Water officials approved a small hike in water bills throughout Sonoma County on Tuesday, despite exceptional conservation measures enacted during the drought and a record amount of rain this season.

Sonoma County supervisors, who also serve as directors of the county Water Agency, unanimously approved regional wholesale rate increases that will hike rates by 5 percent or more in the next fiscal year for the contractors who tap into the Santa Rosa, Petaluma and Sonoma water systems.

The increases come after several months of drought-busting rainfall filled Lake Sonoma and Lake Mendocino, the region’s main reservoirs, beyond their seasonal supply levels. Higher rates are needed because as water use dropped 27 percent in recent years thanks to conservation efforts during the drought, much of the agency’s operation and maintenance costs remained fixed, officials said.

“Even though we have more water to deliver, the deliveries are down because we’ve asked people to conserve over the last six years, and they’ve done a wonderful job,” said Supervisor Shirlee Zane, the board chairwoman.

The agency serves more than 600,000 residents of Sonoma and Marin counties, drawing from the two reservoirs and selling the water to local cities and water districts.

Prior to Tuesday’s board vote, those contractors had already agreed to rate increases of 4.98 percent for the Santa Rosa and Petaluma water systems and 5.58 percent for the Sonoma system, which has a higher rate to fund certain infrastructure improvements.

The increases should translate into an additional 90 cents per month for an average Santa Rosa home, according to water officials.

Residents remain encouraged to conserve water, since efforts to cut back already have avoided or at least delayed the county’s need for costly expansion projects, said Michael Gossman, the agency’s division manager for administration and finance. Also, the county’s steady rate increases in recent years have allowed the region to avoid some of the double-digit hikes seen in other areas, Gossman said.

“Those are rate increases that could be shocking to anybody’s bill,” he said. “We work hard with our contractors to avoid that.”

The rates were considered as part of the agency’s $39.2 million water transmission budget, which is down about $1.7 million from the year before.

Cities and water districts also have their own additional costs on top of the wholesale rates charged by the Water Agency, so the exact amounts paid by homes and businesses in different areas will vary.

You can reach Staff Writer J.D. Morris at 707-521-5337 or jd.morris@pressdemocrat.com. On Twitter @thejdmorris.