The precise application allows drip-irrigated crops to be watered more frequently than with traditional sprinkler methods. Yet farmers waste fewer resources because most water is absorbed through transpiration. As a result, many governments have encouraged drip irrigation as a water-conserving technology that can boost crop yields.
But drip irrigation may have a downside, according to a study published in last week's Proceedings of the National Academies of Science. In traditional flood or sprinkler irrigation, 'wasted' water - the water not absorbed by crops - seeps into the ground and recharges the below-surface aquifers used by area farmers. As drip irrigation becomes more common, recharge of groundwater may be less frequent, the study said.
'I think it's very true that drip irrigation and drip irrigation subsidies definitely contribute to food security and increased farm income,' said Frank Ward, the study's co-author and a professor of water resource economics at New Mexico State University. 'The only downside...is that drip irrigation could be using more water.'
If Ward's study is true for areas beyond the study's focus area - the U.S. Rio Grande Basin - agricultural development organizations may need to evaluate whether costly drip irrigation is truly an efficient technique, the study said. This is of particular concern as global food crises and water shortages force many regions to decide between growing food or conserving water.
The Stockholm International Water Institute estimates that feeding the world's undernourished population and the additional 3 billion people expected by 2050 will require 50 percent more water resources than today.
The vast majority of the world's available fresh water - some 70 percent, according to the United Nations Food and Agriculture Organization (FAO) - is withdrawn for agricultural usage. Most crops are not irrigated with the drip method due to higher costs. In the United States and Spain, where the technology is used most, it comprises 6.75 and 2.75 percent of the total irrigated area, respectively, according to the International Commission on Irrigation and Drainage.
But drip irrigation is becoming more popular. More farmers in the United States, the Middle East, and North Africa are turning to the technology, especially to grow permanent tree and orchard crops.
Ward first became aware of drip irrigation's potential to diminish water reserves when several irrigation engineers, farming consultants, and water agency administrators in the Rio Grande Basin informed him of the problem. They told Ward that farmers who adopted the technology were using a larger portion of the region's groundwater. 'They worried [that] farmers would conserve themselves right out of an aquifer,' he said.
After initially dismissing the concerns, Ward and Spanish researcher Manuel Pulido-Velazquez created a model of the basin's hydrology. The model included the total water diverted from streams, applied to crops, evaporated into the air, and returned to the ground.
At maximum levels of drip irrigation subsidies (the U.S. government offers subsidies to offset the technology's higher cost), the analysis concluded that farmers in the irrigation district would apply 40,000 acre-feet (49.3 million cubic meters) less of water per year. Yet due to the loss of 'wasted water' and the additional water demands associated with the higher yields of drip irrigation, the entire district would face a deficit of 36,700 acre-feet (45.3 million cubic meters) of water per year.
'Farmers themselves believe they are using less water,' Ward said. 'They apply less, but because plants unknowingly may use more, they deplete more.'
Not all researchers are convinced. 'It's an absurd proposition,' said Mahbub Alam, an irrigation specialist at Kansas State University. 'Drip is the answer to be able to manage water much better and still have good production and do more with less.'
In addition to questioning the study's model, Alam said that irrigation water obtained directly from a stream is more valuable than any runoff harnessed downstream, due to the mix of chemicals applied to most farmland. 'Assumption that degraded water from return flow has the same value as the pristine water left in the stream by [increased use of drip irrigation] cannot be correct,' he said.
But the efficiency of drip irrigation varies considerably by region and crop. In some Great Plains states, for instance, farmers who use a spray irrigation method known as central pivot often lose much of their water to evaporation. In contrast, if drip irrigation is used, the application often occurs below the surface and very little water evaporates, said Charles Burt, chairman of the Irrigated Training and Research Center at California Polytechnic State University.
'In Kansas and Texas, it is entirely possible, but not guaranteed ...that evapotranspiration under drip is less than with pivot because it depends on the management and design of the sprinklers,' Burt said. With less water vanishing into the air, he explained, crops would lose less water than Ward estimated in his study of New Mexico.
In California, however, Burt supports the study's findings. 'Let's face it. You don't irrigate to save water,' he said. '[With drip irrigation], overall there is higher consumption.'
Yet drip irrigation's significant boost for crop yields cannot be ignored. California has become the world's No. 1 producer of almonds over the past 30 years, due in part to the increased reliance on drip irrigation. Almond farmers have required some 15-20 percent more water, but average yields nearly doubled, said Blake Sanden, an irrigation farm advisor with the University of California Cooperative Extension.
'You cannot produce high-yield crops without a fairly significant input of water,' Sanden said. 'In the end of the day, hungry people will win the game.'
In addition to irrigation technologies, farmers' crop decisions play a significant role in water conservation outcomes. Rice, for example, uses about twice as much water per hectare as wheat, according to the FAO.