California Ag Network

Tag: AGU

  • Central Valley Groundwater may be Unable to Recover from Past and Future Droughts

    American Geophysical Union — Groundwater in California’s Central Valley is at risk of being depleted by pumping too much water during and after droughts, according to a new study in the AGU journal Water Resources Research, an interdisciplinary journal that focuses on hydrology and water resources.

    The new study shows groundwater storage recovery has been dismal after the state’s last two droughts, with less than a third of groundwater recovered from the drought that spanned 2012 to 2016.

    Under a best-case scenario where drought years are followed by consecutive wet years with above-average precipitation, the researchers found there is a high probability it would take six to eight years to fully recover overdrafted water, which occurs when more groundwater is pumped out than is supplied through all sources like precipitation, irrigation and runoff.

    However, this best-case scenario where California has six to eight consecutive wet years is not likely because of the state’s increasingly hot and dry climate. Under a more likely, drier climate, there is less than a 20% chance of full overdraft recovery over a 20-year period following a drought.

    The Central Valley produces about a quarter of the nation’s food and is home to around 6.5 million people. Using too much groundwater during and after droughts could soon push this natural resource beyond the point of recovery unless pumping restrictions are implemented. The study finds recovery times can be halved with modest caps on groundwater pumping in drought and post-drought years.

    “This is really threatening,” said Sarfaraz Alam, a hydrologist at Stanford and lead study author. “There are many wells that people draw water from for drinking water. Since [groundwater is] always going down, at some point these wells will go dry and the people won’t have water.”

    Measuring Depletion

    The researchers combined NASA satellite data, well level data, detailed groundwater models and calculations of water inflows versus outflows to create a reliable assessment of groundwater storage data. They then used those data to predict how long it would take groundwater to fully recharge after droughts in the region under different climate scenarios.

    California has faced three major droughts since 2000: from 2007 to 2009, 2012 to 2016 and the state’s current drought period, which began in 2020. Researchers found that of the 19 cubic kilometers of groundwater (about 10% of the water volume in Lake Tahoe) lost during the 2006-2009 drought, only 34% was recovered after the drought. For the 2012-2016 drought, only 19% of 28 cubic kilometers lost were recovered.

    The researchers attributed especially low recovery in the post-2016 drought period to significant overdraft compared to limited water availability. 

    “It’s very hard to [measure] the volume of groundwater being pumped by humankind, and we really want to know that because we really want to know how much we’re depleting the groundwater,” said Donald Argus, a geophysicist who researches water resources at the NASA Jet Propulsion Laboratory who was not associated with the study. “If we start to understand how much water is replenished each year or each rainy season, then we get an idea of how much groundwater we’re pumping out, and whether we can sustain it or not.”

    Opportunities for Management 

    Despite the grave predictions of recovery time, researchers found that there is hope for increased water recovery when management practices are put into place. If California’s climate remains at historical levels, rather than worsening with climate change, groundwater extraction caps could significantly improve aquifer resistance to drought. Overdraft recovery times could be reduced by about two times if pumping restrictions are put in place during no-drought years and could be reduced by up to four times with pumping restrictions, according to the study.

    However, these management practices can create complicated trade-offs for laborers in the region, according to Alam. The livelihoods of those for those who depend on the region’s agricultural industry is threatened when pumping for agricultural purposes is capped to prioritize drinking water. But finding a balance of water supply and demand will be necessary to continue to use the Central Valley’s aquifer resource.

    “Drought comes, groundwater goes. It’s super fast,” Alam said. “The policymakers and decision makers need to ensure they are making the right decision to make sure groundwater use is well managed.”

    AGU (www.agu.org) supports 130,000 enthusiasts to experts worldwide in Earth and space sciences. Through broad and inclusive partnerships, we advance discovery and solution science that accelerate knowledge and create solutions that are ethical, unbiased and respectful of communities and their values. Our programs include serving as a scholarly publisher, convening virtual and in-person events and providing career support. We live our values in everything we do, such as our net zero energy renovated building in Washington, D.C. and our Ethics and Equity Center, which fosters a diverse and inclusive geoscience community to ensure responsible conduct.

