The black plastic shade balls used to reduce water evaporation from artificial lakes can achieve conservation only if they remain in use for at least 0.2-2.5 years, depending on the wall thickness of the balls, reports a paper published online this week in Nature Sustainability. This finding casts a shadow on the real sustainability of quick and easy water-management technologies.

Evaporative loss from open-air reservoirs amounts to 25% of the global water consumption from agriculture, industries and households - a problem expected to worsen under climate change. Water managers are therefore forced to develop new conservation solutions. To minimize evaporative water loss during California’s recent severe drought, over 96 million hollow, high-density polyethylene balls - each with a diameter of around 10 cm - were released into the Ivanhoe reservoir in Los Angeles in August 2015.

Erfan Haghighi and colleagues assessed the sustainability of this solution using a water footprint approach, which includes a consideration of the total volume of water consumed or polluted over the entire supply chain of the polyethylene balls. They found that although the balls save about 1.15 million cubic metres of water each year, their manufacture costs around 0.25-2.9 million cubic metres of water, depending on the wall thickness of the balls used. Each drop of water saved by the shade balls in Los Angeles might thus correspond to more than one drop being consumed in other parts of the US or the globe - unless the balls are kept on the reservoir for a sufficient amount of time. The authors conclude that quick technological solutions to water management, such as shade balls, need integrated sustainability analyses to assess their overall viability.