in Nature, "about 1.7 billion people live in areas where groundwater resources and/or groundwater-dependent ecosystems are under threat." Researchers estimate that the amount of water being used is 3.5 times the size of the aquifers.