Battling a record dry spell that has left reservoirs badly drained, the Las Virgenes County water district on the outskirts of Los Angeles has decided to fill its reservoir with hundreds of black plastic balls. The balls sit on top of the water and block the sunlight from hitting it, helping to reduce water evaporation as well as natural contamination and certain chemical reactions that could cause algae blooms in the reservoirs.

"The balls, really they're intended for three reasons: they help to cut down on evaporation, that's helped us save water, and then it also helps to prevent algae from growing in the reservoir, so it improves our water quality and then finally as a side benefit, it helps keep the birds off the surface and birds do their thing and that causes a problem," said the water district's general manager David Pedersen on 12 August.

Las Virgenes County is not the only water district in southern California to use the shade balls. On 11 August, Los Angeles mayor Eric Garcetti helped release 20,000 black balls into the Los Angeles reservoir. Most of the water in Las Virgenes is imported from northern California. Pedersen said the shade balls help cut down on the amount of water the district needs to bring in from outside sources.

"For every gallon of recycled water, it reduces our need by one gallon of potable water," he said.

The shade balls cost 36 cents (23p) each and are expected to save some 300m gallons of water annually. Associate Engineer Eric Schlageter said the life expectancy of the balls was approximately 20 years.

"The HDPE (High-density polyethylene) material itself is inhibitive to UV so these hold up for an extremely long amount of time and we're very happy with the product," he said.

California is in the fourth year of a catastrophic drought that has led the state to issue a series of steps to reduce water consumption, including the first-ever mandatory cutbacks in urban water use.