Water agencies say those facts, coupled with projected population increases, mean that it is time for the state to get serious about building new dams.
"Failure to consider these potential changes and develop the storage to address it will all but guarantee a future that swings from extreme floods to droughts on a regular basis," the Association of California Water Agencies reported in a "blueprint" for statewide water planning in May.
That report, "No Time to Waste," was one of several this year to look at where California's water policy ought to go between now and 2030, when the state's population is expected to soar by 11 million, to nearly 50 million.
By then, global warming also could begin taking a toll: Scientists have predicted dramatic reductions in the Sierra snowpack 100 years from now.
If those predictions prove accurate, the amount of precipitation might be the same, but it would shift from snow to rain. Water would then wash down Sierra rivers faster and in shorter periods of time.
For water users, reservoirs will be all the more important to catch that runoff.
The ACWA report called for new surface and ground water storage, increased pumping capacity out of the Delta, relaxation of Endangered Species Act requirements for development of new water infrastructure and a comprehensive re-evaluation of the Delta's viability, among other things.
"California has been well served by its water supply system, but it has been more than 30 years since improvements were made on the scale required to keep pace with the state's growing population and changing water needs," the report said.
The Oakland-based Pacific Institute, an environmental group, had a different view.
It said that water use could be cut by 20 percent and still sustain continued population and economic growth by improving water use efficiency. That means setting new water-efficiency standards for household appliances and implementing irrigation methods that use less water.
That report recommended phasing out water subsidies to San Joaquin Valley farmers and said those farmers could simply irrigate more efficiently without changing crop patterns.
"Experience has shown that efforts to improve water-use efficiency are consistently successful and cost-effective," said the Pacific Institute's report, "California Water 2030: An Efficient Future."
"If California put as much time, money and effort into water-efficiency programs as has gone into traditional water supply development, a high-efficiency future could be readily achieved -- with benefits to our economy, environment and health."
Meanwhile, the state Department of Water Resources triennial review of state water policy recommends implementing CalFed programs to increase water supplies and protect the ecosystem, sustaining the Delta, and increasing focus on regional initiatives in which individual water agencies band together to increase water supply and reduce water demand, among other steps.
For example, the Bay Area Water Agencies Coalition includes 29 cities, water districts and other agencies in the region and was formed in 2002 to promote water conservation and cooperate in other ways.