Page updated August 13, 2024
Groundwater in California has been in the spotlight recently, and rightly so: we rely heavily on it. In fact, California’s groundwater usage is the highest in the United States, accounting for about 21% of the nation’s total groundwater extraction.
Groundwater, often referred to as a ‘hidden resource’, plays a crucial role in California’s economy. Its vast supply has significantly contributed to the state’s status as the nation’s largest food and agricultural economy and the world’s fifth-largest overall economy. This economic significance underscores the value of groundwater, a resource that is often overlooked due to its invisible nature.
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 such as maintaining wetlands, supporting streamflow, and providing habitat for wildlife, 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 are, at most, the amount replenished by man or by nature. Still, 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 overdraft conditions and threatening this vital resource.
California’s groundwater basins
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 fractured 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 significant 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 Sacramento River drains the northern portion of the valley, the San Joaquin River drains the middle portion, while drainage in the Tulare Lake basin is entirely internal. Nearly three-quarters of the state’s groundwater supplies and 83% of the state’s agricultural groundwater use is extracted from the Central Valley aquifer system.
Coastal aquifers, which include several basins in the Bay Area, Central Coast, and Southern California regions, face a significant threat-seawater intrusion. This is a common problem for coastal aquifers, posing a serious challenge to the sustainability of these water sources. Other alluvial aquifers include the Eastern Sierra basins 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)
California’s groundwater use by the numbers
Groundwater is a critically important part of California’s supply, accounting for 41% of total annual agricultural and urban water uses statewide in an average year and up to 58% or more in drought years. About 83 percent of Californians depend on groundwater for some 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 annually, with 39% going towards agriculture, 41% supporting cities, and 18% being used for managed wetlands. 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 ten 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, creating a stark disparity. 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 90% of its supplies coming from groundwater aquifers.
Groundwater use and overuse
Groundwater can be replenished through various methods, such as precipitation, seepage from surface waters into aquifers, intentional recharge efforts, excess irrigation water, and even accidental leaks from pipelines and canals. The balance is maintained when the amount of water pumped out matches the amount recharged. However, if more groundwater is extracted than replenished, the aquifer can become overdrafted.
Overdraft can have many consequences: it can lower groundwater levels, raise energy costs for pumping, cause overlying land to subside, dry up domestic wells, allow saltwater intrusion in coastal areas, and reduce interconnected surface water supplies.
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.
The San Joaquin Valley is one of the most productive agricultural regions in the nation, producing more than $24 billion in crops and employing around 340,000 people. However, it has a long history of overdraft. Since about 1960, groundwater has been depleted by almost 60 million acre-feet, with the overdraft estimated at 1.5 to 2 MAF per year. During the 2012-2015 drought, the lack of surface water and dwindling groundwater basins caused thousands of domestic wells to go dry, the San Joaquin Valley floor to subside as much as three feet in some areas, and brought California’s agriculture to a halt.
Groundwater use remained unregulated by the state until the passage of the 2014 Sustainable Groundwater Management Act (SGMA).
The Sustainable Groundwater Management Act (SGMA)
In September 2014, Governor Brown signed the Sustainable Groundwater Management Act (SGMA), a three-bill legislative package to sustainably manage California’s groundwater basins. A key aspect of SGMA is the emphasis on local management, mandating the establishment of Groundwater Sustainability Agencies (GSAs) to create and develop groundwater sustainability plans to achieve sustainability within a 20-year timeframe.
SGMA requires all medium- and high-priority groundwater basins, as designated by the Department of Water Resources, to form GSAs and develop localized sustainability plans. Low and very-low-priority basins are not mandated to comply but are encouraged to establish GSAs and create management plans. Adjudicated basins and those with pre-existing, successful groundwater management programs that meet SGMA’s criteria are exempt from forming GSAs and developing new plans, although they must adhere to reporting requirements.
As of June 2024, 71 basins have submitted groundwater sustainability plans that have been approved; thirteen basins had plans deemed incomplete, giving them six months to address deficiencies; and six basins from the 2020 plans submitted by basins determined to be critically overdrafted have been deemed inadequate and referred to the State Water Board for consideration of the state intervention process and probationary status.
SGMA granted GSAs new authorities to manage groundwater and implement their GSPs, including the authority to conduct investigations, determine the sustainable yield of a groundwater basin, measure and limit extraction, impose fees for groundwater management, and enforce the terms of a GSP.
Interestingly, although SGMA grants GSAs the authority to limit groundwater extractions, the legislation explicitly states that it does not alter any water rights: the legislation adds Water Code section 10720.5(b) that states that nothing in the legislation “determines or alters surface water rights or groundwater rights under common law or any provision of law that determines or grants surface water rights.” How this will play out as SGMA lawsuits work through the court system remains to be seen.
- Introduction to SGMA, FAQ from the Groundwater Exchange
- Find your GSA: Visit the California Department of Water Resources’ GSA Map Viewer to view GSAs across the State. Once there, enter your address or place in the upper left corner and click on the magnifying glass icon. You can click on the map in any area to reveal the agency name and other important information.
