The mandatory reduction includes residents and businesses. Commercial water customers are required to reduce irrigation by 30 percent for landscaping. It also bans the washing of cars or boats without a nozzle, and prohibits the washing of outdoor surfaces unless necessary for health and safety.
Roseville says outdoor irrigation accounts for more than half of a typical home's water use, and says reducing that is the quickest way to reach the 20 percent goal.
The city gets most of its water from Folsom Lake, which is extremely low due to the drought. Earlier this year, the city turned on its groundwater wells in order to conserve water from the lake, but turned them off this month after more rain and snow fell in Northern California. The city of Sacramento has also implemented a mandatory 20 percent reduction.
While Roseville is calling the reduction "mandatory," water customers will not be hit with a surcharge or penalty if they do not meet that target.
Roseville, the largest city in Placer County, operates its own utility, which serves most of the 127,000 residents. Officials backtracked on a February proposal that would have used surcharges to compel reductions
Water conservationists criticized Roseville's approach, saying that without an enforcement or penalty structure, customers were unlikely to heed the call. But city leaders expect that education and outreach will translate into cutbacks.
The Associated Press contributed to this report.
California Communities Respond To Drought
Four consecutive years of drought, millions of dead trees and summer heat, are all factors as thousands of firefighters work to control wildfires in California.
Not much change is expected in drought conditions in California during the summer "dry season" but wildfire danger is increasing, with 66 million dead trees in the Sierra Nevada adding potential fuel.
The U.S. Forest Service says 66 million trees are dead in the Sierra Nevada after four consecutive years of drought in California and a bark beetle infestation.
California is in the fifth consecutive year of drought and water providers continue to urge voluntary conservation, as mandatory statewide rules have ended. Sacramento-area residents reduced their water use by 31 percent in May.
As thousands of federal and state firefighters work to contain wildfires in California and other western states, record-setting heat has prompted warnings from the National Weather Service for parts of California, Nevada and Arizona.