This page last updated on 4/1/2020.
Groundwater in California has received a lot of attention lately, and for good reason: We use a lot of it. So much so, California’s groundwater is use is the largest in the United States, with approximately 16% of the nation’s groundwater supplies being extracted from the state’s aquifers.
Groundwater is often described as a ‘hidden resource’; because it cannot be directly seen, it is likely most Californians do not give much thought to the economic benefits that the vast groundwater supply has added to the State, nor consider it’s value. But it is unlikely that California could have achieved its present status as the largest food and agricultural economy in the nation and eighth largest overall economy in the world without the state’s abundant groundwater resources.
These economic benefits have not come without many costs: the diminished quantity and degraded quality of groundwater resources, the infrastructure damaged by land subsidence, the decline in ecosystem services provided by the interaction of groundwater and surface water, and the increased energy required to bring the groundwater to the surface.
Many of the state’s groundwater basins are being managed sustainably – meaning that withdrawals do not exceed the amount replenished by man or by nature; but in some basins, especially those underlying major agricultural regions in the southern Central Valley and Central Coast, groundwater withdrawal far exceeds the amount that is recharged, causing conditions of overdraft and threatening this vital resource.
Groundwater is a critical important part of California’s supply, accounting on average for 40% of total annual agricultural and urban water uses statewide in an average year, and up to 65% or more in drought years. About three-quarters of the state’s residents – about 30 million people – depend on groundwater for at least a portion of their water supply; for 6 million residents, it is their only supply.
The Department of Water Resources estimates that on average, 16.5 million acre-feet of groundwater is extracted per year, with 39% going towards agriculture, 41% supporting urban dwellers, and 18% being used for managed wetlands. The USGS estimated that in 2010, groundwater withdrawals in California were equal to about 13 billion gallons per day. Nearly three-quarters of the state’s groundwater pumping occurs in the Central Valley, with the Tulare Lake region being the largest user of groundwater.
The state’s groundwater storage capacity is more than 10 times that of all its surface reservoirs, although not all that water is accessible or of acceptable quality.
However, the state’s groundwater resources are not evenly distributed. Whether or not there is groundwater in a particular area depends on the geology of the underlying soils. Some of the state’s largest cities, such as San Diego and San Francisco, have very little groundwater resources available, while others, such as Bakersfield, rely solely on groundwater to support their population. The Central Coast region is the most dependent on groundwater, with 80% of its supplies coming from groundwater aquifers.
DWR estimates that on average, 2 MAF is withdrawn from the state’s aquifers per year more than what is being recharged, and much more so during periods of drought. This is nothing new; scientists estimate that since California’s development in the late 1800s, the state’s groundwater reserves have been reduced by 125 MAF, or 4.5 times the capacity of Lake Mead. Most of this groundwater depletion has occurred in the San Joaquin Valley.
Based on estimates by DWR for the period between 2005 and 2010, with respect California’s total water supply, groundwater supplies contributed 39% of the supply to meet the agricultural uses, 41% of the supply to meet urban uses, and 18% of the supply to meet managed wetland uses. During that same time period, with respect to total annual groundwater extraction, about 76% of the groundwater extracted went to meet agricultural uses, 22% to meet urban uses, and 2% for managed wetlands uses.
During the historic drought from 2012 to 2016, estimates from NASA’s Gravity Recovery and Climate Experiment (GRACE) found that approximately 40 km3 or over 32 million acre-feet of water was pumped from Central Valley aquifers over the four year period – which is equivalent to pumping enough water to fill Shasta Dam twice – for each year of the drought. The substantial withdrawal of groundwater from the aquifers caused shallow wells to go dry, exacerbated water quality problems, and increased the rate of subsidence.
Groundwater is held in geologic formations known as aquifers. An aquifer is an underground layer of permeable rock, sediment (usually sand or gravel), or soil that stores water in the spaces and voids between the granules. California’s groundwater supplies come from two types of aquifers: fractured rock aquifers and alluvial aquifers.
Fractured-rock aquifers store groundwater in fractures or other void spaces within the rocks; these types of aquifers are typically found in the mountain and foothill areas of the coastal ranges and Sierra Nevada, as well as in the volcanics of the Modoc Plateau. Wells served by these fractured rock formations typically have much smaller yields than an alluvial groundwater basin with half of all hard rock wells yielding only enough for individual domestic supplies. 
In addition, the limited storage capacity of these fractured rock systems can mean that supplies can vary dramatically over the summer and during dry years. (For more on fractured-rock aquifers, click here.)
