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OpenET: Balancing Water Supply and Demand in the West


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Preparations for Next Moonwalk Simulations Underway (and Underwater)

At the end of 2022, 65 percent of the Western United States was in severe drought, the result of a two decades long mega drought in the Colorado River Basin that had captured headlines around the world. 

However, it was flooding, not drought, that was making headlines when we began our research for this story about OpenET, a revolutionary new online platform geared towards helping farmers and water managers monitor and reduce water use in watersheds where supplies were not keeping up with demand.  

The start of 2023 brought flooding to many counties in California, leaving 68 percent of the state with suddenly little to no drought. And caused Forrest Melton, the NASA Project Scientist for OpenET and Associate Program Manager for agriculture and water resources with the NASA Earth Action program, to pause our video interview after a tree fell down outside his Bay Area home on a rainy day in March, 2023. 

Coming online again after calling the fire department, Melton didn’t seem all too optimistic that the wet conditions would last. “California tends to swing between the two extremes of drought and flood,” Melton said. He referenced the 2016/17 winter which had particularly high precipitation but was followed by dry conditions during the following years, before the relief brought by the heavy rains, and flooding, in early 2023. 

According to NOAA’s National Integrated Drought Information System it will take more than one wet winter to replenish groundwater in many parts of the western United States. Groundwater levels across the California Central Valley and many parts of the Ogallala Aquifer continue to decline. The need for better water management remains essential, and yet the data necessary to support new approaches has not been broadly available. 

Enter the OpenET project, a multi-disciplinary, collaborative effort to make satellite-based evapotranspiration (ET) data available to the public. Melton describes the project as providing invaluable and scientifically robust data at all scales, “that can be used to support day to day decision making and long range planning to try to solve some really long standing and important water management challenges in the West.”

What is Evapotranspiration?

Evapotranspiration is the combined process of evaporation and transpiration, both of which transfer water from the land surface to the atmosphere as water vapor. Evaporation transforms water from the surface of the ground or bodies of water into water vapor, while transpiration is water vapor that is evaporated from plant tissues and escapes through the stomata, the tiny pores in plant leaves and stems. It is a process that is happening all around us almost all the time, but because water vapor is invisible to the human eye, it is very hard to measure on the ground.  

A diagram shows what looks like a cross-section of a potato, with trees coming out of the left side, grass in the middle, two orange arrows point up from the left, and two blue arrows point down from beneath. Text on the diagram labels each section.
A conceptual diagram of near-surface hydrology.
M. W. Toews

To understand the effect evapotranspiration has on a local water cycle, picture a large decorative fountain. Typically, these fountains recycle the same water over and over. As a fountain runs, water is pumped out of the fountain heads, falls back into the fountain’s basin, and then flows back through the pipe system before starting the process all over again. We can think of the water remaining within this fountain’s local water system as non-consumptive water use. Some water, however, will be lost from the fountain’s local water system by evaporating from the pool’s surface or mist from the fountain’s spray.

Imagine the fountain also has lily pads growing in its basin. The lily pads will use the fountain’s water to survive and grow, losing some of that water to transpiration. The total water lost is evapotranspiration, and is considered consumptive water use, because it cannot be reused by the fountain. Tracking evapotranspiration can tell you how much water is removed or “depleted” from a local water system, and how much water needs to be added back in to support plant growth and maintain a healthy balance between water supply and water use. If too much water leaves the fountain, it will stop running. If too much water is added, it will overflow.  

These concepts can be applied more broadly to the hydrologic cycle as a whole, and evapotranspiration data can play an important part in designing and implementing sustainable water management practices to combat larger issues like drought, as well as both short and long-term reductions in water availability. Historically, ET data have been obtained from ground-based instruments and methods, such as weighing lysimeters, which weigh soil and plants to track the water volume lost by evaporation or transpiration. Another common method is called eddy covariance, which calculates the amount of water vapor transported away from the land surface by wind eddies as they move across the land surface. But both are expensive and difficult to install and maintain, and measurements are only representative of a small portion of an individual agricultural field. It is cost prohibitive to collect these measurements over larger areas. 

What makes OpenET different?

