Shade Balls

In recent years, California has been experimenting with "shade balls"  to try to reduce evaporation and protect it from wildlife. Just recently however, the city of Los Angeles dumped 20,000 of the little black balls into their main reservoir. These balls are set to save the city 300 million gallons of each year, which is only a drop in the bucket compared to 13.6 billion gallons of water Los Angeles used last year. The shade balls themselves only costed $34.5 million, but actually saved the city a lot of money. The EPA had mandated earlier that the city cover its' reservoir, which would cost around $300 million, but thanks to shade balls this wasn't necessary. 

http://www.washingtonpost.com/news/morning-mix/wp/2015/08/12/plastic-shade-balls-the-hypnotizing-tool-california-is-using-to-save-water/