https://phys.org/news/2018-07-balls-reservoirs.html

Amid California's latest drought, which lasted from 2011-2017, 96 million 'shade balls' were deployed on the Los Angeles reservoir. These floating, black plastic balls cover the water surface to prevent evaporation.

However, a new study published today in Nature Sustainability shows that producing the balls probably used more water elsewhere than was saved during their deployment—which could have knock-on environmental impacts.