Alluvial aquifers are composed of sand, gravel, and other fine-grained sediments that store the water in the voids and spaces among the sediments. The Department of Water Resources has identified 515 alluvial groundwater basins across the state which cover 62,000 square miles or about 42% of the state’s area. These basins are highly variable in their water yields, geologic origins, physical and hydrogeological characteristics, production properties, and water quality.
The most prodigious of the state’s groundwater basins are found in the Central Valley where a structural trough forms an aquifer system extending from north of Red Bluff to south of Bakersfield, about 400 miles long and from 20 to 70 miles wide. The aquifer system is comprised primarily of sand, gravel, and clay deposits with most of the freshwater found at depths of less than 2,500 feet. The northern portion of the valley is drained by the Sacramento River, the middle portion is drained by the San Joaquin River, while drainage in the Tulare Lake basin is completely internal. Nearly three-quarters of the state’s groundwater use and 83% of the state’s agricultural groundwater use is extracted from the Central Valley aquifer system.
Coastal aquifers include a number of basins in the Bay Area, Central Coast, and Southern California regions; seawater intrusion is a common problem for coastal aquifers. Other alluvial aquifers include basins in the Eastern Sierra, and the Mojave and Colorado deserts.
For more information …
- General facts and concepts about groundwater, by the USGS
- Basic concepts of groundwater hydrology, by Thomas Harter, UC Division of Agriculture and Natural Resources (6 pages)
Bulletin 118, California’s groundwater
The state’s groundwater basins are described in Bulletin 118, California’s Groundwater, which provides groundwater basin maps and descriptions for the state’s 515 alluvial groundwater basins. Information includes geology of the basin, groundwater quality and quantity, and current groundwater practices.
The bulletin also includes guidance and tools to assist local agencies in effectively managing groundwater, such as required and recommended components of effective groundwater management plans and a model ordinance that can be used by local governments, as well as a chapter that describes the roles of state and federal agencies in protecting groundwater quantity and quality.
Last published in 2003, an interim report was issued in 2016, with a full updated report due in 2020.
- Bulletin 118, 2016 update, by Department of Water Resources
- Bulletin 118, 2003 version, by Department of Water Resources
Groundwater can be replenished in many ways: by precipitation, by water that leaks into aquifers from surface waters, by intentional recharge efforts, by irrigation water applied in excess of what crops use, or even inadvertently from leaky pipelines and canals.
There is a balance that occurs when the amount pumped out approximates the amount of recharge; however, if more water is pumped out than is recharged, groundwater levels will fall and the groundwater basin can become overdrafted.
Groundwater overdraft can have numerous impacts:
One of the first and most noticeable impacts of excessive groundwater pumping is the lowering of the water table. As the depth to water increases, the water must be lifted higher to reach the land surface, requiring more energy to drive the pump and increasing energy costs. At the same time, as water levels decline, the rate of water the well can yield may decline as well. If groundwater levels continue to decline, the shallower wells can go dry and might have to be deepened, or even new wells drilled.
Groundwater overdraft can create new water quality problems or make existing groundwater pollution worse. As aquifer levels decline from chronic overdraft, natural and man-made pollutants can concentrate in the remaining groundwater, making it unsafe for irrigation or drinking without costly treatment. In some cases, wells must be shut down.
Groundwater quality can also be impacted as continued pumping may cause polluted groundwater or seawater to migrate or be drawn into areas that would otherwise not be impacted.
For more information …
- The Groundwater Ambient Monitoring and Assessment (GAMA) Program is California’s comprehensive groundwater quality monitoring program that is a colalboration between the State Water Board, the USGS, and others. Click here to learn more.
- Click here to learn more about groundwater quality at the Groundwater Exchange.
Seawater intrusion is the movement of saline water into freshwater aquifers. It occurs naturally to some degree in most coastal aquifers, due to the hydraulic connection between groundwater and seawater; however, seawater intrusion is most often is caused by groundwater pumping from coastal wells causing the groundwater level to drop, reducing water pressure, and slowing the saltwater to flow further inland.
In the 1950s, the post World War II population boom in Southern California led to groundwater overdraft and seawater intrusion. To combat the problem, a seawater barrier was constructed by positioning a series of injection wells between the ocean and the groundwater aquifer that inject water into the aquifer to ensure the water level near the ocean stays high enough to keep the seawater from seeping in.
Along the Central Coast, seawater intrusion remains an issue, with seawater intruding as far as five miles inland in the Salinas Valley, and three miles inland in the Pajaro Valley.
For more information …
When large amounts of groundwater are withdrawn, the aquifer can compact, 
causing the overlying land to sink because the water in the aquifer was part of the structure responsible for holding the ground up. This is known as land subsidence.
