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Drip irrigation may use more water, not less
Drip irrigation supplies plants with only as much water as they need, and none is wasted feeding weeds, leaching into the soil or evaporating into the air. So it must be good, right?
Well, paradoxically, a study published in the Proceedings of the National Academy of Sciences of the USA has found that drip irrigation could well lead to overuse of limited water resources (PNAS 105: 18215–18220; DOI: 10.1073/pnas.0805554105).
Authors Frank Ward and Manuel Pulido-Velazquez studied water use in the dry Upper Rio Grande Basin in the USA and Mexico, and looked at how changes in water use policies would affect actual water use. Their computer modelling showed that subsidies to growers who switched to drip irrigation would encourage the uptake of drip irrigation, as might be expected. But this would have two unintended consequences.
The first consequence would be that the higher productivity (more crop per drop) at less cost (on account of subsidy) would encourage growers to plant more area and so use up to 20% more water than before. The second would be that the hydrologic cycle would be interrupted, as all water applied by drip irrigation is lost to transpiration, and none is returned to the originating basin in groundwater flow or surface flow.
However, further studies taking into account different factors may yet prove otherwise. Nevertheless, water is a scarce and valuable resource, especially here in Australia, and a solution may lie in simultaneously maximising yield while minimising water use, rather than trying to use all the water available.