Sustainable groundwater management means no ‘undesirable results’
The legislation defines ‘sustainable groundwater management’ as the ‘management and use of groundwater in a manner that can be maintained during the planning and implementation horizon without causing undesirable results.’
Undesirable results as defined in the legislation are:
One of the first and most noticeable impacts of excessive groundwater pumping is the lowering of the water table. As the depth of 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 decrease as well. If groundwater levels continue to fall, the shallower wells can go dry and might have to be deepened or even new wells drilled.
Chronic declines in groundwater levels occur when groundwater extraction exceeds recharge on a long-term basis. Chronic groundwater level declines may serve as an indicator or proxy for other undesirable results such as subsidence, compaction, and concentration of pollutants.
Regular groundwater monitoring is essential for tracking changes in storage and flow within a basin.
For more information:
- Lowering groundwater levels, fact sheet from the California Farm Bureau Federation
- Groundwater level declines, webpage from the USGS
Declines in groundwater storage indicate that more groundwater is leaving the system through groundwater pumping, subsurface flows, evapotranspiration, or other factors than is entering the system via natural or artificial recharge, subsurface flows, precipitation, or other sources.
Groundwater storage is a term used to refer to the amount of usable water that remains in an aquifer. Significant and reasonable declines in groundwater storage may threaten the aquifer’s ability to “produce” or provide water long-term.
Changes in groundwater levels in different aquifers can serve as a proxy for changes in groundwater storage over time. Thus, groundwater monitoring networks are critical for tracking changes in groundwater levels and understanding how the basin responds to changing conditions.
Well-developed and calibrated groundwater models can help water managers understand how changes in pumping, land use, or other variables may impact groundwater storage long-term. These models rely on long-term groundwater level data for calibration.
NASA’s Gravity Recovery and Climate Experiment (GRACE) satellites can provide estimates of changes in groundwater storage. However, satellite estimates should be used cautiously, as they are more regional in nature and do not provide the level of accuracy necessary to fully determine the conditions at the basin scale. Land-based gravity methods can measure changes in groundwater storage locally and are more accurate than satellite estimates; however, these methods must be calibrated using local groundwater monitoring networks.
For more information:
- Fact Sheet: Decreasing groundwater storage, from the California Farm Bureau Federation
- Groundwater storage reductions, webpage from the USGS
A host of natural and human-caused sources can impact groundwater quality. Aquifers can become contaminated by a single point source, such as a decommissioned military base, or by runoff from widespread areas, such as agricultural fields using fertilizers or urban stormwater carrying pollutants from roadways and urban landscapes. Even within a single aquifer, groundwater quality can change with the level of the groundwater table itself. Groundwater quality generally degrades with increasing depth within an aquifer. Thus, declining groundwater levels can lead to poorer water quality and concentration of contaminants.
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. Continued pumping can also cause polluted groundwater or seawater to migrate or be drawn into areas that would otherwise not be impacted.
SGMA directs groundwater basins to be managed to avoid ‘significant and unreasonable’ impacts to groundwater quality.
For more information:
- A Guide to Water Quality Requirements Under the Sustainable Groundwater Management Act, from the Stanford Repository
- Guide to Protecting Drinking Water Quality Under the Sustainable Groundwater Management Act, from the Community Water Center
- Protecting Groundwater Quality in California: Management considerations for avoiding naturally occurring and emerging contaminants, from the Environmental Defense Fund
- Water quality degradation, webpage from the USGS
Seawater intrusion is the movement of ocean water into freshwater aquifers. It occurs naturally to some degree in most coastal aquifers due to the hydraulic connection between groundwater and seawater. Under natural conditions, the seaward movement of freshwater prevents seawater from encroaching on coastal aquifers. However, when groundwater is pumped from a coastal aquifer, the lowered water levels can cause seawater to be drawn further inland into the freshwater zones of the aquifer.
In the 1950s, Southern California’s post-World War II population boom 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 sea stays high enough to keep the seawater from seeping in.
Seawater intrusion remains an issue along the Central Coast, with seawater intruding as far as five miles inland in the Salinas Valley and three miles inland in the Pajaro Valley.
For more information:
- Sunshine, beaches and…saltwater intrusion? article from EDF’s Growing Returns blog
- Saltwater Intrusion in Los Angeles Area Coastal Aquifers—the Marine Connection, from the USGS
- Seawater intrusion, webpage from the USGS
Land subsidence is the gradual sinking of the Earth’s surface. Various factors can cause subsidence; however, here in California, the primary causes are groundwater pumping and the decomposition of wetland soils after conversion to farmland. Groundwater extractions that deplete or “drain” confined aquifer systems can cause the sediments to compact and the land surface to sink (or subside). Extensive groundwater withdrawals from aquifer systems have caused land subsidence in many California basins. Land subsidence can damage structures such as wells, buildings, and highways. In most aquifers, this subsidence is permanent, forever reducing the storage capacity of the aquifer.
