Hoori Ajami

Parched Central Valley farms depend on Sierras for groundwater

Tracie Ventimiglia — Tue, 30 Jul 2024 21:22:29 +0000

Parched Central Valley farms depend on Sierras for groundwater Tracie Ventimiglia Tue, 07/30/2024 - 14:22 More News Jules Bernstein July 25, 2024

New research shows that California’s Central Valley, known as America’s breadbasket, gets as much as half of its groundwater from the Sierra Nevadas. This is significant for a farming region that, in some parts, relies almost entirely on groundwater for irrigation.

While it is easy to see above-ground reservoirs rise and fall with the rain and snow, aquifers are a natural water source hidden out of sight, in some cases hundreds of feet underground. “They are like giant bathtubs full of water and sediment,” said UC Riverside associate professor of groundwater hydrology Hoori Ajami.

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Rivers may not recover from drought for years

Tracie Ventimiglia — Mon, 13 Nov 2023 18:02:49 +0000

Rivers may not recover from drought for years Tracie Ventimiglia Mon, 11/13/2023 - 10:02 More News Jules Bernstein October 11, 2023

Lack of rainfall is not the only measure of drought. New UC Riverside research shows that despite a series of storms, the impact of drought can persist in streams and rivers for up to 3.5 years.

There are two measures of drought in streams. One measure is the total water level, which is impacted by snowmelt and rainfall. Many researchers examine this measurement. Another measure is baseflow, which is the portion of streamflow fed by groundwater.

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Hoori Ajami

Why the Salton Sea is turning into toxic dust

Mayra Gandarilla — Fri, 21 Oct 2022 18:30:34 +0000

Why the Salton Sea is turning into toxic dust Mayra Gandarilla Fri, 10/21/2022 - 11:30 More News Jules Bernstein October 06, 2022

"The Salton Sea, California’s most polluted inland lake, has lost a third of its water in the last 25 years. New research has determined a decline in Colorado River flow is the reason for that shrinking.

As the lake dries up, the concentration of salt and chemicals in the remaining water has increased dramatically, causing a mass die-off of fish and birds, including endangered species. The dry lakebed, coated in the salty, toxic water, becomes dust that causes respiratory problems for nearby residents."

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Hoori Ajami

Environmental Research

Hydrology

UCR experts weigh drought's long-term impacts

Anonymous — Fri, 10 Dec 2021 21:13:12 +0000

UCR experts weigh drought's long-term impacts Anonymous (not verified) Fri, 12/10/2021 - 13:13 More News UCR NEWS November 18, 2021

Gov. Gavin Newsom declared a drought emergency for the entire state of California this past month. The period from Oct. 1, 2020 to Sept. 30, 2021 was the second-driest year on record in California. Almost 90% of the state is in throes of extreme drought. Reservoir storage is 60% of average. Said one state water official: "It's almost beyond comprehension." We asked UCR experts what the future holds if drought conditions persist.

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Hoori Ajami

Critical groundwater supplies may never recover from drought

Anonymous — Wed, 13 Oct 2021 21:36:15 +0000

Critical groundwater supplies may never recover from drought Anonymous (not verified) Wed, 10/13/2021 - 14:36 More News September 30, 2021

"Along with hurricanes and wildfires, there's another important, but seldom-discussed effect of climate change — toxic water and sinking land made worse by groundwater drought.

Water from snow and rain seeps deep into the ground between layers of soil and accumulates in sponge-like underground bathtubs, called aquifers. Farmers rely heavily on this groundwater to irrigate their crops when they can’t get enough water from surface water sources."

Radio Interview

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Hoori Ajami

Shrub encroachment on grasslands can increase groundwater recharge

Anonymous — Sun, 09 Aug 2020 17:24:20 +0000

Shrub encroachment on grasslands can increase groundwater recharge Anonymous (not verified) Sun, 08/09/2020 - 10:24 More News Holly Ober | UCR News May 15, 2020

Grasslands across the globe, which support the majority of the world’s grazing animals, have been transitioning to shrublands in a process that scientists call “woody plant encroachment.”

Managed grazing of drylands is the most extensive form of land use on the planet, which has led to widespread efforts to reverse this trend and restore grass cover due to the belief that it results in less water entering streams and groundwater aquifers.

