Community Perspectives and Environmental Justice in California's San Joaquin Valley

University of California, Merced | December 1st, 2022

Critical Aquifer Overdraft Accelerates Degradation of Groundwater Quality in California's Central Valley During Drought

American Geophysical Union (AGU) | September 1st, 2021

It has been established that wells are at risk to run dry when groundwater levels decline during drought, but associated impacts to water quality are poorly understood. 

It has been established that wells are at risk to run dry when groundwater levels decline during drought, but associated impacts to water quality are poorly understood. This study examined 30 yr of data from California's Central Valley to find dramatic groundwater-level declines during drought were associated with worsening groundwater quality. This likely happens because increased pumping during drought can draw shallow, contaminated groundwater to depths commonly tapped by public drinking water wells. This is the first study of its kind to show direct linkages between drought and groundwater quality at such a broad spatial scale

Cuyama Basin Groundwater Sustainability Plan

Cuyama Basin Groundwater Sustainability Agency (CBGSA) | December 4th, 2019

Cuyama Valley, California Hydrologic Study: An Assessment of Water Availability

U.S. Geological Survey (USGS) | May 29th, 2015

Decadal Shifts in Groundwater Age Detected by Environmental Tracers Across California

American Geophysical Union (AGU) | March 23rd, 2026

Groundwater age tells us how long water has been underground since it last infiltrated the land surface. Monitoring changes in groundwater age helps identify aquifer stre

Groundwater age tells us how long water has been underground since it last infiltrated the land surface. Monitoring changes in groundwater age helps identify aquifer stress from pumping or shifts toward new hydrologic conditions—key factors for sustainable water management. In this study, changes in groundwater age were tested by comparing chemical tracer data collected about 10 years apart from hundreds of wells across California. Chemical tracers are dissolved substances with known historical records and decay rates that can be used to infer groundwater age. Using models that estimate expected tracer concentrations under steady-age conditions, this study found that nearly one-third of the wells showed significant changes in groundwater age over time. These changes were most common in areas with heavy agricultural use, where pumping and irrigation have altered how water moves underground. These shifts in age can also affect water quality, especially when younger water carries contaminants like nitrate. By using a statistical modeling approach, this study shows how changes in groundwater age can help identify aquifers under stress and support efforts to manage groundwater sustainably.

Declining Freshwater Availability in the Colorado River Basin Threatens Sustainability of Its Critical Groundwater Supplies

American Geophysical Union (AGU) | May 27th, 2025

Climate change is causing more frequent and intense droughts around the world, including in the Colorado River Basin, which supplies water to seven U.S. states and Mexico

Climate change is causing more frequent and intense droughts around the world, including in the Colorado River Basin, which supplies water to seven U.S. states and Mexico, and is facing severe water shortages. This study investigates how changes in water storage components (e.g., snow, surface water, soil moisture and groundwater) and water use are contributing to these shortages, and impacting agriculture and municipal water supplies, using data from the NASA GRACE and GRACE Follow-On (GRACE-FO) satellite missions, land surface models, and in situ data over the past two decades. Results show significant losses of groundwater: 7.8.0 ± 5.3 km3 in the Upper CRB (UCRB) and 25.5 ± 7.4 km3 in the Lower CRB (LCRB). Progress toward sustainable groundwater could be achieved by reducing groundwater extraction, in line with current rates of depletion (0.35 km3 in the UCRB and 1.15 km3 in the LCRB). This research highlights the importance of understanding the factors affecting groundwater levels to create sustainable water management strategies that can help secure water resources for the region's future.