Subsidence in the San Joaquin Valley has been called the single most significant alteration of the land surface attributable to humankind, with subsidence in some areas reaching nearly 30 feet. Since the 1920s, the extensive extraction of groundwater from numerous wells in California’s San Joaquin Valley has led to the ground sinking in some areas by up to 28 feet (8.5 meters). This subsidence problem worsens during drought periods when farmers increasingly depend on groundwater to support one of the nation’s most productive agricultural regions, as demonstrated during the 2012-15 drought, when estimates from NASA’s Gravity Recovery and Climate Experiment (GRACE) revealed that approximately 32 million acre-feet was pumped from Central Valley aquifers over the four-year period. This is equivalent to pumping enough water to fill Shasta Dam twice each year of the drought.
Irrigation canals and aqueducts typically rely on gravity to deliver water, but the ability to move the water can become impaired when the land sinks. Land subsidence has significantly impacted the Friant-Kern Canal, reducing its ability to deliver water to some contractors by up to 60%; fixing the damage has cost millions. In 2017, a report on San Joaquin Valley subsidence determined areas along the California Aqueduct that have experienced almost two feet of subsidence over three years.
Subsidence has also 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 altered the capacity of flood bypasses and diminished the effectiveness of levees.
For more information:
- Land subsidence: what is it and why is it an important aspect of groundwater management?, from Glenn County
- Measuring land subsidence from space, from the USGS
- San Joaquin Valley: Largest human alteration of the Earth’s surface, from the USGS
- Santa Clara Valley: A case of arrested subsidence, from the USGS
- Land subsidence, webpage from the USGS
Though considered legally separate, groundwater and surface water are physically interconnected resources. Surface waters can gain water from the inflow of groundwater through the streambed, lose water to groundwater by outflow through the streambed, or do both, depending upon the situation. The extent to which this exchange occurs in surface water and groundwater systems depends on the hydraulic gradient between systems.
In much of California, groundwater pumping has lowered the hydraulic gradient in these systems such that surface water bodies largely lose water to underlying groundwater systems. A study by The Nature Conservancy in 2016 estimates that there is approximately 900,000 acre-feet less in the Sacramento River from groundwater pumping and other causes. However, in some groundwater basins, the connection between these systems remains, supporting plant and aquatic species, particularly in dry months and years.
SGMA recognizes the interconnection of groundwater and surface water and requires that GSPs avoid “depletions of interconnected surface water that have significant and unreasonable adverse impacts.” Additionally, GSPs must consider impacts to groundwater-dependent ecosystems.
For more information:
- Paper 1: Depletions of ISW: An Introduction, Part 1 of 3-part series under development by the DWR (Part 2 & 3 expected late 2024-25)
- Surface water depletions, fact sheet from the California Farm Bureau Federation
- Report: Navigating Groundwater-Surface Water Interactions under the Sustainable Groundwater Management Act, report from Berkeley Law
- Guide to Compliance with California’s Sustainable Groundwater Management Act: How to Avoid the “Undesirable Result” of “Significant and Unreasonable Adverse Impacts on Beneficial Uses of Surface Waters”, from Stanford
- Report: Groundwater and Stream Interaction in California’s Central Valley: Insights for Sustainable Groundwater Management, from the Nature Conservancy
- USGS: Streamflow Depletion by Wells—Understanding and Managing the Effects of Groundwater Pumping on Streamflow, from the USGS
Roles and responsibilities for SGMA implementation
The legislation defines roles for local agencies, the Department of Water Resources, and the State Water Resources Control Board.
The Sustainable Groundwater Management Act required local groundwater pumpers to form Groundwater Sustainability Agencies and to develop Groundwater Sustainability Plans to manage their groundwater basins.
Groundwater Sustainability Plans 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 GSA will monitor groundwater conditions in the basin. A GSA may adopt a single plan to cover an entire basin or a number of plans can be combined through a coordination agreement.
To effectively implement the plans, SGMA provided 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.
State Water Board programs to protect groundwater quality
The State Water Board and the nine regional water boards protect groundwater through numerous regulatory and planning programs. The key elements of the water board’s approach include identifying and updating beneficial uses and water quality objectives, regulating activities that can impact the beneficial uses of groundwater, and preventing future groundwater impacts through planning, management, education, monitoring, and funding.
For more information on California’s groundwater
DWR, State Water Board, and USGS Resources
The Department of Water Resources has a wealth of information for GSAs and the public on its website:
- SGMA Portal: View and download information related to groundwater sustainability agencies (GSAs), groundwater sustainability plans (GSPs), alternatives to GSPs, adjudicated areas, and basin boundary modifications.
- Sustainable Groundwater Management Act – Main page
- Best Management Practices and Guidance Documents
- Technical Assistance – Data and Tools
- Outreach and Facilitation Support
- Sustainable Groundwater Management Grant Program
Relevant State Water Board webpages related to groundwater:
