also, why are they black, doesn't that attract more heat?

i remember an article stating that the melting ice at the poles is accelerating global warming because the white of the ice used to reflect the light coming towards the water

The visible color shouldn't matter as much as strong absorbance or reflectance in the UV band. I would guess that they're using a plastic that absorbs UV, or contains a dye that does so.

1: http://www.ncbi.nlm.nih.gov/pubmed/18290477

> And the primary reason for deploying the hollow balls is more to protect the quality of the water rather than to simply stop it evaporating. In the reservoir water, the naturally occurring bromide was mixing with sunlight and chlorine (added to disinfect drinking water) to create dangerous levels of the the carcinogen bromate. Shade balls should stop that harmful chemical reaction from happening at a large scale, and of course by deflecting the Sun's rays, they also keep more of the water in liquid form.

Doesn't that fix the bromate problem at the root?

http://www.thameswater.co.uk/cycles/accessible/water_treatme...

The last stage of the process is to add chlorine to the water.

Bromates are formed many different ways in municipal drinking water. The most common is the reaction of ozone and bromide.

Source: https://en.wikipedia.org/wiki/Bromate

This is what I don't get in this story. How on Earth are plastic balls cheaper than just covering the thing? One would think that a tarp would use less material and be easier to manufacture than the equivalent number of plastic balls.

EDIT: Thanks for all your answers. I guess the heatwaves we have right now in Poland are negatively affecting my imagination :(.

How do you suspend the tarp? What about when it rains on the tarp, increasing its weight substantially? Repair the tarp when sections get damaged? UV resistance over its lifetime?

Using barges/boats and dragging it from shore-to-shore is very slow going, and any suitably durable tarp (i.e. waxed canvas or some plastic) is going to be HEAVY.

The tarp would ideally need to be reflective, like a white roof on modern buildings. The rain issue is the easiest one to solve, since the tarp could be slightly perforated to allow drainage.

I wonder if a permanent structure over the lake would be easier than a tarp? Like literally building a roof.

Either way the balls seem like a solid approach.

"reservoirs exposed to sunlight are now rare. The area's reservoirs -- Silver Lake, Ivanhoe and Elysian -- first registered elevated levels of bromate between June and October 2007.

But state health officials said the dangers were minimal because bromate poses a small cancer risk only after consumed daily over a lifetime."

> And the primary reason for deploying the hollow balls is more to protect the quality of the water rather than to simply stop it evaporating