ITHACA, N.Y. -- Unless the world's food-growing nations improve their resource-management practices, life in the 21st century will be as tough as it is now in the 80 countries that already suffer serious water shortages, a new Cornell University study warns.

As a start, governments should end irrigation subsidies that encourage inefficient use of water and instead reward conservation, according the report, "Water Resources: Agriculture, the Environment and Society," published in the February 1997 issue of the journal, BioScience.

"Undercharging for irrigation water in the U.S. and other nations hides the true cost of food and encourages the planting of low-value crops," said David Pimentel, professor of ecology in Cornell's College of Agriculture and Life Sciences, leader of the 10-researcher team that performed the water-resource analysis. "If farmers paid the full cost of water, they would manage irrigation water more efficiently. We should reward water conservation, not water use."

The study examined factors responsible for a worsening shortage of fresh water, including usage that is increasing out of proportion to population increases. While the world population increased from 3.8 billion to 5.4 billion during the recent two decades, water use worldwide increased three-fold, the analysts found.

Whether or not humans are to blame for global warming, we can expect changes that are difficult to predict, the report observed. For example, a warmer California, which already is experiencing water shortages, might see a 20- to 40-percent decrease in mountain snowpack and water flow through its river basins, the Cornell researchers found. And warmer temperatures would melt the California snowpack earlier in the year, creating severe water shortages in the summer.

Farther north on a warmer globe Canada might benefit from longer growing seasons, but also face water shortages, according to one prediction cited by the Cornell team. Overall, global warming could increase the world's irrigation needs by 26 percent while worsening deforestation, desertification and soil erosion -- all of which affect water resources.

Our taste for meat is costly in terms of water, Pimentel noted. Producing a pound of animal protein requires, on average, about 100 times more water than producing a pound of vegetable protein. But some animals are thriftier, he noted: Whereas growing the grain to feed cattle requires 12,000 gallons of water for every pound of beef, chicken can be produced for "only" 420 gallons of water per pound of meat.

Not only is irrigation becoming more costly, in terms of energy expenditures, as underground aquifers run dry and water has to be pumped from greater depths, but energy production is taking more water, according to the study. Oil shale, coal gasification or coal liquefaction -- the last of the fossil fuel sources as the world's supplies of oil and natural gas are depleted -- require 20 to 50 times more water to produce an equivalent amount of energy, compared to oil and gas.

The study predicts heightened international tensions when shared rivers dwindle as they flow through countries that siphon off their share. Pointing to the Nile, which passes through the Sudan, Ethiopia, Egypt, Burundi, Kenya, Rwanda, Tanzania, Zaire, Eritrea and Uganda on the way to the Mediterranean, the researchers said: "This river is so dammed and overused that for parts of the year little or no fresh water reaches the sea. All these nations are becoming increasingly dependent on the Nile as their populations increase and their food situation worsens."

To make matters worse, much of the water for irrigation never reaches the crops, the study reported, because of losses through pumping and transporting. Worldwide, irrigation efficiency is less than 40 percent, and U.S. growers don't do much better, losing more than 50 percent of irrigation water. Among the technologies and practices suggested to improve efficiency are surge flow irrigation (to replace the traditional method of slow, continuous flooding) as well as night irrigation, low-pressure sprinklers, low-energy precision application and drip irrigation, all help reduce losses by evaporation.

An old agricultural practice -- planting trees as "shelter belts" along with food crops -- can reduce evaporation from soil and transpiration from crops while reducing wind erosion of soils by as much as 50 percent, the researchers suggested. In particular, they recommend intercropping crops with "hydraulic lifter" trees, such as eucalyptus, which draw moisture from deep in the soil at night and make it available to surrounding plants.

Intercropping also reduces soil erosion, the study observed, noting that the loss of topsoil cuts rainwater infiltration by 93 percent and dramatically increases water runoff and loss. When water runs off farmed land, it carries with it not only sediments but nutrients and pesticides, making soil erosion the leading cause of non-point source pollution in the United States.

Some changes in practice will be involuntary, the water-resource study predicted: "In the future, in arid regions where ground water resources are the primary source of water, irrigation probably will have to be curtailed and types of crops and livestock maintained altered to meet the changing water situation."

But policy changes can help, the researchers said: "To encourage conservation, subsidies for irrigation water should be phased out to increase overall efficiency. Irrigation technologies that make efficient use of water for crop production must be encouraged. In general, more effective use of water in agricultural production could be achieved by providing farmers incentives to conserve water and soil resources."

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10 not-so-dry facts about water resources

From "Water Resources: Agriculture, the Environment and Society"

By D. Pimentel, J. Houser, E. Preiss, O. White, H. Fang, L. Mesnick, T. Barsky, S. Tariche, J. Schreck and S. Alpert

-- Thirsty plants. Agriculture consumes 87 percent of the fresh water withdrawn in the world.

-- Lucky break. Earth's atmosphere is the source of all the rain that falls, returning to the surface water that evaporates from the oceans and land areas. But the distribution is not equal, and that's lucky for land-dwellers: Although only 14 percent of evaporation occurs from land, about 24 percent of the world's precipitation falls onto land.

-- Water guzzlers. Humans need only 1 to 2 quarts of water per day to stay alive, but we use much more. Americans use about 100 gallons of water per person per day for drinking, cooking, washing, disposing of wastes and other personal purposes -- much higher than the world average of about 22 gallons per person per day. Counting irrigation and all other uses, Americans account for 1,350 gallons per person per day. The worldwide average is 475 gallons for all uses, and in China the average is even less -- 290 gallons.

-- More thirsty plants. A squash plant transpires 10 times its weight in water per day. Many deciduous trees transpire 2 to 6 times their weight.

-- Water bill. Depending on the water quality and purification treatments used, potable water costs from $1.1- to $1.25 per 1,000 gallons. Add 50 cents per 1,000 gallons if charcoal filtration is needed and another 80 cents if salts have to be removed. To desalinate ocean water costs as much as $7.60 per 1,000 gallons.

-- Out of sight. In the United States, ground water provides about 46 percent of the water used by all households, and agriculture takes even more: About 66 percent of irrigation water in Texas and 38 percent in California is pumped from ground water.

-- Deficit pumping. The Ogallala aquifer that underlies parts of Nebraska, Kansas, Colorado, Oklahoma, New Mexico and Texas is almost one-half depleted. Recharge rates for the Ogallala aquifer are only a small fraction of the withdrawals.

-- No rain, no gain. Some countries are worse off, having little ground water and receiving practically no rain. Egypt, for example, gets 97 percent of its water from the Nile, which flows from its upstream neighbors. Of the 14 Middle Eastern countries, nine face serious water shortages.

-- Dangerous drink. Agricultural and industrial chemicals and the lack of community sanitary facilities are the primary causes of water pollution. In developing countries, about 90 percent of all illnesses result from waterborne parasites and pathogens.