The OpenET team saw the important niche left open by traditional evapotranspiration measurement methods and filled it. They built upon decades of research funded by NASA, USDA and USGS and developed a new platform that can take easily accessible and already available data from satellite programs, like Landsat, and combine it with weather data to calculate the ET for every quarter acre of land. Satellites can record information like the Earth’s surface temperature and how much of the incoming light from the sun is being reflected back out to space. OpenET is able to use physically-based mathematical models to combine the satellite and weather data and output accurate data on evapotranspiration rates and volumes. 

This information is then made easily accessible through OpenET’s Data Explorer, a free web-based tool that allows anyone with an internet connection to access the data OpenET provides. Users begin by selecting an area of interest from a map of the western United States that provides data at the satellite resolution of a quarter-acre, and also broken down into known areas of interest and individual agricultural fields, each color coded with a heat map of evapotranspiration. Cooler colors indicate higher rates of evapotranspiration while warmer colors indicate lower rates. Users can zoom into specific areas on the map, and with just a click, a chart pops up showing the evapotranspiration trends for a given area, for the current year and the past five years. 

The chart can show monthly ET trends, useful for understanding seasonal fluctuations, and also cumulative trends, useful for understanding year-to-year changes in evapotranspiration. “The OpenET team took a user-driven design approach from the beginning, and each element of the Data Explorer and the open data services is there because a water manager or farmer asked for it,” Melton explained. As we played around with the map, it became apparent how much work was put into developing this project. Scientists needed to improve models and assess the accuracy of data, programmers had to develop the user interface and data services, designers needed to make the interface intuitive enough to be impactful, agriculture and environmental groups needed to help validate the model’s accuracy, and users of all types needed to provide requirements and then test the product to make sure their needs were actually met. 

The OpenET consortium includes NASA, USGS, USDA Agricultural Research Service (ARS), Environmental Defense Fund (EDF), Google Earth Engine, California State University Monterey Bay (CSUMB), Desert Research Institute (DRI), Habitat Seven, Chapman University, Cornell University, University of Nebraska-Lincoln and close to a dozen other universities and experts across the U.S. NASA Ames Research Center and CSUMB have played key roles in the scientific and technical leadership of the effort from the outset, working closely with DRI, EDF and the recently formed non-profit OpenET, Inc. In addition, over 100 partners from the water management, agriculture and conservation community provided user requirements and assisted with the design and testing of the OpenET platform and tools.

“OpenET would not be possible without the contributions of each one of those partners,” Melton said. “Both on the implementation side and those who are translating the data from OpenET into solutions to long standing challenges.” 

A satellite image is colored over with squares, like colored pixels, that range from deep blue to green to bright yellow.
Map of farmlands showing ET data for 2024. The cooler colors represent higher levels of evapotranspiration (ET), while warmer colors indicate areas with less ET.
OpenET

Models like those built into OpenET can be extremely useful tools for understanding patterns in ET and water use, but are only helpful if their accuracy is known. The OpenET science team recently completed the largest accuracy assessment to date for field-scale satellite-based ET data, comparing the satellite data to ground-based measurements at more than 150 sites across the U.S. Led by John Volk of the Desert Research Institute, the study was published in Nature Water earlier this year. A key finding was that across all sites, an ensemble value computed from six different ET models performed the best overall, leveraging the strengths of each individual satellite-driven model. 

However, the study also found that some models performed best for particular crop types or regions, which is important information for water managers and farmers who need the most accurate data possible. Publishing the results as an open access study with all data and analysis made publicly available was also important to build trust in the data. While the study highlighted some limitations of the models and priorities for future research, the rigorous and reproducible accuracy assessment helps to build user confidence that they can use the data, while being aware of the expected accuracy for different applications of the data.  

Bridging the Gap Between Farmers and Resource Managers

OpenET has already contributed to one significant win for farmers that affects how water use will be monitored and reported in the Sacramento-San Joaquin Delta. 

This inland river delta covers 750,000 acres and is an important water resource in California, but one where accelerated demand combined with habitat loss and water quality issues has led to major concerns. In the Delta, large portions of the agricultural land are below sea level. Levees protect the fields and contain the river channels that supply water for irrigation. In 2023, the state began requiring farmers to maintain a water meter or measuring device on each diversion, where water is diverted from a river for irrigation. However, this measurement proved challenging and costly as there are thousands of diversions in the Delta, and the measuring equipment was inaccurate and difficult to maintain in this environment. In addition, water users also had to pay for meters at the locations where water that drained from the fields was pumped back over the levees and into the river channels.

