Groundwater and Stream Interaction in California's Central Valley: Insights for Sustainable Groundwater Management

Nature Conservancy | June 16th, 2016

Sustainable water management requires ensuring ecosystem water needs are met in balance with water needs for people. The Nature Conservancy (TNC) is working across Califo

Sustainable water management requires ensuring ecosystem water needs are met in balance with water needs for people. The Nature Conservancy (TNC) is working across California to demonstrate how to achieve this balance in the context of integrated water management programs. Sustainable water management must also recognize that surface water and groundwater resources are interconnected and that proper management of these resources includes understanding this interaction and managing the resources accordingly. Management of groundwater and surface water systems in an integrated manner is impeded by California water law and policy, which has separate rules and regulations for managing groundwater and surface water resources. The Central Valley of California is the hub of the state’s water supply system. An extensive network of dams and canals supplies surface water to users within the Central Valley as well as to the San Francisco Bay Area, the Central Coast, and Southern California. Analysis of information collected in this study reveals that just within the Central Valley, agricultural and urban sectors use an average of 13 million acre-feet (MAF) of surface water and over 8 MAF of groundwater per year. Approximately 2 MAF of water is exported annually to areas outside the Central Valley. Because the availability of surface water supplies varies significantly from year to year, groundwater use also varies considerably on annual basis. In an average year, groundwater meets about 40% of the state’s water demand and up to 60% or more during droughts, when groundwater provides a water supply buffer against economic and environmental harm from water scarcity (DWR, 2014). In some areas, groundwater provides 100% of the supply, even in wetter years, and then is heavily pumped during drought. In other locations, where surface water supplies are reasonably abundant, groundwater use may be relatively light during wetter years, but more heavily pumped during drier years when surface water supplies are cut back. Accordingly, the impact of groundwater use is much more evident during dry years, and especially in drought conditions. A case in point is the drought condition of 2013-15. This drought is exerting significant stress on the state’s water supply. The surface water deliveries have been reduced dramatically, to near zero in many regions; therefore, agricultural and municipal water users in the Central Valley and other parts of the state that normally rely on surface water for much of their supplies, are now greatly increasing their use of groundwater to meet their needs. A 2014 UC Davis report indicates that expected increase in Central Valley groundwater pumping would be in the range of 20% to 25%, during the 2014 irrigation season. To the authors’ knowledge, there are no reports of actual estimates of increase in groundwater use for the recent drought conditions. The long-term increase in groundwater pumping and especially recent drought related increases in pumping would translate into significantly more adverse impacts on the surface water and stream systems, as well as groundwater dependent ecosystems. In addition, this increase in groundwater pumping could potentially result in more land subsidence throughout the valley. The drought has also affected the irrigated lands under cultivation. A November 2014 DWR publication “Public Update for Drought Response” 3 reported that based on NASA satellite imagery, approximately 700,000 acres of land were estimated idled between 2011 and 2014. Majority of these acreages idled are reported to be due to the drought conditions, although, some were also due to normal agronomic practices, crop markets, and other reasons. This report provides the results of a number of analyses and evaluations of Central Valley hydrologic conditions using the California Central Valley Groundwater-Surface Water Simulation Model (C2VSim). The study examines important hydrologic and operational characteristics and behaviors of the California’s Central Valley water system during the historical development in the Valley. In addition, the study presents results of model simulations conducted to assess potential impacts of several water management scenarios. The results of this study contribute significantly to the understanding of how management of groundwater and surface water are interrelated and will inform the development of more effective integrated water management in the Central Valley and throughout California. The interaction between groundwater and surface water resources has been evaluated and illustrated by many researchers. As early as 1941, Theis (1941) documented the hydraulic connection between stream and aquifer systems and quantitatively described the general behavior of stream-groundwater systems. Many others, including Glover & Balmer (1954) and Hantush (1965) built on Theis’ work, which demonstrated and confirmed the basic principles of hydraulic connection between stream and aquifer systems. In recent years, significant work has been conducted regarding regional impacts of groundwater pumping and operation on regional surface water systems. For example, the recent USGS study (Barlow & Leake, 2012) presents original findings of the stream-aquifer interaction in the context of sustainable groundwater management. In an effort to quantify the interaction between the groundwater and the surface water system, DWR’s C2VSim model was employed in this study. The C2VSim model was originally developed in 1990 for DWR, U. S. Bureau of Reclamation (USBR), and California State Water Resources Control Board (SWRCB) as the Central Valley Groundwater and Surface water Model (CVGSM) (James M. Montgomery Consulting Engineers, 1990a). Subsequently, the model was enhanced through numerous key applications, including the Central Valley Project Improvement Act – Programmatic Environmental Impact Statement (CVPIA-PEIS). In 2005, the CVGSM model was upgraded to the newly developed Integrated Water Flow Model (IWFM) platform, and was renamed the C2VSim model. The detailed features of the model are described and presented in Section 2 of this report. The C2VSim model is a leading Central Valley-wide integrated hydrologic model adopted by DWR and many other regional and state-wide agencies, as well as non-governmental organizations (NGOs) to evaluate various water management scenarios throughout the Valley. This report is organized in six sections: Section 1: Provides the background information and lays out the goals and objectives of this study. Section 2: Provides information on the status of C2VSim model, refinements made during this study, and assumptions on the modeling analysis. Section 3: Discusses simulation of the historical hydrologic conditions in the Central Valley. Section 4: Provides the assumptions and results of the existing baseline conditions, assuming that no changes to the facilities and their operations, as well as water management actions, take place over the course of a predetermined hydrologic condition. Section 5: Provides a description of three water management scenarios that may have major impacts on the future of surface water and groundwater interaction in Central Valley. Results of simulation of these management scenarios are presented and discussed in detail. Section 6: Provides conclusions from this study and a list of recommendations for future work.

