With desert cities like Phoenix and Tucson bracing for their allotments of Colorado River water to be slashed dramatically, San Diego County’s water agency could for the first time sell some of its water to other states by drawing on its ample supplies from the nation’s largest desalination plant.

The San Diego County Water Authority’s board unanimously approved an initial agreement last week to consider selling some of its water to Arizona and Nevada, where cities that depend on the over-tapped Colorado River are expected to face substantial cuts in water supplies.

General Manager Dan Denham said the agreement, if approved by other agencies, could clear the way for the first-ever interstate transfers of Colorado River water starting next year.

“It’s just a different way of managing water in the West,” Denham said. “I think it has to happen now, and it has to happen because of the situation on the river.”

The Colorado River provides water for farms, cities and tribal communities across seven states and northern Mexico. Its reservoirs have declined dramatically over the last 25 years as drought compounded by climate change has shrunk the river flow.

Negotiators for the seven states, despite extensive talks, remain at odds on the water cutbacks each should accept to prevent reservoirs from declining further.

Officials at San Diego County Water Authority plan to consider selling water across state lines by letting agencies in Arizona and Nevada purchase some of its Colorado River supplies. The revenue generated, the San Diego County agency says, would be used to reduce costs for its ratepayers.

The agreement — technically a memorandum of understanding — first would need to be approved by the Metropolitan Water District of Southern California, the federal government and agencies in Arizona and Nevada. Then, various water agencies would need to negotiate the details.

The San Diego County Water Authority serves as a wholesaler, delivering water to 22 cities and other agencies that serve 3.3 million people.

The authority is in a position to sell a portion of the region’s water because it has invested heavily in securing additional water supplies from the Claude “Bud” Lewis Carlsbad Desalination Plant, which since 2015 has been drawing in seawater and churning out a portion of the area’s drinking water. The authority secured additional water through a 2003 agriculture-to-urban transfer deal. These and other investments have brought San Diego County plentiful water — though as a result, the region also has some of the most expensive water in the state.

The Water Authority purchases water from the Carlsbad desalination plant under a 30-year agreement. But the plant is currently operating at less than full capacity, Denham said, and its output could be increased to provide a larger share of the region’s water if agencies in other states bought some of the Water Authority’s Colorado River water. Essentially, it’s too expensive to run the plant at full capacity at this time given the availability of other more economical supplies, but out-of-state money could make it worth the agency’s while.

Conservation efforts have also reduced the water needs of many of the cities that the Water Authority serves. And in the next few years, the area will also start recycling wastewater at new facilities, including San Diego’s Pure Water project as well as other recycling projects in Oceanside and eastern San Diego County.

If other states and agencies sign on, Denham said the Water Authority is prepared to sell up to 10,000 acre-feet of water starting next year. That’s nearly 5% of the Las Vegas area’s current water use.

In future years, he said, that could increase to 25,000 acre-feet or more. And with additional investment in upgrades, the Carlsbad desalination plant could be expanded to transform more seawater into drinking water, thereby freeing up additional water to be traded to cities that need it.

“It’s in everyone’s best interest to make this work,” Denham said.

Gov. Gavin Newsom has supported the idea, telling governors of the other six states in a recent letter that California would welcome joint investments in water recycling and desalination.

Denham said Scott Cameron, the Trump administration’s acting head of the U.S. Bureau of Reclamation, has also supported the idea.

The board of the Metropolitan Water District has yet to vote on the proposed agreement.

“I believe these kinds of concepts are what we need to do in regards to thinking about water resources, not from political boundaries, but as a region,” said Shivaji Deshmukh, the district’s general manager.

If the San Diego County Water Authority eventually reaches deals with other agencies, the amount of water sold will be small compared to the gaping water shortfall that is pushing reservoir levels lower along the Colorado River. But Denham said if it works, this approach could be a first step in showing that deals between states can help ease the water shortages.

Superliminous supernovae are miraculous events. For astronomers, they also provide a vital tool for measuring cosmic distances and the rate at which the Universe is expanding. As part of the Cosmic Distance Ladder, these incredibly bright stellar explosions are the “standard candles” for objects billions of light-years away. In a rare event, researchers from the University of Munich, using the Large Binocular Telescope (LBT) in Arizona, witnessed a superluminous supernova 10 billion light-years away that was far brighter than most explosions of its kind.

