Consistently falling groundwater levels are an indicator of overexploitation. They are observed worldwide. In northern China,4 western India5 and South Africa,6 groundwa-ter tables are declining at rates between 1 and 3 m per year. The abstractions in the Sahara and Saudi Arabia deplete aquifers which since the last ice age have not had any recharge to speak of.7 It is estimated that by 2010 the deeper aquifers of Saudi Arabia will have only 40% of their 1985 water reserves left.2 The present net depletion by overdraft of important groundwater reservoirs worldwide is at least 200 cubic kilometers per day.8 In practically all cases, irrigation is responsible for overdrafts.
The best documented case of over pumping due to irrigation is the Ogallala aquifer in the Great Plains region of the United States. It is one of the largest aquifer systems in the world, stretching across parts of eight states, South Dakota, Nebraska, Wyoming, Colorado, Kansas, Oklahoma, New Mexico and Texas, and underlying 174,000 square miles.
Groundwater withdrawal on a large scale started during the 1930 drought. Besides drought conditions, reasons for the fast development of groundwater irrigation were cheap energy, improved well drilling and pumping systems, and high crop prices. Development of groundwater resources for irrigation made this region one of the major agricultural regions in the United States. In the late 1970s, the abstraction rate of the Ogallala Aquifer amounted to more than a quarter of the groundwater used for irrigation in the US. Pumping rates many times in excess of the recharge rate led to a substantial decline in water levels ranging from 3 to 30 meters.9 Up to 1990, the total abstraction exceeded the total recharge by 164 km3.10
Water level declines were greatest in those parts of the aquifer where irrigation was developed first and where it was most intense. Severe depletion of the resource led to rising pumping costs, driving much of the irrigated agriculture in the region out of production by the early 1990s. In the Texas High Plains for example, irrigated area decreased by 34% between 1974 and 1989.2 A large part of the area today has reverted to rainfed agriculture with much lower yields.
Increased pumping costs led to technical and institutional adaptations. Among the technical measures were increased irrigation efficiency and the practice of conservation tillage. Average water-use efficiency in the southern High Plains of Texas improved from about 50% in the mid-1970s to approximately 75% in 1990 due to improved technology. Current state of the art low pressure, full dropline center pivot systems are about 95% efficient. Buried drip lines approach 100% efficiency.11 Institutional measures include groundwater laws that made possible the institution of regionally controlled ground-water management units, which set limits on the spacing and number of wells and carried out metering of water use and promotion of water conservation.12
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