It may seem to defy the logic of a closed planetary system, but the supply of water available for irrigation is indeed diminishing - at an alarming rate.
In 1970, farmers in rural Deaf Smith County in the Texas panhandle encountered a small but definite sign that local agriculture was seriously out of balance. An irrigation well that had been drilled in 1936 went dry. After more than 30 years of heavy pumping, the water table had dropped 24 meters. Soon other wells began to dry up too.
Water tables were falling across a wide area of the Texas High Plains, and when energy prices shot up in the 1970s, farmers were forced to close down thousands of wells because they could no longer afford to pump from such depths.
During the last three decades, the depletion of underground water reserves, known as aquifers, has spread from isolated pockets of the agricultural landscape to large portions of the world's irrigated land. Many farmers are now pumping groundwater faster than nature is replenishing it, causing a steady drop in water tables. Just as a bank account dwindles if withdrawals routinely exceed deposits, so will an underground water reserve decline if pumping exceeds recharge. Groundwater overdrafting is now widespread in the crop-producing regions of central and northern China, northwest and southern India, parts of Pakistan, much of the western United States, North Africa, the Middle East, and the Arabian Peninsula.
Many cities are overexploiting groundwater as well. Portions of Bangkok and Mexico City are actually sinking as geologic formations compact after the water is removed. Albuquerque, Phoenix, and Tucson are among the larger U.S. cities that are overdrafting their aquifers.
Globally, however, it is in agriculture where the greatest social risks lie. Irrigated land is disproportionately important to world food production. Some 40 percent of the global harvest comes from the 17 percent of cropland that is irrigated. Because of limited opportunities for expanding rainfed production, we are betting on that share to increase markedly in the decades ahead, in order to feed the world's growing population. As irrigation goes deeper and deeper into hydrologic debt, the possibilities for serious disruption grow ever greater. Should energy prices rise again, for example, farmers in many parts of the world could find it too expensive to irrigate. Groundwater overpumping may now be the single biggest threat to food production.
Our irrigation base is remarkably young: 60 percent of it is less than 50 years old. Yet a number of threats to its continued productivity are already apparent. Along with groundwater depletion, there is the buildup of salts in the soil, the silting up of reservoirs and canals, mounting competition for water between cities and farms and between countries sharing rivers, rapid population growth in regions that are already water-stressed - and on top of all that, the uncertainties of climate change. Any one of these threats could seriously compromise agriculture's productivity. But these stresses are evolving simultaneously-making it increasingly likely that cracks will appear in our agricultural foundation.
Few governments are taking adequate steps to address any of these threats and, hidden below the surface, groundwater depletion often gets the least attention of all. Yet this hydrologic equivalent of deficit financing cannot continue indefinitely. Groundwater withdrawals will eventually come back into balance with replenishment - the only question is whether they do so in a planned and coordinated way that maintains food supplies, or in a chaotic and unexpected way that reduces food production, worsens poverty, and disrupts regional economies.
It is true that there are enormous inefficiencies elsewhere in the agricultural sector - and tackling these could take some of the pressure off aquifers. A shift in diets, for example, could conserve large amounts of irrigation water. The typical U.S. diet, with its high share of animal products, requires twice as much water to produce as the nutritious but less meat-intensive diets common in some Asian and European nations. If U.S. consumers moved down the food chain, the same volume of water could produce enough food for two people instead of one, leaving more water in rivers and aquifers. But given the rates of groundwater depletion, there is no longer any reasonable alternative to tackling the problem directly. Aquifer management will be an essential part of any strategy for living within the limits imposed by a finite supply of fresh water.
The Groundwater Revolution
During the first century of the modern irrigation age - roughly from 1850 to 1950 - efforts to develop water supplies focused mainly on rivers. Government agencies and private investors constructed dams to capture river water and canals to deliver that water to cities and farms. By the middle of this century, engineers had built impressive irrigation schemes in China, India, Pakistan, and the United States, and these nations became the world's top four irrigators. The Indus River system in South Asia, the Yellow and Yangtze Rivers in China, and the Colorado and Sacramento-San Joaquin river systems of the western United States were each irrigating sizable areas by 1950. The global irrigation base then stood at 100 million hectares, up from 40 million in 1900.
Between 1950 and 1995, world irrigated area increased to more than 250 million hectares. Even as the construction of large dams for hydroelectric power, water supply, and flood control picked up pace, a quiet revolution in water use unfolded during this period. Rural electrification, the spread of diesel pumps, and new well-drilling technologies allowed farmers to sink millions of wells into the aquifers beneath their land. For the first time in human history, farmers began to tap groundwater on a large scale.
Aquifers are in many ways an ideal source of water. Farmers can pump groundwater whenever they need it, and that kind of availability typically pays off in higher crop yields. Compare this with the standard scenario for irrigating with river water: river flow is erratic, so a reservoir is usually required to store flood water for use in the dry season. And reservoirs - especially arid-land reservoirs such as Lake Nassar behind Egypt's High Aswan Dam - can lose 10 percent or more of their water to evaporation. In addition, the large canal networks that move water out of reservoirs are often unreliable - they may not deliver enough water when farmers actually need it. Aquifers, on the other hand, have a fairly slow and steady flow that is usually available year-round and they don't lose water to evaporation. Finally, groundwater is generally less expensive to develop than river water. Data from 191 irrigation projects funded by the World Bank show that groundwater schemes cost a third less on average than surface schemes.
Not surprisingly, huge numbers of farmers and investors turned to groundwater as soon as they acquired the means to tap into it. In China, the number of irrigation wells shot up from 110,000 in 1961 to nearly 2.4 million by the mid-1980s. In India, government canal building nearly doubled the area under surface irrigation between 1950 and 1985, but the most impressive growth was in groundwater development: the area irrigated by tubewells ballooned from 100,000 hectares in 1961 to 11.3 million hectares in 1985 - a 113-fold rise, most of it privately funded. (A tubewell is a narrow well that is drilled into an aquifer, as opposed to a larger-diameter well that is excavated, either by hand or with machinery.) In neighboring Pakistan, groundwater was the fastest-growing form of irrigation from the mid-1960s through the 1980s. A public program of tubewell development failed miserably, but private groundwater investments climbed steeply. The total number of tubewells in that country rose from some 25,000 in 1964 to nearly 360,000 in 1993.
After World War II, the United States experienced a groundwater...