Declining Groundwater Level Effects on Supply Well Operations

University of California, Davis (UC Davis) | December 1st, 2017

Pumping more from groundwater basins than they can sustainably supply decreases groundwater levels, often by undesirable amounts. Economic impacts from overdraft

Pumping more from groundwater basins than they can sustainably supply decreases groundwater levels, often by undesirable amounts. Economic impacts from overdraft and drought include increased costs for supplying water and lost revenue from inability to meet water demands. The combination of decreased groundwater level with existing well depth may limit well production as pumping costs increase and wells run dry. Declining groundwater elevations may also incur costs to move pumps deeper in the wells. Further falling groundwater elevation increases costs for production and maintenance, and decreases production capacity, as pumping levels drop into the screened intervals of wells and cause screen clogging and corrosion. In more extreme cases, well replacement costs may be incurred if groundwater drops so low that adequate submergence of pumps cannot be maintained, and wells become unusable. The potential impacts of decreasing groundwater elevation on supply well operations were evaluated for a study area in California’s Central Valley (greater vicinity of Tulare, CA). Well construction data from logs provided by the California Department of Water Resources (DWR) were characterized through statistical analysis of the elevations for the tops and bottoms of screened intervals. Groundwater elevation time series were obtained from the DWR Water Data Library. Estimated trends in well capacity loss were developed from estimates of the fraction of wells over time that had standing water levels (minus an estimated pumping drawdown) below 1) the top of the screened interval and 2) the bottom of the screened interval. The end result is a method for estimating the costs of water level declines over time from drought or groundwater overdraft due to effects on proper operation and pumping of wells. The conceptual approach, data, methods, and conclusions of this thesis draw from a research report funded by the California Department of Food and Agriculture (CDFA).

Delano Earlimart Irrigation District Groundwater Sustainability Plan

Delano Earlimart Irrigation District | January 27th, 2020

Depletion and Capture: Revisiting “The Source of Water Derived from Wells”

National Groundwater Association (NGWA) | May 28th, 2014

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

A natural consequence of groundwater withdrawals is the removal of water from subsurface storage, but the overall rates and magnitude of groundwater depletion and capture relative to groundwater withdrawals (extraction or pumpage) have not previously been well characterized. This study assesses the partitioning of long‐term cumulative withdrawal volumes into fractions derived from storage depletion and capture, where capture includes both increases in recharge and decreases in discharge. Numerical simulation of a hypothetical groundwater basin is used to further illustrate some of Theis' (1940) principles, particularly when capture is constrained by insufficient available water. Most prior studies of depletion and capture have assumed that capture is unconstrained through boundary conditions that yield linear responses. Examination of real systems indicates that capture and depletion fractions are highly variable in time and space. For a large sample of long‐developed groundwater systems, the depletion fraction averages about 0.15 and the capture fraction averages about 0.85 based on cumulative volumes. Higher depletion fractions tend to occur in more arid regions, but the variation is high and the correlation coefficient between average annual precipitation and depletion fraction for individual systems is only 0.40. Because 85% of long‐term pumpage is derived from capture in these real systems, capture must be recognized as a critical factor in assessing water budgets, groundwater storage depletion, and sustainability of groundwater development. Most capture translates into streamflow depletion, so it can detrimentally impact ecosystems.

Depletion and response of deep groundwater to climate-induced pumping variability

Nature Portfolio (Springer Nature) | January 23rd, 2017

Groundwater constitutes a critical component of our water resources. Widespread groundwater level declines have occurred in the USA over recent decades, including

Groundwater constitutes a critical component of our water resources. Widespread groundwater level declines have occurred in the USA over recent decades, including in regions not typically considered water stressed, such as areas of the Northwest and mid-Atlantic Coast. This loss of water storage reflects extraction rates that exceed natural recharge and capture. Here, we explore recent changes in the groundwater levels of deep aquifers from wells across the USA, and their relation to indices of interannual to decadal climate variability and to annual precipitation. We show that groundwater level changes correspond to selected global climate variations. Although climate-induced variations of deep aquifer natural recharge are expected to have multi-year time lags, we find that deep groundwater levels respond to climate over timescales of less than one year. In irrigated areas, the annual response to local precipitation in the deepest wells may reflect climate-induced pumping variability. An understanding of how the human response to drought through pumping leads to deep groundwater changes is critical to manage the impacts of interannual to decadal and longer climate variability on the nation’s water resources.