The top of the San Vicente Dam in San Diego County. The county’s water agency estimated that it would have sustainable water supplies through 2045, even if dry conditions persisted for years.Credit...Ariana Drehsler for The New York Times.

But not in San Diego County.

On a recent afternoon, boats sped over the silvery surface of San Vicente Reservoir, a key water storage site for the county about 25 miles northeast of downtown San Diego.

It was about as full as usual, cutting a sharp contrast with the desiccated lake beds where state officials have appeared in recent months, pleading with Californians to save water.

The San Diego County Water Authority estimated that it would have sustainable water supplies through 2045, even if dry conditions persisted for years.

Now, with San Diego facing the prospect of orders to use even less water, its relative water plenty has become a case study in the uneven ways that the Western drought is affecting the nation’s most populous state.

Nevertheless, state leaders have said they were not ruling out mandatory restrictions if the voluntary efforts do not save enough water through the fall.

San Diego water officials have said that state-mandated cutbacks this time would be an unfair punishment for residents of a region who have already ponied up for higher water bills and willingly changed their habits to save water.

Francisco Pantaleon, 40, even remembered his mother instructing him to save pool water as a child in Oceanside, a suburb in the northern part of the county: “Don’t come in and out and splash.”.

Their supplies were effectively controlled by water officials in Los Angeles — a contentious relationship that seeded long-running legal battles.

In 1996, the San Diego County Water Authority struck a landmark agreement to buy water from farmers in the Imperial Valley, in California’s southeastern corner, that heralded the beginning of the region’s water divorce from Los Angeles.

In 2010, the authority lined canals in the Imperial Valley with concrete to prevent water from seeping into the earth, and made a deal to take the water saved by the process — some 26 billion gallons a year?

The authority finished raising the San Vicente Dam in 2014, adding more capacity to San Vicente Reservoir in the biggest water storage increase in the county’s history.

Across the county, restrictions and conservation pushes have led per capita water use to fall by half over the past three decades.

San Diego has provided a road map for others now scrambling for water, said Toni Atkins, who is the president pro tem of the California Senate and previously served on the San Diego City Council.

Although San Diego has come up with more ways to get water locally, it still gets most of its water from outside the county, including from the shrinking Colorado River.