SACRAMENTO, Calif. — Californians voted last fall to reduce penalties for certain crimes, a decision that abruptly lowered jail populations and promised more money for jail alternative programs starting next year.

But the state is now poised to spend $500 million for new county jail construction this year, on top of $2 billion spent for new jails over the last eight years.

Critics say the Board of State and Community Corrections should delay Wednesday's vote and give policymakers time to gauge the long-term effects of Proposition 47. The law approved in November treats certain drug and property crimes as misdemeanors instead of felonies, leading to fewer inmates in both state prisons and county jails.

County sheriffs say the jail populations will rebound and the money is needed now to expand program and treatment space in antiquated jails.

It's too soon to tell, argued Steven Meinrath, an advocate with the ACLU of California's Center for Advocacy and Policy.

The sheriffs could be wrong and the lower jail populations could remain, he said. What's more, the nonpartisan Legislative Analyst's Office projects Proposition 47 could save the state up to $200 million, most of which will go to programs intended to eventually reduce crime and keep people out of jail. That, in turn, could change the types of facilities that are needed in years to come.

"We could have a lot of empty jail beds," Meinrath said. "It would make sense for us to hit the pause button and let these things play out for a period of time before we rush to spend another half-billion dollars on more jail construction."

The view is supported by at least one state lawmaker as well as by the organizer who helped persuade 60 percent of the state's voters to back Proposition 47. At least three other advocacy groups also would prefer the money went to another purpose, but are more concerned that the $500 million could go to replacing outdated cells instead of aiding jail rehabilitation programs.

"We are in a moment of change," said Lenore Anderson, who led the initiative campaign as executive director of Californians for Safety and Justice. She said the state should reconsider whether the $500 million could be better spent elsewhere to "give us the biggest bang for the buck in reducing recidivism."

Cory Salzillo, a spokesman for the California State Sheriffs' Association, said the money is needed to provide space in antiquated jails for the kind of rehabilitation programs envisioned under Proposition 47, including those dealing with mental health, substance abuse, jobs and education.

"We're not doing this so we can warehouse new people, we're doing this so sheriffs can meet their desires and obligations to provide better treatment," he said.

However, he said the drop in jail populations is unlikely to continue. Aside from any short-term changes, he pointed to a 2014 report by the nonpartisan Public Policy Institute of California that says many jails are outdated and will eventually need to be expanded simply because of the state's growing population.

One of the study's authors, Magnus Lofstrum, said it is unclear how many jail beds will be freed up because of Proposition 47, but many of those empty beds are likely to be filled with inmates who otherwise would have been released before completing their full sentences.

In the long term, the need for more jail space will be less because of Proposition 47 but will still exist, Lofstrum said. He said there also is a need to provide treatment space at jails that in many cases were built decades ago.

The money was approved by state lawmakers a year ago, before Proposition 47 passed, with just two lawmakers opposed.

One was Sen. Loni Hancock, D-Berkeley, who heads a corrections oversight committee. She said in a statement that there are better ways to expand treatment for criminals, as well as alternatives that could reduce the need for more jail construction.

Proposition 47's effects haven't been part of the discussions because the jail money was approved before the measure passed, said Magi Work, a deputy director at the state board who oversees its construction division.

"I think it's important to recognize that this money is focused on programming space and really replacing old, outdated facilities. That's what the legislation says. We haven't been told to stop yet," she said.