SACRAMENTO — Thanks to a rainy December, California’s drought-suffering residents met Gov. Jerry Brown’s call to slash water consumption by 20 percent for the first time last month, using 22 percent less water than in December 2013.

But water officials are only cautiously optimistic, noting that in an otherwise dismal year for water, last month’s rains made watering lawns less necessary. California’s overall precipitation was 6 inches, compared to 0.5 inches in December 2013.

“It reinforces what we thought all along, that the extent of outdoor water use is a huge driver of water conservation and water use,” said Felicia Marcus, chairwoman of the State Water Resources Control Board.

The governor called on Californians to use 20 percent less water when he declared a drought emergency last year. The closest they came before to reaching that goal was in August, when water use dropped 11.6 percent.

The new data show some Californians may be getting the message. While seasonal rains are a major factor, per-capita water use has been cut in half in the last six months, with Californians using an average of 67 gallons a day per person in December, compared with 140 gallons per person in June.

Water board officials lauded large water providers that managed to meet the target, such as the city of San Diego, where December water use fell by 24 percent, and the city’s nearly 1.3 million residents cut their average daily water use to 46 gallons per person a day.

“San Diego has really stepped up. They were the largest” among the agencies that exceeded the standard, water board staffer Eric Oppenheimer told the meeting.

That said, California is by no means out of trouble as it enters the fourth year of a drought. The Sierra Nevada snowpack, which provides nearly a third of the state water, is 88 percent below normal. And for the first time in recorded history, there was no measurable rainfall in downtown San Francisco in January, when winter rains usually come.

More than 400 local water departments must report their residential per-capita water monthly water use, numbers publicized by the water board to encourage conservation.

The board imposed mandatory limits on watering lawns and washing cars last summer. The restrictions are set to expire in April, but the board is considering extending and expanding those rules later this month.