"We found that crop water productivity varies a lot, even between places that have about the same climate," Kate Brauman of the University of Minnesota, US, told environmentalresearchweb. "This means that some areas have a 'water gap' – they could be getting a lot more crop per drop. Governments and aid organizations around the world can use this information to help decide which crops to focus on, in which places, to best boost water sustainability and food security."

The team calculated the improvements in water use that could result if crop water productivity in precipitation-limited areas was raised to the 20th percentile for all crops. In such croplands around 40% of water consumption currently goes to production of just 20% of food calories.

Rain-fed croplands could feed an extra 110 million people each year without consuming more water, Brauman and colleagues from the University of Bonn, Germany, and University of Minnesota found. Irrigated areas, meanwhile, could save enough water to meet the annual domestic water demands of nearly 1.4 billion people without decreasing food production.

In many cases low water productivity is likely to be a function of low yield, the researchers believe. Techniques such as terracing, nutrient addition or furrowing could improve crop yields even when limitations are due to soil or slopes, they said. Measures such as reducing wind-driven erosion, changing planting dates, rainwater harvesting and local water storage, drip irrigation, or changing tilling practices to reduce evaporation could also help.

"The on-the-ground changes farmers make – things like using more fertilizer to increase yields – will differ from place to place," said Brauman. "What a global study like this shows is that those changes could add up to something big."

To come up with their findings, the researchers analysed a combination of modelled and empirical data for crop production, water use and crop water productivity for wheat, maize, rice, barley, rye, millet, sorghum, soybean, sunflower, potato, cassava, sugarcane, sugar beet, oil palm, rapeseed and groundnut. They defined crop water productivity as edible kilocalories produced per litre of evapotranspiration, which was evaluated separately for rain-fed and irrigated crops.

There was a large range in crop water productivity; in large parts of sub-Saharan Africa the value for maize was less than one kcal per litre, compared with values of more than six for the US corn belt and northern China.

"We chose to focus on places with really low water productivity that were in precipitation-limited regions – low water productivity cropland because it should be easier to improve really poor performers than really high performers, and precipitation-limited regions because they plausibly could really benefit from using less water to produce food," said Brauman.

Raising crop water productivity to the 20th percentile in precipitation-limited regions could increase the total rain-fed food production in Africa by more than 10% without exploiting additional cropland, according to the team. And similar improvements in water productivity on irrigated cropland could reduce total water consumption by around 8–15% in precipitation-limited regions of Africa, Asia, Europe and South America.

So what's next? "Efficiency is obviously most important in places where resources are scarce, so I'm excited to be working now on integrating this research on crop per drop with research about water stress," said Brauman.

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

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