Subsidence impacts can occur over a wide area, damaging roads, canals, buildings, pipelines, and other infrastructure. In some aquifers, this subsidence is permanent, forever reducing the storage capacity of the aquifer.
Subsidence in the San Joaquin Valley has been called the single largest alteration of the land surface attributable to humankind with subsidence in some areas reaching nearly 30 feet. In the current drought, a NASA report measured land subsidence rates in some areas of the San Joaquin Valley at as much as 2 inches per month.
Irrigation canals and aqueducts typically rely on gravity to deliver water, but when the land sinks, the ability to move the water can become impaired. This has particularly affected the San Joaquin Valley where land subsidence has significantly impacted the Friant-Kern Canal, reducing it’s ability to deliver water to some of their contractors by up to 60%; the California Aqueduct has been similarly affected. Fixing the canals will cost hundreds of millions of dollars.
The San Joaquin Valley is not the only region in California to experience subsidence; subsidence has occurred to a lesser extent in the Sacramento Valley, the Santa Clara Valley, the Antelope Valley and other southern desert groundwater basins. A 2014 report by the California Water Foundation found that land subsidence has cost billions of dollars in damage to canals, roads, bridges, building foundations, pipelines, and other infrastructure, as well as altering the capacity of flood bypasses and diminishing the effectiveness of levees.
For more information …
Traditionally, management of water resources has focused on surface water or groundwater as if they were separate entities. Surface water commonly is hydraulically connected to groundwater, but these interactions are difficult to observe, hard to measure, and therefore commonly have been ignored in water-management considerations and policies.
However, surface water and groundwater systems are connected in most landscapes. Surface waters can gain water from the inflow of groundwater through the streambed, lose water to groundwater by outflow through the streambed, or they can do both, depending upon the situation. It is groundwater that generally keeps streams flowing between precipitation events or after snowmelt.
For more information …
California depends on groundwater for a major portion of its annual water supply, and sustainable groundwater management is essential to a reliable and resilient water system. In recognition of this, Governor Brown signed a three-bill legislative package of laws collectively known as the Sustainable Groundwater Management Act (SGMA) in September of 2014 that set in motion a plan to sustainably manage the state’s groundwater basins.
The legislation defines roles for local agencies, the Department of Water Resources, and the State Water Resources Control Board.
The Sustainable Groundwater Management Act requires local groundwater pumpers to form Groundwater Sustainability Agencies and to develop Groundwater Sustainability Plans to manage their groundwater basins. All groundwater basins designated as high and medium priority must be managed by a
groundwater sustainability plan by 2022, or by 2020 if the groundwater basin has been determined to be in conditions of critical overdraft. A GSA may adopt a single plan to cover an entire basin or a number of plans can be combined through a coordination agreement.
Groundwater Sustainability Plans must identify when and where groundwater conditions are causing undesirable results; the specific projects and actions that the GSA will undertake to prevent those impacts; and milestones to track plan progress. Plans must include measurable objectives, as well as describe how the GSAs will groundwater conditions in the basin.
In order to effectively implement the plans, SGMA provides new tools and authorities which were modeled on specific authorities that the Legislature historically has granted to specific groundwater management agencies, such as requiring registration of groundwater wells, measuring and limiting groundwater extractions, imposing fees for groundwater management, and enforcing the terms of a groundwater sustainability plan.
The Sustainable Groundwater Management Act defined new and expanded responsibilities for the Department of Water Resources,
which included developing regulations to revise groundwater basin boundaries; adopting regulations for evaluating and implementing Groundwater Sustainability Plans (GSPs) and coordination agreements; identifying basins subject to critical conditions of overdraft; identifying water available for groundwater replenishment; and publishing best management practices for the sustainable management of groundwater. DWR has also been tasked with evaluating groundwater sustainability plans for adequacy.
On January 31, 2020, Groundwater Sustainability Agencies overseeing groundwater basins designated as critically-overdrafted were required to submit their adopted Groundwater Sustainability Plans to the Department of Water Resources. The remaining high and medium priority basins that are subject to SGMA must submit their adopted Groundwater Sustainability Plans by January 31, 2022. Failure to submit a plan or submitting a plan that is deemed inadequate by the Department of Water Resources could result in intervention by the State Water Resources Control Board.
After the GSP is submitted, the Department then has two years to evaluate the GSP and issue a written assessment. The GSP will be evaluated per the criteria outlined in the regulations, which includes whether best available science and best available information was used in the development of the plan, a demonstration that the beneficial uses and users of water of groundwater in the basin have been engaged, the identification of data gaps, and whether those projects and management actions proposed are reasonable and feasible and likely to achieve the sustainability goal.