Groundwater and Urban Growth in the San Joaquin Valley

Public Policy Institute of California (PPIC) | September 20th, 2021

The San Joaquin Valley is ground zero for the Sustainable Groundwater Management Act (SGMA). It has the largest groundwater deficit in California and faces some of the

The San Joaquin Valley is ground zero for the Sustainable Groundwater Management Act (SGMA). It has the largest groundwater deficit in California and faces some of the worst impacts from overdraft, including land subsidence and drying wells. SGMA will help the region address these issues—while also building its climate resilience. But this will require hard work, not only by farmers—the largest groundwater users—but also by the valley’s urban water utilities, which serve a growing population while relying mostly on groundwater. As the valley confronts limits on groundwater pumping, urban utilities, local land use authorities, other water users, and state agencies must all engage to ensure a smooth transition for the region’s residents.

Groundwater Availability of the Central Valley Aquifer

U.S. Geological Survey (USGS) | July 15th, 2009

California’s Central Valley covers about 20,000 square miles and is one of the most productive agricultural regions in the world. More than 250 different crops are grow

California’s Central Valley covers about 20,000 square miles and is one of the most productive agricultural regions in the world. More than 250 different crops are grown in the Central Valley with an estimated value of $17 billion per year. This irrigated agriculture relies heavily on surface-water diversions and groundwater pumpage. Approximately one-sixth of the Nation’s irrigated land is in the Central Valley, and about one-fifth of the Nation’s groundwater demand is supplied from its aquifers. The Central Valley also is rapidly becoming an important area for California’s expanding urban population. Since 1980, the population of the Central Valley has nearly doubled from 2 million to 3.8 million people. The Census Bureau projects that the Central Valley’s population will increase to 6 million people by 2020. This surge in population has increased the competition for water resources within the Central Valley and statewide, which likely will be exacerbated by anticipated reductions in deliveries of Colorado River water to southern California. In response to this competition for water, a number of water-related issues have gained prominence: conservation of agricultural land, conjunctive use, artificial recharge, hydrologic implications of land-use change, and effects of climate variability. To provide information to stakeholders addressing these issues, the USGS Groundwater Resources Program made a detailed assessment of groundwater availability of the Central Valley aquifer system, that includes: (1) the present status of groundwater resources; (2) how these resources have changed over time; and (3) tools to assess system responses to stresses from future human uses and climate variability and change. This effort builds on previous investigations, such as the USGS Central Valley Regional Aquifer System and Analysis (CV-RASA) project and several other groundwater studies in the Valley completed by Federal, State and local agencies at differing scales. The principal product of this new assessment is a tool referred to as the Central Valley Hydrologic Model (CVHM) that accounts for integrated, variable water supply and demand, and simulates surface-water and groundwater-flow across the entire Central Valley system. The development of the CVHM comprised four major elements: (1) a comprehensive Geographic Information System (GIS) to compile, analyze and visualize data; (2) a texture model to characterize the aquifer system;(3) estimates of water-budget components by numerically modeling the hydrologic system with the Farm Process (FMP); and (4) simulations to assess and quantify hydrologic conditions. Chapter A: Introduction, Overview of Hydrogeology, and Textural Model of California’s Central Valley Chapter B: Groundwater Availability in California’s Central Valley Chapter C:  Numerical Model of the Hydrologic Landscape and Groundwater Flow in California’s Central Valley Appendix 1: Supplemental Information—Modifications to Modflow-2000 Packages and Processes

