Ninety-five percent of the United States' fresh water is underground. One crucial source is a huge underground reservoir, the 800-mile Ogallala aquifer which stretches from Texas to South Dakota and waters one fifth of US irrigated land.
The aquifer was formed over millions of years, but has since been cut off from its original natural sources. It is being depleted at a rate of 12 billion cubic metres a year – amounting to a total depletion to date of a volume equal to the annual flow of 18 Colorado Rivers. Some estimates say it will dry up in as little as 25 years.
Many farmers in the Texan High Plains, which rely particularly on the underground source, are now turning away from irrigated agriculture as they become aware of the hazards of overpumping.