- For more on the roles of DWR and the State Water Board, go here: DWR and State Water Board discuss their roles in implementing the Sustainable Groundwater Management Act at public meeting in Sacramento
- To learn more about the evaluation process for Groundwater Sustainability Plans, click here.
While the Sustainable Groundwater Management Act is centered on the concept that local governance is the key to successful groundwater management,
in cases where the local and regional agencies are unable or unwilling to sustainably manage the groundwater basin, the legislation also authorizes the State Water Resources Control Board to step in to protect groundwater resources in cases. State intervention only occurs when local efforts are not successful, and remain in place until local efforts are able to sustainably manage groundwater resources.
The legislation allows for intervention by the State Water Board instances where no local agency is willing to serve as a groundwater sustainability agency; the groundwater sustainability agency does not complete a groundwater sustainability plan by the mandated deadline (2020 or 2022); the groundwater sustainability plan is deemed inadequate by DWR and remains so after efforts to cure the deficiencies; or the groundwater sustainability plan is being implemented and simply does not work. In these cases, the State Water Board is authorized to create an interim plan that will remain in place until the GSA is able to reassume responsibility. Of course, all that State Water Board love doesn’t come for free: the legislation authorizes the State Water Board to establish fees to recover costs incurred in connection with the intervention.
- Fact Sheet: Overview of Submittal and Evaluation of Groundwater Sustainability Plans (1/23/2020) (Espanol)
- Fact Sheet: Probationary Designation and Groundwater Regulation by the State Water Board (4/11/2019) (Espanol)
- For more from the State Water Board on their role in implementing the Sustainable Groundwater Management Act, click here.
- To learn more about the evaluation process for Groundwater Sustainability Plans, click here.
Information on the state’s groundwater levels is available through the California Statewide Groundwater Elevation Program, or CASGEM.
CASGEM is the state’s long-term program to collect and track groundwater elevations statewide. The program is a collaborative effort between the Department of Water Resources and the local agencies (called Monitoring Entities) who are responsible for collecting and reporting the data; the Department’s role is to coordinate, maintain, and make the data available in a publicly-accessible database. The intent of the CASGEM program is to establish a permanent, locally-managed program of regular and systematic monitoring in all of California’s alluvial groundwater basins.
The data compiled from the CASGEM program is made available to the public through on online system. The system uses reports, search tools, and GIS to allow users to view a wide-range of data, such as find information on the Monitoring Entities; search and view groundwater elevation data and monitoring well information, and download summary reports, well information, groundwater data, hydrographs and maps.
For more information …
As part of the SGMA legislation, DWR was required to determine which basins would be subject to SGMA by considering eight criteria: the overlying population; projected population growth; number of public supply wells; total number of wells; overlying irrigated acreage; reliance on groundwater as the primary source of water; and impacts on the groundwater; including overdraft, subsidence, saline intrusion, and other water quality degradation; as well as any other information determined to be relevant by the Department. The basins were then scored on the criteria and ranked as either high, medium, low, or very low priority.
The most recent basin prioritization was completed in December 2019. Ninety-four basins and/or sub-basins were identified as medium or high priority and are required to form GSAs and develop GSPs. These 94 basins, in combination with adjudicated areas which have existing governance and oversight in place, account for 98 percent of the pumping (20 million acre-feet), 83 percent of the population (25 million Californians), and 88 percent of all irrigated acres (6.7 million acres) within the state’s groundwater basins.
It is important to note that the basin priority is more a reflection of the importance of groundwater to the cities, farms, and other overlying land uses and not necessarily a reflection of poor basin management. Many groundwater basins designated by DWR as high priority are well managed.
The final basin prioritization is available by interactive map, summary tables, and reports. Individual basin summary sheets provide the ranking values for the eight components and the basin’s total ranking score.
For more information …
Provisions in the Sustainable Groundwater Management Act directed the Department of Water Resources to identify groundwater basins and subbasins in conditions of critical overdraft, which is defined by the legislation as a condition ‘when continuation of present water management practices would result in significant adverse overdraft-related environmental, social, or economic impacts.’ Those adverse impacts can include seawater intrusion, land subsidence, groundwater depletion, and/or chronic lowering of groundwater levels.
In January of 2016, the Department of Water Resources released the final list of groundwater basins designated in critical overdraft. SGMA requires that all groundwater basins designated as high or medium priority and considered critically overdrafted must be managed by groundwater sustainability plan by January 31, 2020; that deadline for all other high and medium priority basins is January 31, 2022.