A new study led by Adam Schreiner-McGraw, a postdoctoral hydrology researcher at the University of California, Riverside, modeled shrub encroachment on a sloping landscape and reached a startling conclusion: Shrub encroachment on slopes can increase the amount of water that goes into groundwater storage. The effect of shrubs is so powerful that it even counterbalances the lower annual rainfall amounts expected during climate change.

Until now, researchers have thought that because woody plants like trees and shrubs have deeper roots than grass, woody plant encroachment resulted in less water entering streams and groundwater aquifers. This belief stemmed from scientists performing their related studies on flat ground.

“It is striking that ecosystem composition is what controls projected future changes to groundwater recharge,” Schreiner-McGraw said. “This does not mean that climate change is not important, but that vegetation change is potentially more important and something that scientists and land managers should focus more effort on understanding.”

Co-author Hoori Ajami, an assistant professor of groundwater hydrology at UC Riverside, said the paper looks at the combined effects of climate and vegetation change on groundwater-recharge processes in arid environments.

“Most studies to date have looked at these changes in isolation,” Ajami said. “Here we illustrate that the combined effects of vegetation change and climate change could be greater or less than the sum of its parts.”

The flume at the outlet of the watershed that measures streamflow. (Adam Schreiner-McGraw/UC Riverside)

The intrusion of shrubs into grasslands is often considered a problem because it reduces the amount of forage available for livestock grazing and can lead to more bare ground patches and subsequent increase in soil erosion. This process of creating more bare ground is called “xerification.” Climate change contributes to xerification, but fire suppression and overgrazing play the biggest roles.

It makes sense that shrubs, which have deep root systems along with thick stems and many leaves, capture more water than grass does as it percolates down through the soil, leaving less available water to replenish the underground aquifers. Research on “diffuse recharge,” the process by which water replenishes groundwater supplies over a large area, seems to bear this out for flat landscapes. Xerification of grasslands has thus been viewed as bad for both livestock and the water cycle.

“We approached this research with a simple premise that topography plays a role in redistributing available water, and this should affect the outcomes of xerification,” said co-author Enrique R. Vivoni, a professor at Arizona State University.

The group looked at focused recharge, which occurs when hillslopes funnel water into concentrated areas, such as streambeds. Streambeds often have sandy bottoms, which allow water to quickly infiltrate and prevent the deep-rooted shrubs from sucking it up.

Data from a highly monitored desert mountain slope in New Mexico was used to simulate the effects of woody plant encroachment and climate change on water resources. The team discovered that not only did the shrubs increase focused groundwater recharge, but that they did so even under conditions where climate change reduced the amount of rainfall.

They also modeled a more extensive form of shrub encroachment called thicketization, in which plants grow in dense stands with no bare patches, and found, as in prior flat landscape research, the shrubs reduced the amount of groundwater recharge on slopes as well.

On hillslopes, bare soil in between patches of shrubs is necessary to drive water into streambeds. Increased runoff increases focused groundwater recharge.

“We were surprised to find that a transition from grassland to shrubland can increase sustainability of groundwater aquifers,” said Schreiner-McGraw. “The best way to increase focused recharge in this system is to increase the amount of runoff from hillslopes that gets concentrated in the streambeds.”

Climate change will most likely increase groundwater recharge by making rainstorms larger, but less frequent. Larger storms increase the amount of runoff that reaches sandy-bottom channels and increases groundwater recharge. Findings from this study suggest vegetation will also play an important part in groundwater recharge in the future.

Though the study took place in New Mexico, Schreiner-McGraw said it applies to similar environments. Large parts of California are also desert savannahs. Southern California and the Central Valley have landforms similar to those found in the New Mexico study site. These areas could experience similar hydrological processes, though atmospheric rivers create storms very different from monsoon storms, so more research is required.

“The study highlights the role of long-term monitoring in understanding water balance dynamics of watersheds, and the role that process-based modeling plays in understanding system dynamics,” Ajami said.

The paper, “Woody Plant Encroachment has a Larger Impact than Climate Change on Dryland Water Budgets,” is published in Nature Scientific Reports. Other authors include Osvaldo E. Sala and Heather L. Throop of Arizona State University, and Debra P.C. Peters with the USDA Agricultural Research Service.

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Adam Schreiner-McGraw