Results from a study performed in response to the California drought show that certain pool covers can significantly reduce water loss due to evaporation, when they are in place.

The National Pool Industry Research Center compared the water loss in pools covered by various kinds of products, including solid track covers, foam covers, bubble covers, solar disks and liquid evaporation suppressants. The study was performed on test pools at NPIRC's facility at California Polytechnic State University, San Luis Obispo.

The three highest performing covers fell within 1 percentage point of each other: Foam covers showed a 95.9 percent efficiency; bubble covers clocked in at 94.9 percent; and solid track covers at 93.9 percent In the mid-range were solar disks, at 50.1 percent. The liquid evaporation suppressants rated lowest, between 14.4 and 15.8 percent.

The authors of the study report offered a caveat: Once installed, the covers remained in place throughout the study, except during cleaning and water level measurements. "In reality, the covers will have to be removed, possibly for extended hours, when the pools are occupied," the study said. "This suggests that the efficiencies reported here for the solid covers should be considered as maximum possible efficiencies."

They also reported unusually high winds and storms during the liquid evaporation suppressant tests, which may have had a negative effect on the products' performance. However, a counterweight was presented by the lack of swimmers, which removed normal water disturbance from the equation.