Groundwater Data for Selected Wells Within the Eastern San Joaquin Groundwater Subbasin, California, 2003–8

U.S. Geological Survey (USGS) | June 1st, 2012

Data were collected by the U.S. Geological Survey from 2003 through 2008 in the Eastern San Joaquin Groundwater Subbasin, 80 miles east of San Francisco, California, as p

Data were collected by the U.S. Geological Survey from 2003 through 2008 in the Eastern San Joaquin Groundwater Subbasin, 80 miles east of San Francisco, California, as part of a study of the increasing chloride concentrations in groundwater processes. Data collected include geologic, geophysical, chemical, and hydrologic data collected during and after the installation of five multiple-well monitoring sites, from three existing multiple-well sites, and from 79 selected public-supply, irrigation, and domestic wells. Each multiple‑well monitoring site installed as part of this study contained three to five 2-inch diameter polyvinyl chloride (PVC)-cased wells ranging in depth from 68 to 880 feet below land surface. Continuous water-level data were collected from the 19 wells installed at these 5 sites and from 10 existing monitoring wells at 3 additional multiple-well sites in the study area. Thirty‑one electromagnetic logs were collected seasonally from the deepest PVC-cased monitoring well at seven multiple-well sites. About 200 water samples were collected from 79 wells in the study area. Coupled well‑bore flow data and depth-dependent water-quality data were collected from 12 production wells under pumped conditions, and well‑bore flow data were collected from 10 additional wells under unpumped conditions.

Groundwater depletion and sustainability of irrigation in the US High Plains and Central Valley

National Academy of Sciences (NAS) | June 12th, 2012

Aquifer overexploitation could significantly impact crop production in the United States because 60% of irrigation relies on groundwater. Groundwater depletion in

Aquifer overexploitation could significantly impact crop production in the United States because 60% of irrigation relies on groundwater. Groundwater depletion in the irrigated High Plains and California Central Valley accounts for ∼50% of groundwater depletion in the United States since 1900. A newly developed High Plains recharge map shows that high recharge in the northern High Plains results in sustainable pumpage, whereas lower recharge in the central and southern High Plains has resulted in focused depletion of 330 km3 of fossil groundwater, mostly recharged during the past 13,000 y. Depletion is highly localized with about a third of depletion occurring in 4% of the High Plains land area. Extrapolation of the current depletion rate suggests that 35% of the southern High Plains will be unable to support irrigation within the next 30y. Reducing irrigation withdrawals could extend the lifespan of the aquifer but would not result in sustainable management of this fossil groundwater. The Central Valley is a more dynamic, engineered system, with north/south diversions of surface water since the 1950s contributing to ∼7×higher recharge. However, these diversions are regulated because of impacts on endangered species. A newly developed Central Valley Hydrologic Model shows that groundwater depletion since the 1960s, totaling 80 km3, occurs mostly in the south (Tulare Basin) and primarily during droughts. Increasing water storage through artificial recharge of excess surface water in aquifers by up to 3 km3 shows promise for coping with droughts and improv ing sustainability of groundwater resources in the Central Valley.

