According to United Nations estimates, by 2025 roughly 1.8 billion people will be living in regions of absolute water scarcity, while two-thirds of the world's population will be living in water-stressed conditions. In Australia, the government considers the problem to lie in part with old irrigation systems. These include furrow irrigation, in which water is syphoned from nearby distribution channels, and hand-shift sprinklers, in which water is pumped through aluminium piping with sprinklers attached.

Newer irrigation systems, on the other hand, use increased pressure to deliver water more efficiently: they include pressurized sprinkler systems, which move mechanically over a crop, and trickle systems, which sit under the earth and deliver water straight to crop roots.

In the hope of tackling increasing water scarcity, the Australian government is encouraging the adoption of these newer irrigation systems. But there's a trade-off; the newer systems are likely to need increased use of diesel and electricity to power water pumps, which means increased greenhouse-gas emissions.

Kate Reardon-Smith and colleagues at the University of Southern Queensland decided to find out how big this trade-off is by assessing five different Australian farms that were upgrading their irrigation systems. To do this, they collected information about the running of the farms in order to calculate the expected water usage, the carbon-dioxide equivalent greenhouse emissions and the financial return.

All the newer irrigation systems used less water. However, the researchers found that it was only by replacing the oldest irrigation systems, such as hand-shift sprinklers, that farmers could also save energy. This is a "win-win" upgrade, they explain. "[Our] results suggest that priority should be given, in implementing on-farm infrastructure investment priority, to replacing inefficient and energy-intensive sprinkler systems," said Reardon-Smith. "Our conclusions regarding tradeoffs are applicable globally."

Despite this clear recommendation, however, the details of the results are more mixed. Although irrigation-related energy emissions went down only for the upgrading of very old systems, the total emissions also went down for the upgrading of some newer systems. Reardon-Smith and colleagues believe that this is because the newest systems allow farmers to perform so-called precision agriculture – such as distributing fertilizer, which has a high carbon footprint, through the irrigation system. So does this mean that there is no drawback to upgrading irrigation systems after all?

Reardon-Smith warns not to be too hasty in drawing that conclusion. "It is important to acknowledge though that our case-study approach essentially raises this as a hypothesis, and that this should be tested through more comprehensive analysis," she said.

The study is published in Environmental Research Letters.