A satellite image shows a flat brown area of land, with a river that goes from left to right and splits into many tributaries that make it look like branches coming off a tree. The river has been colored in a bright teal.
The Sacramento-San Joaquin River Delta is a major water resource in California.
Matthew Trump

“Mostly, what the state was interested in was the consumptive use: how much (water) was actually removed from the supply in that region,” Melton said. “So, it’s the perfect place for using OpenET because evapotranspiration really is the majority of the consumptive use in the Delta, if not all of it.”

After the launch of OpenET, farmers in the Delta worked with the Delta Watermaster, the California State Water Resources Control Board, the OpenET team and the Delta Measurement Consortium to develop an alternative compliance plan that used OpenET data to help streamline the water use required reporting for this complex region. Once the alternative compliance plan was approved, Forrest Melton and Will Carrara of NASA worked with the state Water Resources Control Board, the Delta Watermaster and water management agencies, and Jordan Harding of HabitatSeven to implement this solution. The Delta Alternative Compliance Plan, also known as the Delta ACP, allows farmers to use OpenET data to estimate their water usage; enabling farmers to complete their use reports in a matter of minutes. 

“It’s the first time that satellite-based evapotranspiration data has been automatically integrated with a state-managed water reporting system,” Melton said. 

Last year, more than 70% of farmers in the Bay-Delta region chose to use OpenET and to report their water use through the Delta ACP website, and they expect this percentage to continue to increase over time. 

“The best part is that it is saving farmers hundreds of hours on preparing and submitting reports, avoiding millions of dollars in costs for farmers to deploy and maintain meters, and giving the state consistent and reproducible data on water use that has been reviewed and approved by the water user,” Melton said. 

According to Delta Watermaster, Jay Ziegler, this approach has a clear benefit in the unique water flow setting of the Delta. “In reality, OpenET – and the use of publicly accessible data measuring ET is the only way to really discern consumptive use of water in the Delta on a reliable basis,” Ziegler said. “Candidly, we don’t really have a viable “plan B” in the absence of applying Open ET for water use reporting.”

In reality, OpenET – and the use of publicly accessible data measuring ET is the only way to really discern consumptive use of water in the Delta on a reliable basis.

Jay ziegler

Jay ziegler

Sacramento-San Joaquin Delta Watermaster

Water Beyond Borders

As water scarcity is increasingly becoming an urgent issue all around the world, it’s easy to imagine how many countries could benefit from OpenET data. 

OpenET’s first international partnership is led by Anderson Ruhoff, a professor in Hydrology and Remote Sensing at the Federal University of Rio Grande do Sul, Brazil, where his team developed an evapotranspiration model called geeSEBAL for Brazil’s Water Agency.

Ruhoff learned about OpenET while he was in the US on a visiting professorship in Nebraska. He was intrigued and reached out to Melton who encouraged him to attend an upcoming conference in Reno, Nevada, where OpenET would be featured. The conference was due to start in just a few days time.

“So I had to find a last minute ticket to Reno and I’m glad I bought it, because when I arrived there they invited me to join Open ET. It was quite a coincidence,” Ruhoff said, smiling as he remembered the spontaneous decision. “We adapted our model for the US and started to participate in their work.”

In March, 2024, Ruhoff and OpenET launched an extension of the tool, called OpenET Brazil, with financial support from the Agência Nacional de Águas e Saneamento Básico (ANA), the Brazilian national water agency. The tool, called OpenET Brazil, will have similar goals as OpenET in the U.S., and the data collected will help improve Open ET’s accuracy overall.

Melton feels this will be a “great test case” for both working with new environmental conditions (in Brazil there frequently is more cloud cover than in the US during key parts of the growing season) and also developing new collaborations.

“The partnership will help us figure out how we can work with international partners to make the ET data useful,” Melton said. “The key aspect of our approach to geographic expansion is that leading scientists in each country and region, like Dr. Ruhoff, will lead the implementation, accuracy assessment, and the development of applications and partnerships for their country.”