Groundwater depletion in California’s Central Valley accelerates during megadrought

Nature Portfolio (Springer Nature) | December 19th, 2022

Groundwater provides nearly half of irrigation water supply, and it enables resilience during drought, but in many regions of the world, it remains poorly, if at all mana

Groundwater provides nearly half of irrigation water supply, and it enables resilience during drought, but in many regions of the world, it remains poorly, if at all managed. In heavily agricultural regions like California’s Central Valley, where groundwater management is being slowly implemented over a 27-year period that began in 2015, groundwater provides two–thirds or more of irrigation water during drought, which has led to falling water tables, drying wells, subsiding land, and its long-term disappearance. Here we use nearly two decades of observations from NASA’s GRACE satellite missions and show that the rate of groundwater depletion in the Central Valley has been accelerating since 2003 (1.86 km3/yr, 1961–2021; 2.41 km3/yr, 2003–2021; 8.58 km3/yr, 2019–2021), a period of megadrought in southwestern North America. Results suggest the need for expedited implementation of groundwater management in the Central Valley to ensure its availability during the increasingly intense droughts of the future.

Groundwater Depletion in the United States (1900-2008)

U.S. Geological Survey (USGS) | June 1st, 2013

A natural consequence of groundwater withdrawals is the removal of water from subsurface storage, but the overall rates and magnitude of groundwater depletion in the Unit

A natural consequence of groundwater withdrawals is the removal of water from subsurface storage, but the overall rates and magnitude of groundwater depletion in the United States are not well characterized. This study evaluates long-term cumulative depletion volumes in 40 separate aquifers or areas and one land use category in the United States, bringing together information from the literature and from new analyses. Depletion is directly calculated using calibrated groundwater models, analytical approaches, or volumetric budget analyses for multiple aquifer systems. Estimated groundwater depletion in the United States during 1900–2008 totals approximately 1,000 cubic kilometers (km3). Furthermore, the rate of groundwater depletion has increased markedly since about 1950, with maximum rates occurring during the most recent period (2000–2008) when the depletion rate averaged almost 25 km3 per year (compared to 9.2 km3 per year averaged over the 1900–2008 time-frame). 

Groundwater development leads to decreasing arsenic concentrations in the San Joaquin Valley, California

Science of the Total Environment (Elsevier) | January 18th, 2021

In the San Joaquin Valley (SJV), California, about 10% of drinking water wells since 2010 had arsenic concentrations above the U.S. maximum contaminant level of 10μg/L.

In the San Joaquin Valley (SJV), California, about 10% of drinking water wells since 2010 had arsenic concentrations above the U.S. maximum contaminant level of 10μg/L. High concentrations of arsenic are often associated with high pH (greater than 7.8) or reduced geochemical conditions. Although most wells have low arsenic(<3μg/L) and do not have changing arsenic concentrations, this study found that most wells with concentrations above 10μg/L had arsenic trends. Overall, about 24% of wells had time-series trends since 2010 and 59% had paired-sample trends since 2000. Most wells had decreasing arsenic trends, even in wells with higher arsenic concentrations. These wells often had co-detections of increasing nitrate and sulfate trends that reflect oxic groundwater likely derived from agricultural recharge. Wells with increasing arsenic trends were deeper or located in the valley trough where aquifer materials are more fine-grained and where reducing conditions favor arsenic mobility. Wells with arsenic trends also tend to be clustered near areas of higher well density. Groundwater pumping in these areas has likely increased the contribution of younger, more oxic groundwater in wells with declining arsenic or, less frequently, increased the contribution of higher pH or reduced groundwater in wells with rising arsenic. Projections of arsenic trends indicate that 37 wells with high arsenic presently will be below 10μg/L in ten years. Unfortunately, these improvements will be largely offset by 31 wells that are expected to increase above 10μg/L in addition to expected rises in nitrate in wells where arsenic decreased. This study shows how human-altered flow systems can impact the natural geochemical character of water in both beneficial and deleterious ways.

