California may be experiencing the worst drought in 1,200 years. Covering the surface of a 12-billion-liter reservoirs with black plastic balls will block sunlight, preventing more than 1 billion liters of water from evaporation.They will also avoid the production of dangerous chemicals than can form as a result of reactions triggered by sunlight in chlorinated water. The state has now nearly 100 million “shade balls” ready to tip into its reservoirs.