Brazil has one of the world’s largest sources of freshwater, the Amazon River, and yet it can still be affected by drought. This is partly due to the fact that deforestation in the Amazon Rainforest has an impact on the entire region’s water cycle. Trees draw water up from the soil and during photosynthesis they release vapor into the atmosphere. This water vapor will accumulate and form precipitation. Trees are “basically a huge water pump,” Ruhoff said, and the Amazon Rainforest is large enough that it helps to produce the rainy season. But when deforestation is allowed to happen over large areas, that mechanism is interrupted. As a result of this disruption, the dry season is predicted to intensify, becoming longer and dryer, which in turn can affect crop production in Brazil as well as the rainfall that is critical for sustaining water supplies in Brazil and other areas of South America.

“Water doesn’t see borders. It doesn’t follow our rules,” Ruhoff said. “Deforestation in one place can affect people thousands of kilometers away.”

Water doesn’t see borders. It doesn’t follow our rules. Deforestation in one place can affect people thousands of kilometers away.

Anderson Ruhoff

Anderson Ruhoff

Professor of Hydrology and Remote Sensing, Federal University of Rio Grande do Sul, Brazil

Studying evapotranspiration can reveal the impacts of deforestation with even more clarity. And importantly, it’s also public information. “So not only the farmers and water managers but every citizen can check how much water is being used in their area, especially during drought. It’s democratic information in that way,” Ruhoff said. “I think it’s important to have this information openly available and to try and reach as many people as possible.”

Melton feels there’s the potential to expand the project, if more people like Ruhoff are there to lead the way.

“There’s huge potential, but there do need to be stakeholders that come to the table and say that this is something that they’re interested in,” Melton said. “Water is so important and at times so contentious that it’s really important the data is seen as trusted. When there is a local leader, that substantially increases the likelihood that it will be trusted, and most importantly, used to bring people together to develop solutions.”

An application interface shows a top-down view of a section of Brazil in pale tans and greens, with one square (at and angle) in bright colors: dark blue, teals, greens, and yellows.
The geeSEBAL application that Anderson Ruhoff’s team developed, which now informs the OpenET platform.
Science Direct/Anderson Ruhoff

Even when you live in a water-scarce region like California it’s easy to take water for granted. What platforms like OpenET can do for us, however, is make water, even in its most diffuse form, more visible to everyone.

Written by Jane Berg and Rachel Sender, co-published with the Bay Area Environmental Research Institute

To learn more about OpenET, visit https://etdata.org/

Program Contact:
Forrest Melton
NASA Ames Research Center
forrest.s.melton@nasa.gov

