California’s largest reservoir isn’t Lake Shasta or the Sierra snowpack.

Ten times as much water is stored in cracks and crevices beneath the ground. In a typical year, almost 40 percent of the water used in California comes from underground wells. In dry years, that figure increases to 60 percent or more.

Think of it as drought insurance. But it exists only as long as groundwater basins are regularly replenished.

For far too long, California hasn’t kept up its premiums. Use of surface water has been regulated for more than 100 years, but landowners and public agencies are mostly free to pump as much groundwater as they want.

The results of putting so many straws in the ground include water tables falling, saltwater intrusion, land subsidence and, in some places, the permanent loss of groundwater storage capacity.

After a five-year drought that amplified the effects of a laissez-faire approach to groundwater management, California is beginning to tighten the tap. Slowly.

As Staff Writer Guy Kovner explained in Sunday’s paper, groundwater sustainability agencies are being established for 119 water basins, including three in Sonoma County. These local agencies will be responsible for a regulatory system that’s expected to include registering wells and monitoring and metering the extraction of groundwater. In certain critical areas, pumping could be restricted.

The program includes a series of deadlines, some of them as far off as 2040, to develop and implement groundwater sustainability plans.

California, despite its worldwide reputation as a leader in conservation, is the last state to regulate groundwater use. It took a crisis — a sustained drought that came on the heels of a second, shorter drought — to overcome longstanding opposition, especially in the agricultural community, to monitoring and potentially limiting the use of groundwater.

Even then, deeply rooted suspicion of state regulators resulted in the creation of dozens of local agencies and a timetable for action measured in decades.

It is, nonetheless, a historic change in course. For the first time, groundwater users will be required to balance extraction with replenishment of a resource vital to human life.

More than 3,500 wells went dry during the most recent drought, and some communities still must rely on bottled water for all of their needs. In some parts of the state, the surface is sinking a foot a year, and a recently posted state Department of Water Resources map shows dozens of areas where groundwater levels dropped, sometimes by well over 100 feet, during the drought.

Sonoma County is fortunate, with most of the declines in the 6-12 foot range. But saltwater intrusion is a growing concern in the Sonoma Valley, one of the three local groundwater basins where oversight agencies have been formed.

As surface reservoirs fell to critical levels during the drought, it became clear that California needs more water storage for this generation and future generations. Exponentially more water can be stored underground, and recharging aquifers is an environmentally sound and less expensive option than building more dams. The time to start saving is now.