Huh, I Had This Idea a While Back

Source of picture here.The idea, and I do not think that it is particularly original or merits a patent is floating solar panels above reservoirs and hydroelectric lakes to minimize evaporation losses.

It's a neat idea, though I would use translucent photovoltaic cells to only allow light light in the 400-700 nm range to pass through.

If you block all light, you end up with dead lakes because there is no photosynthesis, though it's unlikely that one would be able to cover the whole surface, as fluctuations in depth would damage some of the panels.

Still, this does two good things, increases water supplies and reduces carbon footprint:

Sun-baked Morocco, grappling with its worst drought in decades, has launched a pilot project aimed at slowing water evaporation while simultaneously generating green energy using floating solar panels.

At a major reservoir near the northern city of Tangier, thousands of so-called “floatovoltaic” panels protect the water’s surface from the blazing sun and absorb its light to generate electricity.

………

Water ministry official Yassine Wahbi said the Tangier reservoir loses around 3,000 cubic metres (106,000 cubic feet) a day to evaporation, but that figure more than doubles in the hot summer months.

The floating photovoltaic panels can help cut evaporation by about 30 per cent, he said. 

It's a win-win, unless, of course, you are Dick Cheney.