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Oct 14, 2024
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      To test whether this is true, Yeo and her colleague, Jason McLain, a research scientist at NASA Goddard, built a custom apparatus to examine Apollo lunar samples. In a first, the apparatus held all experiment components inside: a solar particle beam device, an airless chamber that simulated the Moon’s environment, and a molecule detector. Their invention allowed the researchers to avoid ever taking the sample out of the chamber — as other experiments did — and exposing it to contamination from the water in the air. 
      “It took a long time and many iterations to design the apparatus components and get them all to fit inside,” said McLain, “but it was worth it, because once we eliminated all possible sources of contamination, we learned that this decades-old idea about the solar wind turns out to be true.” 
      Using dust from two different samples picked up on the Moon by NASA’s Apollo 17 astronauts in 1972, Yeo and her colleagues first baked the samples to remove any possible water they could have picked up between air-tight storage in NASA’s space-sample curation facility at NASA’s Johnson Space Center in Houston and Goddard’s lab. Then, they used a tiny particle accelerator to bombard the dust with mock solar wind for several days — the equivalent of 80,000 years on the Moon, based on the high dose of the particles used. 
      They used a detector called a spectrometer to measure how much light the dust molecules reflected, which showed how the samples’ chemical makeup changed over time. 
      In the end, the team saw a drop in the light signal that bounced to their detector precisely at the point in the infrared region of the electromagnetic spectrum — near 3 microns — where water typically absorbs energy, leaving a telltale signature.  
      While they can’t conclusively say if their experiment made water molecules, the researchers reported in their study that the shape and width of the dip in the wavy line on their graph suggests that both hydroxyl and water were produced in the lunar samples.  
      By Lonnie Shekhtman
      NASA’s Goddard Space Flight Center, Greenbelt, Md.
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      Preparations for Next Moonwalk Simulations Underway (and Underwater)
      The Crew Module Test Article (CMTA), a full scale mockup of the Orion spacecraft, is seen in the Pacific Ocean as teams practice Artemis recovery operations during Underway Recovery Test-12 onboard USS Somerset off the coast of California, Saturday, March 29, 2025. NASA/Bill Ingalls Preparations for NASA’s next Artemis flight recently took to the seas as a joint NASA and Department of Defense team, led by NASA’s Exploration Ground Systems Program, spent a week aboard the USS Somerset off the coast of California practicing procedures for recovering the Artemis II spacecraft and crew.
      Following successful completion of Underway Recovery Test-12 (URT-12) on Monday, NASA’s Landing and Recovery team and their Defense Department counterparts are certified to recover the Orion spacecraft as part of the upcoming Artemis II test flight that will send NASA astronauts Reid Wiseman, Victor Glover, and Christina Koch, as well as CSA (Canadian Space Agency) astronaut Jeremy Hansen, on a 10-day journey around the Moon.  
      “This will be NASA’s first crewed mission to the Moon under the Artemis program,” said Lili Villarreal, the landing and recovery director for Artemis II. “A lot of practice led up to this week’s event, and seeing everything come together at sea gives me great confidence that the air, water, ground, and medical support teams are ready to safely recover the spacecraft and the crew for this historic mission.”
      A wave breaks inside the well deck of USS Somerset as teams work to recover the Crew Module Test Article (CMTA), a full scale replica of the Orion spacecraft, as they practice Artemis recovery operations during Underway Recovery Test-12 off the coast of California, Thursday, March 27, 2025.NASA/Joel Kowsky Once Orion reenters Earth’s atmosphere, the capsule will keep the crew safe as it slows from nearly 25,000 mph to about 325 mph. Then its system of 11 parachutes will deploy in a precise sequence to slow the capsule and crew to a relatively gentle 20 mph for splashdown off the coast of California. From the time it enters Earth’s atmosphere, the Artemis II spacecraft will fly 1,775 nautical miles to its landing spot in the Pacific Ocean. This direct approach allows NASA to control the amount of time the spacecraft will spend in extremely high temperature ranges.
      The Artemis II astronauts trained during URT-11 in February 2024, when they donned Orion Crew Survival System suits and practiced a range of recovery operations at sea using the Crew Module Test Article, a stand -in for their spacecraft.
      For the 12th training exercise, NASA astronauts Deniz Burnham and Andre Douglas, along with ESA (European Space Agency) astronaut Luca Parmitano, did the same, moving from the simulated crew module to USS Somerset, with helicopters, a team of Navy divers in small boats, NASA’s open water lead – a technical expert and lead design engineer for all open water operations – as well as Navy and NASA medical teams rehearsing different recovery scenarios.
