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In a drought, every drop of water is precious, including those lost to evaporation in the hot summer. But in a massive open reservoir, how do you prevent that from happening?

A solution (hem hem) floated by the Los Angeles Department of Water and Power took real balls.

Over the course of seven years, the department has been gradually covering the reservoir with "shade balls," 4-inch diameter plastic balls that float on the surface of the 175-acre reservoir. This week, the final 20,000 balls were added, bringing the total number to 96 million balls. This measure reduces surface evaporation by 85 percent to 90 percent.

"As the drought continues, it has never been more important to focus on innovative ways to maintain the highest quality drinking water for our 4 million residents," said council member Mitchell Englander.

"In addition to cutting back on the need to chemically treat our water to prevent natural occurrences like algae, these shade balls are a cost-effective way to reduce evaporation each year by nearly 300 million gallons, enough to provide drinking water for 8,100 people for a full year."

The reservoir, which holds a total of 3.3 billion gallons, is protected from more than just evaporation and algae. According to a LADWP release, the balls also help keep the water cool, deter birds and other wildlife, and reduce the amount of wind-blown dust entering the water.

Each ball, which is partially filled with water, costs $0.36 to make, bringing the cost of the project to a total of $34.5 million. This, the department said, is a savings of $250 million compared to alternative water saving techniques, such as splitting the reservoir in two or installing floating covers.

In addition, a $100 million ultraviolet treatment plant is planned to further prevent water loss. Both measures together are expected to save money and time spent on purifying the water.

"Shade balls are a great example of how engineering meets common sense," said LADWP general manager Marcie Edwards. " As we make updates, we are mindful to be wise and practical with our investments. Shade balls are an affordable and effective way to comply with regulations, and helps us continue to deliver the best drinking water to our customers."

California is currently facing one of the most severe droughts on record to affect the state.