What materials stop water evaporation?

How Plastic Balls Help Prevent Water Evaporation

Water conservation is becoming increasingly vital as climate pressures grow, and one surprisingly effective solution involves something that looks more like science fiction than practical engineering: black plastic balls floating on reservoirs.

Officially known as “shade balls,” these small, carbon-black spheres are deployed in large numbers across open water surfaces. Their primary purpose? To significantly reduce water evaporation. By forming a continuous floating barrier, they shield the water below from direct sunlight, cutting down on surface evaporation caused by heat and wind.

Their color isn't just for show. The carbon black pigment absorbs and blocks ultraviolet (UV) radiation, which does more than just protect the water—it prevents chemical reactions that could harm public health. In particular, UV light can trigger the formation of bromate, a carcinogenic compound, when sunlight interacts with naturally occurring bromine in treated water. By blocking UV rays, shade balls help maintain water safety without additional chemical treatments.

First used in large-scale applications in California, these balls were originally developed to combat both evaporation and contamination in drinking water reservoirs. A single reservoir can require millions of shade balls to be fully covered, but the investment pays off in saved water and improved quality.

While not a permanent fix for drought or water scarcity, shade balls represent a clever, low-tech intervention that works with existing infrastructure. They’re a reminder that sometimes, the simplest solutions—like a layer of black plastic floating on water—can have a profound impact on conservation and public health.