  • Dryer, Warmer Night Air Is Worsening Some Western Wildfires

    Firefighters have reported that Western wildfires are starting earlier in the morning and dying down later at night, hampering their ability to recover and regroup before the next day’s flareup.

    A new study suggests why: The drying power of nighttime air over much of the Western U.S. has increased dramatically in the past 40 years. The paper was published in Geophysical Research Letters, the American Geophysical Union (AGU)’s journal for high-impact, short-format reports with immediate implications spanning all Earth and space sciences.

    “Nighttime is an important time in fire management. When fires die down at night it gives firefighters a chance to rest, move equipment and strategize. The problem firefighters are reporting is an unexpected increase in nighttime fire activity,” said lead author Andy Chiodi, a University of Washington research scientist at the Cooperative Institute for Climate, Ocean & Ecosystem Studies, a joint center with the National Oceanic and Atmospheric Administration. “Our findings support that this has been going on over the last 40 years over much, but not all, of the Western U.S.”

    Earth’s atmosphere is warming due to climate change and warming in many places has been greater at night. Warmer night air had been suspected as the culprit altering the daily pattern of wildfire activity, with burns continuing later into the night.

    The new study, however, shows it’s not just that the night air is warmer. The study found a dramatic shift from 1980 to 2019 in its drying power—how much moisture the nighttime air can carry away from the fuels—over much of the Western U.S. This shift is not captured in climate models, and the authors say it could be related to natural long-term cycles rather than to climate change.

    “We paid special attention to the change in recent years compared to the conditions seen in the ’80s and ’90s, which is when many of the current firefighters started their careers, and presumably formed their ideas about what normal fire behavior should look like,” Chiodi said. “We tried to quantify the changes that we were hearing about from firefighters.”

    MOISTURE DEFICIT

    The study looks at the “vapor pressure deficit,” or the difference between the moisture in the air and the saturation moisture level at that air temperature. This difference is a measure of the air’s drying power.

    “In the southern Sierra Nevada, the average summer nighttime vapor pressure deficit for the recent decade was 50% higher than the average in the ’80s and ’90s,” Chiodi said. “I was surprised—it’s unusual to see geophysical data change that dramatically.”

    Some of this shift in vapor pressure deficit is happening because warmer nighttime air, caused by climate change, produces higher saturation values. But part of the drying power is happening because the nighttime air in some regions has less moisture, and that effect is not predicted by climate change models, at least this much or in this pattern. The authors find a possible connection to the Pacific Decadal Oscillation, a long-term cycle that can influence inland weather.

    The increased drying power of nighttime air is especially pronounced in California’s San Fernando Valley and in the Bitterroot-Blue Mountain Region—including parts of the Idaho Panhandle, southeast Washington, northeast Oregon and western Montana.

    “Firefighters had been saying for several years that they feel some fires burn later into the evening than they used to,” said co-author Brian Potter at the U.S. Forest Service’s Pacific Wildland Fire Sciences Laboratory. “We found that in some areas, the amount of water in the air is decreasing, sort of doubling up on the warmer nights. These areas, including where the Snake River Complex and Lick Creek fires are burning right now, are much more likely to have fires burn late into the night.”

    The analysis used hourly weather outputs from the European Centre for Medium-Range Weather Forecasts. The recently released hourly reconstructions of historical weather allowed investigation of daily cycles.

    The next step, Chiodi said, is to further explore the causes of these changes in nighttime vapor pressure deficit. After that, he hopes to connect the atmospheric conditions more directly to fuel moisture and fire behavior.

    The other co-author is Narasimhan ‘Sim’ Larkin at the U.S. Forest Service’s Pacific Wildland Fire Sciences Laboratory in Seattle. The research was funded by the U.S. Forest Service through its AirFire research team and by NOAA (grants: 100007298, NA15OAR4320063). — By the University of Washington & the American Geophysical Union