Groundwater Quality Data for the Madera/Chowchilla–Kings Shallow Aquifer Study Unit, 2013–14: Results from the California GAMA Program

U.S. Geological Survey (USGS) | February 2nd, 2017

Groundwater quality in the 2,390-square-mile Madera/Chowchilla–Kings Shallow Aquifer study unit was investigated by the U.S. Geological Survey from August 2013 to April

Groundwater quality in the 2,390-square-mile Madera/Chowchilla–Kings Shallow Aquifer study unit was investigated by the U.S. Geological Survey from August 2013 to April 2014 as part of the California State Water Resources Control Board Groundwater Ambient Monitoring and Assessment Program’s Priority Basin Project. The study was designed to provide a statistically unbiased, spatially distributed assessment of untreated groundwater quality in the shallow aquifer systems of the Madera, Chowchilla, and Kings subbasins of the San Joaquin Valley groundwater basin. The shallow aquifer system corresponds to the part of the aquifer system generally used by domestic wells and is shallower than the part of the aquifer system generally used by public-supply wells. This report presents the data collected for the study and a brief preliminary description of the results. Groundwater samples were collected from 77 wells and were analyzed for organic constituents, inorganic constituents, selected isotopic and age-dating tracers, and microbial indicators. Most of the wells sampled for this study were private domestic wells. Unlike groundwater from public-supply wells, the groundwater from private domestic wells is not regulated for quality in California and is rarely analyzed for water-quality constituents. To provide context for the sampling results, however, concentrations of constituents measured in the untreated groundwater were compared with regulatory and non-regulatory benchmarks established for drinking-water quality by the U.S. Environmental Protection Agency, the State of California, and the U.S. Geological Survey. Of the 319 organic constituents assessed in this study (90 volatile organic compounds and 229 pesticides and pesticide degradates), 17 volatile organic compounds and 23 pesticides and pesticide degradates were detected in groundwater samples; concentrations of all but 2 were less than the respective benchmarks. The fumigants 1,2-dibromo-3-chloropropane (DBCP) and 1,2-dibromoethane (EDB) were detected at concentrations above their respective regulatory benchmarks in samples from a total of four wells. Most detections of inorganic constituents were at concentrations or activities less than the respective benchmark levels. Five inorganic constituents were detected in groundwater samples from one or more wells at concentrations or activities greater than their respective regulatory, health-based benchmarks: arsenic, uranium, nitrate, adjusted gross alpha particle activity, and gross beta particle activity. Four inorganic constituents were detected in samples from one or more wells at concentrations or activities greater than their respective non-regulatory, health-based benchmarks: manganese, molybdenum, vanadium, and radon-222. Three inorganic constituents were detected in groundwater samples from one or more wells at concentrations greater than their respective non-regulatory, aesthetic-based benchmarks: iron, sulfate, and total dissolved solids. Microbial indicators (Escherichia coli, total coliform, and enterococci) were analyzed for presence or absence. The presence of Escherichia coli (E. coli) was not detected; the presence of total coliform was detected in samples from 10 of the 72 grid wells for which it was analyzed, and the presence of enterococci was detected in samples from 5 of the 73 grid wells analyzed.

Groundwater Quality in the Shallow Aquifers of the Madera– Chowchilla and Kings Subbasins, San Joaquin Valley, California

U.S. Geological Survey (USGS) | January 8th, 2018