My city has a catchment area for water but due to drought conditions it's getting lower and lower in current capacity, and water restrictions have been placed into effect for residents and businesses to conserve the water we have left.

I know of tarps or pool covers are used to combat evaporation of small water sources like pools and ponds. Is there a scaled up version of this for large dams? Or are there different/better technologies available to do this at a large scale?