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In a sign of the ongoing threats to its precious groundwater stores, half a dozen regions in California rank among the world鈥檚 most rapidly declining aquifers, .

Globally, lack of local water drives migration, poverty, starvation and violence 鈥� while in California, it drives over how to stop over-pumping by growers.

Aquifers in Spain, Iran, China and Chile top the list of the 100 most rapidly dropping groundwater levels. , north of Santa Barbara, ranked 34th worldwide. Its underground basin has been dropping almost 5 feet a year, and residents, farmers and even the school district are locked in a court battle with carrot growers who .

Four other basins in the San Joaquin Valley and one in northeastern San Diego also netted spots in the top 100, with water levels falling up to almost four feet a year, according to the study, which was led by University of California and Swiss researchers and published in the journal Nature.

Only two other basins in the United States made the top 100: Gila Bend near Phoenix and Mill Creek in Idaho.

鈥淪ome of the rates of groundwater level decline occurring in California really are some of the highest in the world,鈥� said , a co-author of the study and an associate professor of hydrology, water resources and groundwater at UC Santa Barbara.

鈥淚t鈥檚 a sobering finding,鈥� he said. 鈥淲e鈥檝e got a lot of work to do here in California.鈥�

The research revealed that rapidly declining groundwater basins are virtually non-existent in places with no farming. Heavily-farmed regions in drier climates, such as the San Joaquin Valley, Iran and parts of India, are especially hard hit.

Plummeting groundwater levels . Streams and the 鈥� shrinking the storage capacity of aquifers and damaging roads, buildings, levees and other structures above ground.

In California, after 鈥� spreading from the San Joaquin Valley to the during the most recent drought.

Land in parts of the San Joaquin Valley has subsided so much that it has damaged the , which carries river water to Southern California, forced at least , and required around the sinking town of Corcoran to protect it from floodwaters.

The researchers analyzed more than 170,000 groundwater wells in more than 40 countries, and reported 鈥渨idespread acceleration in groundwater level deepening,鈥� which they said 鈥渉ighlights an urgent need for more effective measures.鈥�

The study provides a global database that backs up observations that have long worried water watchers.

鈥淭he major contribution is to bring into much sharper focus this global problem of groundwater depletion and over-pumping,鈥� said , a professor emeritus of hydrogeology at UC Davis who was not involved with the research.

鈥淲ith groundwater, if it鈥檚 left unmanaged and unregulated, it鈥檚 going to be abused in many, many cases. And if that abuse goes on long enough, some basins will be exhausted of water.鈥�

Violence over water is flaring around the globe. Water is a trigger, casualty and weapon in 鈥� from Russian troops destroying a Ukrainian dam to and Israeli military forces seizing or destroying Palestinian water sources. Clashes over water safety and scarcity have led to injuries and deaths around the world.

In California, disputes over water roil the state, from to and beyond.

San Joaquin Valley growers are still over-pumping

Ten years ago, alarmed by record declines in groundwater and thousands of dried up wells, California lawmakers passed a law aimed at stopping overpumping. The requires local agencies to achieve sustainable groundwater use by 2040 for the most critically overdrafted basins, and 2042 for basins considered less depleted.

But wells have and with few protections in place. So far, California water officials deemed plans for six San Joaquin Valley basins .

In addition to the Cuyama Valley, the Nature paper鈥檚 top 100 includes the (52nd), the in northeastern San Diego (55th), the straddling Merced and Madera counties (65th), the Northern Kern Basin (69th) and in Kings and Tulare counties (93rd).

Jasechko and his colleagues set out to understand how groundwater depletion in California compared to other aquifers globally. It took them six years to scour the literature for water level measurements, download it from databases and request it from water managers around the world.

For more than 540 aquifers, the researchers had enough data to compare groundwater levels over 40 years. Of those, about a third showed accelerating groundwater declines. Another 21% percent had increases in the 1980s and 1990s turn to losses over the past 23 years.

But Jasechko found some reasons for hope: 20% of aquifers saw groundwater declines slow down in the 21st century. Another 16% pivoted from groundwater decline to recovery, while 13% saw groundwater levels continue to increase.

鈥淟ong-term groundwater losses are neither universal nor inevitable,鈥� the researchers wrote.

Groundwater depletion in parts of Saudi Arabia slowed, for instance 鈥� possibly due to policies , including cultivation that . In Bangkok, Thailand, pumping slowed after officials increased fees.

And in the Coachella Valley, groundwater levels introduced a new pricing structure, increased recharge and improved access to Colorado River and recycled water supplies.

But UC Davis鈥檚 Fogg said that the research also brought clarity to what he called one of the existential challenges for the nexus between food, energy and water: how reining in groundwater depletion will affect the global food system. About 70% of water worldwide is used for agriculture and irrigation.

鈥淲e鈥檝e built a food supply system that relies in large part on irrigated agriculture, which in turn relies in many areas鈥�on pumped groundwater,鈥� Fogg said. 鈥淪o that has to change. That change will likely result in effects on the food supply. So it鈥檚 a major challenge to see how civilization can deal with that in the future.鈥�

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