What was especially amazing about this supernova was that it appeared five times in the night sky due to gravitational lensing by two foreground galaxies. These galaxies bent the path of the supernova’s light, causing it to take different paths. Because these paths have different lengths, the light appeared in different places around the galaxies at different times. By measuring the time delays between the multiple images, the researchers were able to obtain measurements of how fast the Universe is expanding – aka the Hubble-Lemaitre Constant.

The team consisted of researchers from the Technical University of Munich (TUM), the Max Planck Institute for Astrophysics (MPG), the Harvard & Smithsonian Center for Astrophysics (CfA), the E.O. Lawrence Berkeley National Laboratory, ETH Zurich, the Research Center for the Early Universe (RESCEU), the Cosmic Dawn Center (DAWN), the Ulugh Beg Astronomical Institute, the Chinese Academy of Sciences (CAS), Institute of Space Sciences (ICE, CSIC), the Excellence Cluster ORIGINS, the National Astronomical Observatory of Japan (NAOJ), the European Southern Observatory (ESO), the Space Telescope Science Institute (STScI), and multiple universities.

*Large Binocular Telescope on Mount Graham in Arizona, USA. Credit & ©: Dr. Christoph Saulder/MPE*

The paper describing their observations has been accepted for publication in Astronomy & Astrophysics

Only a few such measurements have been attempted to date because gravitationally lensed supernovae are so rare. It is also a challenging process, where astronomers must determine the masses of the lensing galaxies because this dictates how strongly the light from the background object is bent. To determine the masses of the two galaxies, the team obtained images with the LBT, using its two 8.4-meter (27.5 ft) mirrors and an adaptive optics system. The observations revealed two foreground lens galaxies at the center surrounded by five bluish images of the supernova explosion, making it look like fireworks!

Sherry Suyu, Associate Professor of Observational Cosmology at TUM and Fellow at the Max Planck Institute for Astrophysics, explained in an MPG press release:

We nicknamed this supernova SN Winny, inspired by its official designation SN 2025wny. It is an extremely rare event that could play a key role in improving our understanding of the cosmos. The chance of finding a superluminous supernova perfectly aligned with a suitable gravitational lens is lower than one in a million. We spent six years searching for such an event by compiling a list of promising gravitational lenses, and in August 2025, SN Winny matched exactly with one of them.

The image came as a surprise to the team since galaxy-scale lens systems normally produce only two or four copies. Using the positions of all five, junior researchers Allan Schweinfurth (TUM) and Leon Ecker (LMU) built the first model of the lens mass distribution. Said Allan Schweinfurth:

Until now, most lensed supernovae were magnified by massive galaxy clusters, whose mass distributions are complex and hard to model. SN Winny, however, is lensed by just two individual galaxies. We find overall smooth and regular light and mass distributions for these galaxies, suggesting that they have not yet collided in the past despite their close apparent proximity. The overall simplicity of the system offers an exciting opportunity to measure the Universe’s expansion rate with high accuracy.

This, in turn, could help astronomers and cosmologists relieve the ongoing issue of the Hubble Tension. To date, scientists have relied primarily on two methods to measure cosmic expansion: the Cosmic Distance Ladder and measurements of the Cosmic Microwave Background (CMB). The former is the local method, combining parallax, supernovae, and redshift measurements of bright objects to establish distances one step at a time. Since every step is dependent on the previous one, even small errors can add up and affect the final result.

In contrast, CMB measurements look back to the beginning of cosmic time by examining the “relic radiation” left over from the Big Bang. This approach is highly precise and relies on models of the early Universe to calculate its current rate of expansion. It relies heavily on assumptions about how the Universe evolved, however, which are still subject to debate. This study presents a third possible method in which astronomers use gravitationally lensed supernovae and measure the time delays between the multiple copies of the same image.

By calculating the mass distribution of the lensing galaxy, scientists can directly calculate the Hubble-Lemaitre Constant. “Unlike the cosmic distance ladder, this is a one-step method, with fewer and completely different sources of systematic uncertainties,” said Stefan Taubenberger, a leading member of Professor Suyu’s team and first author on their study.

Meanwhile, astronomers worldwide are observing SN Winny in detail with ground-based and space-based telescopes. Their results will provide new insights into cosmic expansion that could help resolve the Hubble Tension.

Further Reading: MPG