Other Groundwater Topics
Groundwater recharge is the process of adding water to an aquifer.
Recharge occurs naturally as precipitation falls on the surface of the land, infiltrates through the soil and down into the water table; recharge can also occur through leakage from rivers, streams, lakes, and wetlands.
Aquifers can also be recharged artificially by redirecting surface water into spreading basins or ponds, through irrigation, or by directly injecting water into the subsurface through injection wells. Artificial recharge is a way to store water underground in times of water surplus to meet demand in times of drought; it is also an important component of sustainable groundwater management, as increasing the amount of recharge can boost groundwater levels and improve conditions in overdrafted basins.
Water for artificial recharge can come from many different sources, such as surface water, stormwater runoff, recycled water, or remediated groundwater. When low quality water is used for recharge, the underground formations can act as natural filters to remove many physical, biological, and chemical pollutants from the water as it moves through.
For more information …
Conjunctive management refers to the planned use and management of both surface and groundwater resources to maximize the availability and reliability of water supplies in a region. With the right infrastructure in place, water districts and agencies can manage surface water and groundwater as a single source, using one to balance the other when surface water or groundwater levels are low, thus reducing diversions and groundwater pumping while enhancing supplies.
Conjunctive management involves the efficient use of both surface and groundwater resources through the managed operation of a groundwater basin and a surface water storage system where water is stored in the groundwater basin to be used later by intentionally recharging the basin when excess water supply is available. There are many methods an agency can use, such as in-lieu recharge programs, natural infiltration, managed infiltration such as spreading basins, or by direct injection.
Conjunctive management in California is nothing new; many agencies have implemented conjunctive management programs for decades to meet demands when surface water is cutback, to replenish declining groundwater levels, and to help control subsidence. Conjunctive management programs can range from the Semitropic Water Storage District at 2.1 MAF to the small 2289 AF program run by the Compton Water Department.
For more information …
Groundwater banking essentially uses the empty space in aquifers for storage purposes, storing water during wet periods that is later recovered for use during droughts.
There are two basic methods for putting water into storage:
- direct recharge: adding surface water supplies either by using recharge ponds to percolate water into the aquifer or by injection wells to place the water directly there, or
- in-lieu agreements: surface supplies are provided to groundwater users to be used in lieu of groundwater pumping, and the amount of water that otherwise would have been pumped becomes the banked water.
The groundwater that is stored is then later recovered from the aquifer when needed through recovery wells.
The Central Valley, especially the southern portion of Kern County, is home to numerous groundwater banking operations which have played a critical role in recent droughts, delivering millions of acre-feet of banked supplies to their customers. However, groundwater banking is not just for the wide-open spaces of the Central Valley; when properly managed, the same techniques can be applied to groundwater basins in urban areas, providing a reliable local source of water.
For more information …
More than one million Californians are exposed to unsafe drinking water each year, and approximately 300 communities have been unable to meet safe drinking water standards for several years or even more than a decade.
Ensuring safe, clean water supplies for California’s residents requires managing a variety of water quality challenges across the state, which vary by region. In the Central Valley and other farming areas, nitrate from manure and use of fertilizers can pollute local groundwater basins. In Southern California, aquifers have become contaminated with perchlorate and other volatile organic chemicals, byproducts of industrial processes and petroleum production. Overpumping of groundwater near the coast can cause seawater intrusion into coastal aquifers. Rural areas can experience bacterial contamination from septic tanks, which are often used in rural areas that don’t have a sewage-treatment system.
The most common contaminants are nitrates, arsenic, coliform bacteria, pesticides, disinfectant byproducts, perchlorate, and 1,2,3-TCP, and are primarily the result of agricultural and industrial activities such as the use of fertilizers and pesticides, manufacturing, and mining. Some contaminants are also the result of naturally occurring processes in the groundwater.
There are many ways in which water management actions can inadvertently impact groundwater quality, such as groundwater recharge projects leaching nitrates or other contaminants into the aquifer, or pumping from a well that mobilizes a contaminant plume. Water trading programs can alter pumping regimes, resulting in changes to irrigation return flows and contaminant loading. However, if water management actions are planned with water quantity and quality considerations in mind, unintended contamination can be avoided and groundwater quality possibly improved.
For more information …
- Learn more about drinking water quality in the Central Valley at the Community Water Center website.
- Learn more about groundwater quality from the USGS Water Science School.
- Fact Sheet: California’s water quality challenges, from the PPIC.
- The State Water Board’s Groundwater Ambient Monitoring and Assessment program has a wealth of information on groundwater quality throughout the state. Click here for more on the GAMA program. Click here to access the program’s online tools.