      Grant Bruner, left, and Gary Kirkendall, right, Orion suit technicians, are seen with ESA (European Space Agency) astronaut Luca Parmitano, second from left, and NASA astronauts Deniz Burnham, center, and Andre Douglas, as they prepare to take part in Artemis recovery operations as part of Underway Recovery Test-12 onboard USS Somerset off the coast of California, Thursday, March 27, 2025. NASA/Joel Kowsky “Allowing astronauts to participate when they are not directly involved in a mission gives them valuable experience by exposing them to a lot of different scenarios,” said Glover, who will pilot Artemis II. “Learning about different systems and working with ground control teams also broadens their skillsets and prepares them for future roles. It also allows astronauts like me who are assigned to the mission to experience other roles – in this case, I am serving in the role of Joe Acaba, Chief of the Astronaut Office.” 
      NASA astronaut and Artemis II pilot Victor Glover, right, speaks to NASA astronauts Andre Douglas and Deniz Burnham as they prepare to take part in practicing Artemis recovery procedures during Underway Recovery Test-12 onboard USS Somerset off the coast of California, Friday, March 28, 2025.NASA/Joel Kowsky NASA astronaut Deniz Burnham smiles after landing in a Navy helicopter onboard USS Somerset during Underway Recovery Test-12 off the coast of California, Thursday, March 27, 2025.NASA/Bill Ingalls As the astronauts arrive safely at the ship for medical checkouts, recovery teams focus on returning the spacecraft and its auxiliary ground support hardware to the amphibious transport dock.
      Navy divers attach a connection collar to the spacecraft and an additional line to a pneumatic winch inside the USS Somerset’s well deck, allowing joint NASA and Navy teams to tow Orion toward the ship. A team of sailors and NASA recovery personnel inside the ship manually pull some of the lines to help align Orion with its stand, which will secure the spacecraft for its trip to the shore. Following a safe and precise recovery, sailors will drain the well deck of water, and the ship will make its way back to Naval Base San Diego.
      The Artemis II test flight will confirm the foundational systems and hardware needed for human deep space exploration, taking another step toward missions on the lunar surface and helping the agency prepare for human missions to Mars.
      About the Author
      Allison Tankersley
      Public Affairs Specialist
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      Last Updated Mar 31, 2025 Related Terms
      Missions Artemis 2 Exploration Ground Systems Exploration Systems Development Mission Directorate Orion Multi-Purpose Crew Vehicle Explore More
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    • By NASA
      6 min read
      Preparations for Next Moonwalk Simulations Underway (and Underwater)
      Located off the coast of Ecuador, Paramount seamount is among the kinds of ocean floor features that certain ocean-observing satellites like SWOT can detect by how their gravitational pull affects the sea surface.NOAA Okeanos Explorer Program More accurate maps based on data from the SWOT mission can improve underwater navigation and result in greater knowledge of how heat and life move around the world’s ocean.
      There are better maps of the Moon’s surface than of the bottom of Earth’s ocean. Researchers have been working for decades to change that. As part of the ongoing effort, a NASA-supported team recently published one of the most detailed maps yet of the ocean floor, using data from the SWOT (Surface Water and Ocean Topography) satellite, a collaboration between NASA and the French space agency CNES (Centre National d’Études Spatiales).
      Ships outfitted with sonar instruments can make direct, incredibly detailed measurements of the ocean floor. But to date, only about 25% of it has been surveyed in this way. To produce a global picture of the seafloor, researchers have relied on satellite data.
      This animation shows seafloor features derived from SWOT data on regions off Mexico, South America, and the Antarctic Peninsula. Purple denotes regions that are lower relative to higher areas like seamounts, depicted in green. Eötvös is the unit of measure for the gravity-based data used to create these maps.
      NASA’s Scientific Visualization Studio Why Seafloor Maps Matter
      More accurate maps of the ocean floor are crucial for a range of seafaring activities, including navigation and laying underwater communications cables. “Seafloor mapping is key in both established and emerging economic opportunities, including rare-mineral seabed mining, optimizing shipping routes, hazard detection, and seabed warfare operations,” said Nadya Vinogradova Shiffer, head of physical oceanography programs at NASA Headquarters in Washington.
      Accurate seafloor maps are also important for an improved understanding of deep-sea currents and tides, which affect life in the abyss, as well as geologic processes like plate tectonics. Underwater mountains called seamounts and other ocean floor features like their smaller cousins, abyssal hills, influence the movement of heat and nutrients in the deep sea and can attract life. The effects of these physical features can even be felt at the surface by the influence they exert on ecosystems that human communities depend on.
