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In irrigation, more isn't (necessarily) better
Running a water
deficit at the right time may be
best thing for a crop
February 12, 2009
Counterintuitive as it may seem, the best way to irrigate a crop may be to leave it a bit thirsty. That's according to deficit irrigation proponents, who are trying to reconcile the world's increasing need for food with its simultaneously growing demand for residential and industrial water. "In the next generation, the world is really going to come up short on water," said Marshall English, professor emeritus at Oregon State University who specializes in the theory. "If you don't get a handle on irrigation, you're going to be in trouble." By 2025, global demand for non-irrigation water uses is projected to be 62 percent greater than it was three decades earlier, according to the International Food Policy and Research Institute. "Expanding water supplies can help alleviate water scarcity, but the results show that the most promising avenue is likely to be water management reforms, incentive policies and investments in infrastructure and technology to enhance efficiency in existing uses," according to the institute's water supply outlook. Over-irrigating crops is obviously inefficient for growers, since it induces leaching of nutrients, promotes pathogens and deprives roots of aeration, said English. "If you're drowning them, they're not getting the air they need," he said. Farmers can usually go a step further than simply cutting out wasteful irrigation, though. Beyond a certain threshold, the ratio of applied water to yield begins dropping, said English. In other words, achieving the optimum yield requires a great deal more water than just obtaining an adequate yield. In one study, for instance, reducing irrigation by 70 percent cut wheat yields by half, which would clearly be undesirable, he said. However, decreasing irrigation by 25 percent cut yields by only 7 percent. "The last increment of yield takes a lot of water, and it's generally not worth it," he said. For a wheat grower, the question would be if that 7 percent increase is worth 25 percent more water use. That's not just a consideration for water-poor farmers, either. In light of the electricity lost in pumping the added water and the fertilizer flushed from the soil, it's often more economical for growers to settle for a slightly diminished yield, said English. "Keeping the water from moving below the root zone is key," he said. There's more to deficit irrigation than the amount of water applied, however. Timing is also critical. In some tree fruit and nut crops, for instance, it makes sense to pull back on irrigation during the time of year when trees are geared toward branch and foliage growth, he said. Irrigation can then be focused on the stage of vigorous fruit and nut development before harvest, English said. "You end up with a good crop of nuts. You don't end up with a lot of leaves and shoots you've got to get rid of anyway," he said. Though the concept is easy enough to understand, deciding precisely how much stress a plant can withstand in the field without seriously disrupting production is more challenging, according to a report on deficit irrigation practices from the UN's Food and Agricultural Organization. "Under conditions of scarce water supply and drought, deficit irrigation can lead to greater economic gains than maximizing yields per unit of water for a given crop," the FAO report said. "However, this approach requires precise knowledge of crop response to water as drought tolerance varies considerably by species, cultivar and stage of growth." For that reason, English and specialists are cooperating on an online advisory service for optimum irrigation management that helps farmers in Oregon, Washington, Idaho and California - and eventually elsewhere - make informed deficit irrigation decisions. "This type of irrigation management can't be done on the back of an envelope," he said. The modeling system allows growers to see the effect different water-use strategies will have on their crop production and finances, but it doesn't dictate which outcome is most desirable, English said. "The system does not define best," he said. "It's up to the farmer to see which one he likes best." |
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