But researchers are warning that, despite making water savings, these modern irrigation systems are increasing the food-production system's contribution to climate change.

"These pressurized irrigation systems use more energy and therefore have higher carbon emissions than traditional gravity-operated systems," said Andre Daccache, from Cranfield University, UK. "We have carried out the first large-scale assessment of agricultural water demand in the Mediterranean region and estimated the carbon-dioxide emissions associated with water abstraction for irrigation."

Daccache and his colleagues developed an integrated assessment, combining a gridded water-balance model with a geodatabase and geographic information system (GIS). They used it to assess the water demand and energy footprint of irrigated production in the Mediterranean region, and linked these outputs with crop-yield and water-resource data to estimate water and energy productivity.

The researchers calculated that 61 km3 of water is needed to irrigate the major crops grown in the Mediterranean region. While irrigation modernization could save around 8 km3 of water per year, it would increase the amount of carbon dioxide produced by 135%, they found.

The calculations also revealed that Spain, which is becoming more reliant on groundwater and on pressurized systems to irrigate crops, uses three times the energy of Egypt, a country that almost exclusively employs water from the River Nile and is still largely dependent on traditional gravity-fed systems.

"While Spain is saving water with these new irrigation systems, it is clear that its carbon-dioxide emissions will be higher," said Daccache. "We were expecting to see a difference in energy use between countries but we were surprised by the size of the difference. The problem is that Spain with a dry climate and scarce surface-water resources does not have many options when it comes to irrigation technologies."

The researchers also found that, for each tonne produced, sunflowers have the highest carbon-dioxide emission from irrigated production (73 kg CO2e) followed by cotton (60 kg CO2e) and olives (57 kg CO2e). Citrus has very high water productivity, and is the crop with the lowest carbon emissions per tonne. But these values vary significantly between individual countries, depending on local climate and reported levels of crop productivity.

"Other studies have looked at water usage in the area, but this is the first time that water usage, irrigation technology, crop yields and carbon-dioxide production have been integrated in this way," said Daccache. "Our work highlights vulnerable areas, and we hope it will help policy makers to understand the implications of trade-offs between strategies to save water, reduce carbon-dioxide emissions and/or intensify food production."

The team reported the results in Environmental Research Letters (ERL).

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