Shading is used in California to prevent evaporation, see for example the well-documented use of "shade balls": https://en.wikipedia.org/wiki/Shade_balls https:

Shading is used in California to prevent evaporation, see for example the well-documented use of "shade balls": https://en.wikipedia.org/wiki/Shade_balls https://en.wikipedia.org/wiki/Shade_balls Their use in Los Angeles was covered in a Veritasium video that was all over YouTube for a while (97M views): https://www.youtube.com/watch?v=uxPdPpi5W4o https://www.youtube.com/watch?v=uxPdPpi5W4o The estimate given when they were added in 2015 – in the middle of a record drought – was that they would prevent the loss of 300 million gallons of water each year.