In 2009, the most recent year for which global data are available from the United Nations Food and Agriculture Organization (FAO), 311 million hectares in the world was equipped for irrigation but only 84 percent of that area was actually being irrigated, according to new research conducted by the Washington DC-based Worldwatch Institute. As of 2010, the countries with the largest irrigated areas were India (39 million hectares), China (19 million), and the United States (17 million).

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Around 70 percent of the freshwater withdrawals worldwide are for irrigation. Although this is a huge amount of water, the process is essential for feeding a growing population; especially in arid areas, where crop yields two to four times greater can be grown with irrigation compared to rain-fed farming. The process currently provides 40 percent of the world's food from approximately 20 percent of all agricultural land, according to Worldwatch.

Since the late 1970s, irrigation expansion has experienced a marked slowdown. The FAO attributes the decline in investment to the unsatisfactory performances of formal large canal systems, corruption in the construction process, and acknowledgement of the environmental impact of irrigation projects.  The increasing availability of inexpensive individual pumps and well construction methods has led to a shift from public to private investment in irrigation, and from larger to smaller-scale systems.

The individualised approach to irrigation has been concentrated in India, China, and other parts of Southeast Asia. This has been spurred on by the promise of higher outputs and incomes and better diets. Worldwatch researcher Judith Renner, who has investigated irrigation and its effect on water supplies said, "The option is often made even more appealing with offers of government subsidies for energy costs of running groundwater pumps and support prices of irrigated products."

She continued, "In India's Gujarat state, for example, energy subsidies are structured so that farmers pay a flat rate, no matter how much electricity they use. But with rising numbers of farmers tapping groundwater resources, more and more aquifers are in danger of overuse."

If groundwater resources are overexploited, aquifers will be unable to recharge fast enough to keep pace with water withdrawals. In August, scientists conducting a global analysis of water extraction reported in the journal Nature that demand for water has outpaced supply in the majority of the world’s principal agricultural regions. 

Nevertheless, Renner said, "It should be noted that not all aquifers are being pumped at unsustainable levels----in fact, 80 percent of aquifers worldwide could handle additional water withdrawals." Though Worldwatch warned, "One troubling aspect of groundwater withdrawals is that the world's major agricultural producers (particularly India, China, and the United States) are also the ones responsible for the highest levels of depletion."

Alternative irrigation methods explored

As well as pointing out the danger that high levels of water extraction in some of the world’s key food production areas is beginning to present problems, Worldwatch researchers suggested pumping water from aquifers and redirecting flows for irrigation is impacting on delicate environmental balances. Salinisation occurs when water moves past plant roots to the water table due to inefficient irrigation and drainage systems; as the water table rises, it brings salts to the base of plant roots. Plants take in the water, and the salts are left behind, degrading soil quality and therefore the potential for growth.

Worldwide, the most commonly used irrigation technique is flood irrigation, even though plants often use only about half the amount of water applied in that system. A potentially better alternative to current widely used irrigation methods is drip irrigation, a form of micro-irrigation that waters plants slowly and in small amounts either on the soil surface or directly on roots.

Using these techniques has the potential to reduce water use by as much as 70 percent while increasing output by 20-90 percent. Within the last two decades, the area irrigated using drip and other micro-irrigation methods has increased 6.4-fold, from 1.6 million hectares to over 10.3 million hectares.

With predictions of a global population exceeding 9 billion by 2050, demand for higher agricultural output will put more strain on already fragile water reserves. Even without the effects of climate change, water withdrawals for irrigation will need to rise by 11 percent in the next three decades to meet crop production demands.

Worldwatch said key policy and grassroots moves will need to be made to meet the challenge of reconciling increasing food demands with decreasing water security. The research organisation said efficient systems that produce more food with less water and minimise water waste are necessary.

Worldwatch researchers posited, “Intelligent water management is crucial especially in the face of climate change, which will force the agriculture industry to compete with the environment for water.” 

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