      This map of seafloor features like abyssal hills in the Indian Ocean is based on sea surface height data from the SWOT satellite. Purple denotes regions that are lower relative to higher areas like abyssal hills, depicted in green. Eötvös is the unit of measure for the gravity-based data used to create these maps.NASA Earth Observatory This global map of seafloor features is based on ocean height data from the SWOT satellite. Purple denotes regions that are lower compared to higher features such as seamounts and abyssal hills, depicted in green. Eötvös is the unit of measure for the gravity-based data used to create these maps.NASA Earth Observatory This map of ocean floor features like seamounts southwest of Acapulco, Mexico, is based on sea surface height data from SWOT. Purple denotes regions that are lower relative to higher areas like seamounts, indicated with green. Eötvös is the unit of measure for the gravity-based data used to create these maps.NASA Earth Observatory Mapping the seafloor isn’t the SWOT mission’s primary purpose. Launched in December 2022, the satellite measures the height of water on nearly all of Earth’s surface, including the ocean, lakes, reservoirs, and rivers. Researchers can use these differences in height to create a kind of topographic map of the surface of fresh- and seawater. This data can then be used for tasks such as assessing changes in sea ice or tracking how floods progress down a river.
      “The SWOT satellite was a huge jump in our ability to map the seafloor,” said David Sandwell, a geophysicist at Scripps Institution of Oceanography in La Jolla, California. He’s used satellite data to chart the bottom of the ocean since the 1990s and was one of the researchers responsible for the SWOT-based seafloor map, which was published in the journal Science in December 2024.
      How It Works
      The study authors relied the fact that because geologic features like seamounts and abyssal hills have more mass than their surroundings, they exert a slightly stronger gravitational pull that creates small, measurable bumps in the sea surface above them. These subtle gravity signatures help researchers predict the kind of seafloor feature that produced them.
      Through repeated observations — SWOT covers about 90% of the globe every 21 days — the satellite is sensitive enough to pick up these minute differences, with centimeter-level accuracy, in sea surface height caused by the features below. Sandwell and his colleagues used a year’s worth of SWOT data to focus on seamounts, abyssal hills, and underwater continental margins, where continental crust meets oceanic crust.
      Previous ocean-observing satellites have detected massive versions of these bottom features, such as seamounts over roughly 3,300 feet (1 kilometer) tall. The SWOT satellite can pick up seamounts less than half that height, potentially increasing the number of known seamounts from 44,000 to 100,000. These underwater mountains stick up into the water, influencing deep sea currents. This can concentrate nutrients along their slopes, attracting organisms and creating oases on what would otherwise be barren patches of seafloor.
      Looking Into the Abyss
      The improved view from SWOT also gives researchers more insight into the geologic history of the planet.
      “Abyssal hills are the most abundant landform on Earth, covering about 70% of the ocean floor,” said Yao Yu, an oceanographer at Scripps Institution of Oceanography and lead author on the paper. “These hills are only a few kilometers wide, which makes them hard to observe from space. We were surprised that SWOT could see them so well.”
      Abyssal hills form in parallel bands, like the ridges on a washboard, where tectonic plates spread apart. The orientation and extent of the bands can reveal how tectonic plates have moved over time. Abyssal hills also interact with tides and deep ocean currents in ways that researchers don’t fully understand yet.
      The researchers have extracted nearly all the information on seafloor features they expected to find in the SWOT measurements. Now they’re focusing on refining their picture of the ocean floor by calculating the depth of the features they see. The work complements an effort by the international scientific community to map the entire seafloor using ship-based sonar by 2030. “We won’t get the full ship-based mapping done by then,” said Sandwell. “But SWOT will help us fill it in, getting us close to achieving the 2030 objective.”
      More About SWOT
      The SWOT satellite was jointly developed by NASA and CNES, with contributions from the Canadian Space Agency (CSA) and the UK Space Agency. NASA’s Jet Propulsion Laboratory, managed for the agency by Caltech in Pasadena, California, leads the U.S. component of the project. For the flight system payload, NASA provided the Ka-band radar interferometer (KaRIn) instrument, a GPS science receiver, a laser retroreflector, a two-beam microwave radiometer, and NASA instrument operations. The Doppler Orbitography and Radioposition Integrated by Satellite system, the dual frequency Poseidon altimeter (developed by Thales Alenia Space), the KaRIn radio-frequency subsystem (together with Thales Alenia Space and with support from the UK Space Agency), the satellite platform, and ground operations were provided by CNES. The KaRIn high-power transmitter assembly was provided by CSA.
      To learn more about SWOT, visit:
      https://swot.jpl.nasa.gov
      News Media Contacts
      Jane J. Lee / Andrew Wang
      Jet Propulsion Laboratory, Pasadena, Calif.
      818-354-0307 / 626-379-6874
      jane.j.lee@jpl.nasa.gov / andrew.wang@jpl.nasa.gov
      2025-040
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      Last Updated Mar 19, 2025 Related Terms
      SWOT (Surface Water and Ocean Topography) Earth Jet Propulsion Laboratory Oceans Explore More
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