Humans depend on water in many ways, well beyond the few liters needed daily for drinking. Water is also essential for the production of food. Various forms of agriculture, practiced on about half of Earth's land surface, provide the vast majority of food that over 6 billion people eat. Agriculture also provides much of the fiber for cotton, wool, and linen clothing.
One of the primary ways in which humans use water is by planting important crops in places where they can capture natural rainfall as rain-fed agriculture. Some forms of agriculture, such as intensive rice and corn production, can be practiced only in rainy climates. Such agricultural forms are much more productive than others, such as cattle and sheep herding, which are usually relegated to semiarid climates.
One of the primary reasons rain-dependent forms of agriculture are more productive than dry-land forms is that they have sufficient water to allow plants to grow to their maximum potential. Therefore, the most agriculturally productive regions of the world are all regions where natural rainfall is sufficient to allow rain-fed agriculture to flourish: for example, the
Up to a point, the more rain, the more productive the crops. For example, in the United States, corn is a productive grain that typically yields over 100 bushels per acre, but requires a climate where rainfall is at least 76 centimeters (30 inches) per year. Wheat yields only about 20 bushels per acre under the same conditions, but can still yield nearly this much if rainfall is only 50 centimeters (20 inches). Therefore, farmlands that receive at least 76 centimeters (30 inches) per year, as they do in the Corn Belt states of Ohio, Indiana, Illinois, and Iowa, are much more agriculturally productive than the Wheat Belt lands of the Dakotas, Montana, Nebraska, and Kansas, which receive less than 76 centimeters (30 inches). And the monsoonblessed rice lands of southeast Asia are more productive yet.
Because agricultural crops are so dependent on water, purposely adding water, beyond what naturally falls as rain, is widely practiced to increase agricultural production. This critical practice is known as irrigation.
Irrigation is an ancient practice that originated along the Tigris and Euphrates Rivers in what is now Iraq, and spread in ancient times to the desert valley of the Nile River in Egypt, the Indus River in Pakistan, and all the way to China. Native Americans also practiced irrigation long ago, especially in the areas now called Mexico and the southwestern United States.
It is estimated that 40 percent of all crops grown in the world today are grown using irrigation. The practice of irrigation can increase the
|LEADING IRRIGATION COUNTRIES|
|Country||irrigated Area (1,000 Square Kilometers)||Irrigated Land as a Percentage of Cropland||Water Used in Irrigation (Cubic Kilometers)||Percentage of all Water Withdrawals used for Irrigation|
productivity of crops on what would otherwise be rain-fed agriculture. It can also expand agriculture into areas where it would not otherwise be practiced due to aridity.
In the twentieth century, the practice of irrigation was greatly increased to provide food for the world's growing population. Globally, irrigation now accounts for 69 percent of the 3,240 cubic kilometers (772 cubic miles) of water withdrawn for human use, and 87 percent of all water consumed. Proportions in the United States are 42 percent of all withdrawals and 87 percent of all consumption. (A consumptive use of water means that the water is no longer available for another use.)
Asia, with the majority of Earth's population, remains the world's most irrigated region. The three most populous countries—China, India, and the United States—are also the leading countries in irrigated area and in water used in irrigation, as shown in the table. Pakistan and especially Egypt remain the countries most dependent on irrigation, although other countries of dry southwestern and central Asia (e.g., Saudi Arabia, Iran, Iraq, and Uzbekistan) greatly depend on irrigation for their food and fiber supplies.
In the United States, the state of California is by far the most important irrigated region. * But irrigation also is critical to the relatively small agricultural outputs of arid states such as Utah, Arizona, New Mexico, and Colorado. Irrigation is used extensively in parts of the Great Plains to mirror the Corn Belt style of agriculture farther east, and in Arkansas along the Mississippi River to maximize yields.
In every irrigated region, water supplies are a limitation on further expansion of irrigated agriculture. In many regions, renewable supplies have already been exceeded, resulting in falling groundwater levels and greatly reduced river flow. In some regions, the depletion of water resources due to irrigation has reached crisis proportions. Today the Colorado River rarely reaches the Sea of Cortez (often called the Gulf of California), and even the Yellow River of China is sometimes drained completely. The Aral Sea in central Asia has lost half its surface area, most of its volume, and all of its once-enormous fishing industry. These are all examples of the depletion of regional water resources by irrigated agriculture.
In the United States, especially in California, attempts have been made to move water away from irrigation and toward urban, industrial, and environmental purposes. Farmers can now "rent" their annual water allocation without losing their permanent water rights. Irrigation efficiency is improving with drip irrigation systems such as those pioneered in water-short Israel. In the southern Great Plains, some farmers are simply reverting back to dry-land agriculture as groundwater levels fall so low that they cannot afford to pump the water to the surface.
Agriculture uses vast quantities of water and also causes extensive pollution, primarily by introducing nonpoint-source contaminants. Runoff from agricultural fields often contains eroded soil, fertilizers, animal manure, or pesticides that together form a major source of water pollution.
This form of nonpoint-source pollution remains remarkably unregulated by governments. Yet recent initiatives include Total Maximum Daily Load requirements that are imposed on whole watersheds . Moreover, the U.S. Department of Agriculture now utilizes two methods to induce farmers to decrease polluted runoff:
Despite their good intentions, these policies have succeeded only in chipping away at the growing environmental issue of polluted agricultural runoff.
Hydroponics is a method of gardening without soil. Instead of soil, plants are grown in chemical nutrient solutions and supported by porous materials such as peat moss and sand. The most commonly grown hydroponic produce are tomatoes, lettuce, herbs, cucumbers, and peppers.
A commercial hydroponic operator uses about 5 percent of the water and a fraction of the land needed to produce an equivalent amount of produce in traditional agriculture. For example, with just 650 square meters (7,000 square feet) of greenhouse space, or about one-eighth of an acre, a hydroponic farmer can grow as much as 50,000 pounds of hydroponic tomatoes annually.
In the 1960s, commercial U.S. hydroponics developed in the arid Southwest. Of over 30,000 hydroponic acres in world production as of 2002, about 800 acres were in the United States. Most of these U.S. facilities are family or small business operations that cover less than an acre, with produce sold locally. There are also several larger U.S. facilities that cover over 60 acres, with produce sold nationally. Recently, U.S. production surpassed Canadian production, with the tomato as its leading hydroponic product, followed by the bell pepper and the cucumber.
The U.S. demand for hydroponic produce is growing, and has grown so high that domestic supply in 2002 was inadequate. Consequently, hundreds of thousands of pounds of hydroponic produce are imported daily.
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Pimentel, David et al. "Water Resources: Agriculture, the Environment and Society." Bioscience 47(2):97–106.
Postel, Sandra. Pillar of Sand: Can the Irrigation Miracle Last? New York: W.W. Norton & Company, 1999.
Reisner, Marc. Cadillac Desert: The American West and Its Disappearing Water, 2nd ed. New York: Penguin Books, 1993.
Agriculture. Food and Agriculture Organization of the United Nations. <http://www.fao.org/ag/> .
Water Quality and Management Program. U.S. Department of Agriculture, Agricultural Research Service. <http://www.nps.ars.usda.gov/programs/programs.htm?NPNUMBER=201> .
* See the "Irrigation Management" and "Ogallala Aquifer" entries for photographs of U.S. irrigation.