How much of California water comes from groundwater during a drought
How much of California water comes from groundwater during a drought decided the shape of a state law that most residents have never read. Groundwater is water sitting in natural underground layers of rock, sand and clay called aquifers, pulled to the surface through wells. In an average year, groundwater supplies approximately 41% of the water used in California, according to the Department of Water Resources, source 4. In a drought year, when rivers and reservoirs run low, that share climbs to 60% or more, the same state agency states, source 4. About 83% of Californians depend on groundwater for at least part of their supply, according to the Water Education Foundation, source 5.
Source 4 states the drought year figure as up to 60% or more, an open floor rather than a fixed ceiling.
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| Average precipitation year | 41 |
| Drought year | 60 |
A 2014 law splits the deadline into 2 steps
California signed the Sustainable Groundwater Management Act into law in 2014 to stop decades of overdraft, pumping more water out of the ground than rain and snowmelt put back in, source 1. The law does not set 1 deadline, it sets 2. A basin already designated as subject to critical conditions of overdraft had to have a groundwater sustainability plan, a formal local plan to balance pumping with recharge, in place by January 31, 2020, source 2. Every other high or medium priority basin had until January 31, 2022, to put its own plan in place, source 2.
Then Governor Jerry Brown, signing the law in 2014, described why the state built it around hundreds of separate local agencies rather than 1 statewide order, source 1.
groundwater management in California is best accomplished locally
Jerry Brown, at the time serving as Governor of California, signing the Sustainable Groundwater Management Act into law in 2014. Source 1.
Why the real deadline is 2040 or 2042
Neither statute states 2040 or 2042 outright. Section 10727.2 of the California Water Code requires each basin, once its plan is in place, to reach the sustainability goal, pumping no faster than the water underground refills, within 20 years of that plan taking effect, source 3. Add 20 years to the plan deadline for each basin and the result is 2040 for the basins that started the clock in 2020 and 2042 for the basins that started it in 2022, the same 2 years the Water Education Foundation states directly, source 5. That calculation, adding the 20 year window written into the statute to the plan deadlines already in the law, is arithmetic performed by MAOWCE on the text of the law, not a figure either statute prints, and it independently confirms the 2 years rather than only repeating them.
Years are a calculation stacking the plan deadlines in California Water Code section 10720.7 with the 20 year window in section 10727.2, matching the years stated directly by the Water Education Foundation.
Show the numbers
| Basins already critically overdrafted | 2040 |
| Other high and medium priority basins | 2042 |
Hundreds of local agencies, not 1 statewide regulator
California recognizes 515 separate groundwater basins and subbasins, each run by its own local water agency rather than a single statewide regulator, source 4, source 5. That local structure matches the reasoning Brown gave when he signed the law, source 1. The basins can hold more than 850 million acre feet of storage capacity, more than 10 times the capacity of every surface reservoir in California combined, according to the Water Education Foundation, source 5, which is part of why missing a 2040 or 2042 deadline falls on a local agency rather than the state alone.