GRACE Groundwater Drought Index: Evaluation of California Central Valley groundwater drought

Remote Sensing of Environment (Elsevier) | July 3rd, 2017

Quantitative approaches to assess the complexity of groundwater drought are hindered by the lack of direct observations of groundwater over space and time. Here,

Quantitative approaches to assess the complexity of groundwater drought are hindered by the lack of direct observations of groundwater over space and time. Here, we present an approach to evaluate groundwater drought occurrence based on observations from NASA's Gravity Recovery and Climate Experiment (GRACE) satellite mission. Normalized GRACE-derived groundwater storage deviations are shown to quantify groundwater storage deficits during the GRACE record, which we define as the GRACE Groundwater Drought Index (GGDI). As a case study, GGDI is applied over the Central Valley of California, a regional aquifer undergoing intensive human activities and subject to significant drought periods during the GRACE record. Relations between GGDI and other hydrological drought indices highlight our ability to capture drought delays unique to groundwater drought. Further, GGDI captures characteristics of groundwater drought that occur as a result of complex human activities and natural changes, thus presenting a framework to assess multi-driver groundwater drought characteristics.

Green Light for Adaptive Policies on the Colorado River

Water (MDPI) | December 21st, 2021

The Colorado River is a critical source of water supply for 40 million people in nine states spanning two nations in western North America. Overallocated in the 20th cent

The Colorado River is a critical source of water supply for 40 million people in nine states spanning two nations in western North America. Overallocated in the 20th century, its problems have been compounded by climate change in the 21st century. We review the basin’s hydrologic and water management history in order to identify opportunities for adaptive governance to respond to the challenge of reduced system flows and distill the ingredients of past successes. While significant advances have been made in the first two decades of the 21st century, these past actions have not been sufficient to halt the declines in the basin’s reservoirs. We find that the mix of federal, state, and local responsibility creates challenges for adaptation but that progress can be made through a combination of detailed policy option development followed by quick action at hydrologically driven moments of opportunity. The role of directives and deadlines from federal authorities in facilitating difficult compromises is noted. The current state of dramatically decreased overall flows has opened a window of opportunity for the adoption of water management actions that move the river system toward sustainability. Specific measures, based on the existing institutional framework and on policy proposals that have circulated within the Colorado River community, are suggested.

Groundwater and Remotely Sensed Phenology Reveal Vulnerability of Riparian Trees to Drought

Wiley | October 27th, 2025

The increasing frequency and magnitude of climatic extremes are altering water availability in dryland ecosystems globally. However, riparian vulnerability to hydroclimat

The increasing frequency and magnitude of climatic extremes are altering water availability in dryland ecosystems globally. However, riparian vulnerability to hydroclimate whiplash remains poorly understood. Here, we examined how riparian willow, cottonwood, and valley oak trees respond to groundwater fluctuations and drought through their water use patterns and phenology. To this end, we combined time-series analysis of in situ, high-frequency groundwater monitoring with high-resolution PlanetScope satellite imagery of a drought-prone and relatively pristine watershed in California (Chalone Creek, Pinnacles National Park). We found that flow regime dictates the potential for trees to access groundwater, while the identity of tree species determines the timing and magnitude of their use. Machine-learning models revealed that at intermittent sites groundwater depth predominantly controlled vegetation greenness, represented by the Normalized Difference Vegetation Index (NDVI). In contrast, variation in photoperiod length dominated at the perennial site where water was more reliably available. During the severe 2020–2022 drought, all species experienced reduced greenness, but phenological responses differed by flow regime. While the start of the season was delayed across all sites, trees at intermittent reaches exhibited a substantially earlier end of season during drought, resulting in growing seasons shortened by as much as 37 days. These phenological shifts vastly exceed those documented across aridity classifications in global datasets from satellite observations, ground-based monitoring networks, and experimental precipitation manipulations. Although riparian trees in drylands have been shaped by exposure to drought over evolutionary timescales, our findings show that trees in these intermittent systems may be operating close to critical groundwater thresholds, rendering them particularly vulnerable to increasingly long and severe droughts.

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-dependent ecosystem map exposes global dryland protection needs

Nature Portfolio (Springer Nature) | July 17th, 2024

Guidelines for the Small Community Drought Relief Program

California Department of Water Resources (DWR) | September 20th, 2022

Health of Native Riparian Vegetation and its Relation to Hydrologic Conditions along the Mojave River, Southern California

U.S. Geological Survey (USGS) | July 15th, 1999

Height-related changes in forest composition explain increasing tree mortality with height during an extreme drought

Nature Portfolio (Springer Nature) | July 7th, 2020

Recently, Stovall et al. showed that during an extreme drought, remotely sensed mortality of tall trees was more than double that of short trees. They interpreted this t

Recently, Stovall et al. showed that during an extreme drought, remotely sensed mortality of tall trees was more than double that of short trees. They interpreted this to be a consequence of inherently greater hydraulic vulnerability of tall trees, and suggested that tall-tree vulnerability should generalize more broadly. Here we reassess their conclusions using contemporaneous, ground-based data from near their study sites. We find that 90% of trees belong to taxonomic groups showing declining, not increasing, mortality with height, and that the overall increase in mortality with height is instead a consequence of height-related changes in forest composition, not intrinsically greater vulnerability of tall trees. Similar mechanisms likely explain mortality patterns at Stovall et al.’s sites, and, regardless, we show that their conclusions should not be accepted in the absence of robust tests of alternative mechanisms.

Hot Drought of Summer 2023 in Southwestern North America

American Geophysical Union (AGU) | September 17th, 2025

Hot droughts, or compound drought and heatwaves, have a significant impact on arid regions in southwestern North America (SWNA). In the summer of 2023, SWNA experienced u

Hot droughts, or compound drought and heatwaves, have a significant impact on arid regions in southwestern North America (SWNA). In the summer of 2023, SWNA experienced unusually intense hot droughts during both the daytime and nighttime, with their severity amplified three to five times above values during the prior four decades. Over this period, a significant increase was found in the coupling between nighttime and daytime conditions at local scales and across different regions of the SWNA. The hot drought was due to a suppressed North American monsoon (NAM) in response to synoptic-scale subsidence and moisture divergence. Nevertheless, we identified a remote connection between soil moisture in upwind areas and hot drought conditions in downwind regions across the US-Mexico border. Our findings indicate that hot droughts need to be analyzed over complete diurnal cycles and over regions that are connected through atmospheric pathways to improve their prediction, response, and mitigation.

How Much Snow?

Scientific American | March 1st, 1939

Last winter, at 682 different locations in the Rockies and high Sierras, more than 30,000 exact measurements of snow depth and water content were taken, in order to e

Last winter, at 682 different locations in the Rockies and high Sierras, more than 30,000 exact measurements of snow depth and water content were taken, in order to estimate how much water the mountain snowfields would supply in the run-off during summer months. To make this survey cost more than $100,000. On it was predicated the expenditure, and saving, of millions. While the science of surveying snow is still in its infancy, its extent and importance are increasing rapidly. Ridiculed at first, in the last few years it has taken on world-wide aspects. To grasp its significance in our western states, picture a series of snow-covered mountain ranges that by their streams make life possible in dry intervening valleys. Abundant snow means abundant water. Abundant water means power, and cheap electricity. More important still, it means abundant crops. Dearth of snowfall in the mountains means dry river beds in August, machinery standing idle, thirsty cattle, and dying plants.