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Plastic balls have been added to a reservoir in drought stricken California in order to reduce evaporation. (Link to news story here.)

Why are the balls colored black instead of white? I would think that white would decrease heating and therefore evaporation.

Edit: Apparently the balls are there to block sunlight from converting bromide to carcinogenic bromate.

Youtube video explaining the purpose of the balls

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    $\begingroup$ Cynical me thinks the reason might not be scientific, but perhaps someone here can dissuade me from such a thought. $\endgroup$ – user7652 Aug 13 '15 at 1:47
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    $\begingroup$ It's all about blocking the UV wavelengths. Those photons carry more energy than visible wavelength photons and will consequently cause more rapid evaporation. Black or white plastic balls, we can't really know what their UV absorption\reflection properties are without an actual spectrum of the colorant. I'm guessing that the black colorant is cheap and permits less UV to pass through (ideally reflects UV better) than other colorants. $\endgroup$ – ron Aug 13 '15 at 2:41
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    $\begingroup$ I'm thinking the balls may have been sold by the lowest bidder or, like Ron says, it's the UV issue. $\endgroup$ – Brinn Belyea Aug 13 '15 at 2:54
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    $\begingroup$ The primary means of lowering evaporation is just lowering air-exposed surface area. I think the black color may have to do with not wanting to change the thermal properties of the water body, but I would love to know the real reason. $\endgroup$ – Curt F. Aug 13 '15 at 4:50
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    $\begingroup$ Stumbled across a quote which suggests that the balls were not originally made with the purpose of reducing evaporation: "The reason they were originally referred to as shade balls is it was an issue of mitigating bromide in algae bloom..." $\endgroup$ – chipbuster Aug 13 '15 at 6:29
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This piece in Bloomberg News is about Sydney Chase, the founder of XavierC, the company which makes the "shade balls" in question (there are two other suppliers, Orange Products, Inc. and Artisan Screen Printing).

From the section of the XavierC site on how they work:

Two main causes of water evaporation in reservoirs are temperature and surface area. Our hollow plastic conservation balls address these two issues effectively and inexpensively. The black conservation balls provide a floating cover on the reservoir taking up approximately 90% of the surface area. Simultaneously the balls prevent UV rays reaching the water and therefore reducing algae.

From the above-mentioned Bloomberg piece:

The Los Angeles Department of Water and Power has now dumped 96 million balls into local reservoirs to reduce evaporation and block sunlight from encouraging algae growth and toxic chemical reactions. The balls are coated with a chemical that blocks ultraviolet light and helps the spheres last as long as 25 years. Las Virgenes, north of L.A., now uses shade balls, too.

From the Los Angeles Times story on August 12, 2015:

What are they made of?

The 4-inch-diameter balls are made from high-density polyethylene, which is the same material you would find in a one-gallon milk jug. This plastic is approved to come into contact with drinking water.

Why are they black?

Carbon black was added to the plastic to stabilize the balls in UV light from the sun.

and, from another LA Times piece about the "shade balls" written in 2008:

The balls, which cost 40 cents each, are made of polyethylene. The coating contains carbon. Black is the only color strong enough to deflect ultraviolet rays, said Paul Sachdev, president of Orange Products.

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    $\begingroup$ So the carbon black protects the ball itself from the UV, right? $\endgroup$ – Dmitry Grigoryev Aug 13 '15 at 13:51
  • $\begingroup$ @DmitryGrigoryev - It appears that way. And since the balls cover the surface of the reservoir, that means that the UV radiation making it to the underlying water is attenuated, too. If I had to guess, I'd say the design of these was pretty well thought-out to optimize both the lifespan of the item as well as its function. $\endgroup$ – Todd Minehardt Aug 13 '15 at 13:56

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