Water, water everywhere - but how to find it?
North Carolina State University
A new study finds that commercial satellite imagery data often outperforms public data sets when identifying surface water, but that public data sets may be better at detecting water hidden by forest cover.
Satellite imagery is a powerful tool for mapping surface water, from the movement of rivers and streams to water levels and even water temperatures. The effectiveness of those satellites depends on their ability to identify water in the images they capture. To do this, satellites use machine-learning algorithms to analyze color data across spectral bands, many of which are not visible to the human eye. This information comes from data sets that are either purchased commercially or available to the public, with commercial data typically having higher-resolution images with far more detail at the pixel level.
To understand the impact of higher-resolution imagery in detecting surface water, researchers compared the commercial PlanetBasemap data set to the Dynamic Surface Water Extent, a public data set built from the United States Geological Survey Landsat program. Lead author Mollie Gaines, who led the study as Ph.D. candidate at North Carolina State University, said that Planet Basemap’s higher resolution made it more capable of detecting small bodies of water.
“The Planet data is approximately four-meter resolution, which means that each pixel is approximately a four-by-four-meter square. That leads to a much more detailed image compared to the DSWE’s 30-meter resolution,” she said. “We’re seeing that the commercial data set often identifies more of the smaller water bodies, as well as river extents.”
However, Gaines said, that changes during seasons when high levels of vegetation obscure the water. The public DSWE data captures a wider portion of the electromagnetic spectrum resolution than PlanetBasemap, which makes it particularly good at detecting water hidden underneath vegetation.
“The Planet Scope data, which is what PlanetBasemap is built on, is limited to red, blue and green, or what the human eye can see, and near infrared,” she said. “DSWE includes the shortwave infrared band, which is the best option for this kind of water detection.”
This benefit was most pronounced when researchers included all three of DSWE’s “confidence classes,” categories that the classified satellite imagery data is sorted into based on how likely it is to contain water. With all three classes included, DSWE data captured more water in places like streams and rivers, where their winding paths can sometimes throw off imagery classifications.
These results show that both data sets have legitimate use cases, and that publicly available data is a strong option when used in the right circumstances.
“When studying very small bodies of water like ponds, the commercial data is the more reliable product,” she said. “But if you're looking at a larger study area, the publicly available product is a really good option.”
The paper, “Impact of spatial scale on optical Earth observation-derived seasonal surface water extents,” is published in Geophysical Research Letters. Co-authors include Mirela G. Tulbure, Darcy Boast, Rebecca Composto, Varun Tiwari and Júlio Caineta, NC State University; Vinicius Perin, Planet Labs Inc.; and Henry Castellanos Quiroz, Colombia Institute of Hydrology, Meteorology and Environmental Studies.
This research was supported by NASA FINESST Grant 80NSSC21K1606, NASA CSDA Grant 80NSSC24K0053, and MGT's funding through NC State. This work utilized data made available through the NASA Commercial Satellite Data Acquisition (CSDA) program.
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Note to editors: The abstract of the paper follows.
Impact of Spatial Scale on Optical Earth Observation-Derived Seasonal Surface Water Extents
Authors: Mollie Gaines, Mirela G. Tulbure, Darcy Boast, Rebecca Composto, Varun Tiwari and Júlio Caineta, NC State University; Vinicius Perin, Planet Labs Inc.; and Henry Castellanos Quiroz, Colombia Institute of Hydrology, Meteorology and Environmental Studies.
Published: Feb. 5, 2026 in Geophysical Research Letters
DOI: 10.1029/2025GL119880
Abstract: Landsat-derived products are the most prominent, publicly available sources of large-scale surface water extent data. However, few studies have assessed the limitations of spatial scale on such products. Here, we mapped seasonal surface water extents utilizing high-resolution (4.77 m) PlanetScope Basemap imagery and machine learning. We conducted a pixel-wise comparison of these high resolution classifications with a set of classifications from a moderate resolution (30 m) Landsat product. The vast majority (< 93%) of areas classified as water by the Landsat product were similarly classified by PlanetBasemap; however, only 65%–75% of the PlanetBasemap water area was also classified by the Landsat classes. Of the Landsat classes, only the partial surface water class comparably detects smaller water bodies (widths > 50–70 m) with PlanetBasemaps. Our results indicate that higher resolution imagery detects more small water bodies, which are instrumental to better understanding flood dynamics, methane emissions, and downstream water volume and quality.
Journal
Geophysical Research Letters
Method of Research
Imaging analysis
Subject of Research
Not applicable
Article Title
Impact of Spatial Scale on Optical Earth Observation-Derived Seasonal Surface Water Extents
Experimental discovery of a new critical point in water
image:
Image: POSTECH University, South Korea
view moreCredit: Image: POSTECH University, South Korea
Using x-ray lasers, researchers at Stockholm University have been able to determine the existence of a critical point in supercooled water at around -63 °C and 1000 atmosphere. Ordinary water at higher temperatures and lower pressures is strongly affected by the presence of this critical point, causing the origin of its strange properties. The findings are being published in the journal Science.
Water, both omnipresent and essential for life on earth, behaves very strangely in comparison with other substances. How water’s density, specific heat, viscosity and compressibility respond to changes in pressure and temperature is completely opposite to other liquids that we know.
All matter shrinks when it is cooled resulting in an increase in its density. One would therefore expect that water would have high density at the freezing point. However, looking at a glass of ice water, everything is upside down since ,as we all know, ice cubes float. Strangely enough for the liquid state, water is the densest at 4 degrees C, and therefore it stays on the bottom whether it’s in a glass or in an ocean.
If you chill water below 4 degrees, it starts to expand again. If you continue to cool pure water (where the rate of crystallization is low) to below 0 degrees, it continues to expand – the expansion even speeds up when it gets colder. Many more properties such as compressibility and heat capacity become increasingly strange as water is cooled. Now researchers at Stockholm University, with the help of ultra-short x-ray pulses at x-ray lasers in South Korea, have succeeded in determining that water has a critical point upon deep supercooling and that critical point is the source of the strange properties.
“What was special was that we were able to X-ray unimaginably fast before the ice froze and could observe how the liquid-liquid transition vanishes and a new critical state emerges”, says Anders Nilsson, Professor of Chemical Physics at the Department of Physics at Stockholm University. “For decades there has been speculations and different theories to explain these remarkable properties and one theory has been the existence of a critical point. Now we have found that such a point exists”.
Water is unique, as it can exist in two liquid macroscopic phases that have different ways of bonding the water molecules together at low temperature and high pressure. When the temperature increases and pressure decreases there is a state where distinction between the two liquid phases vanishes and only one phase is present. It is a point of large instability, causing fluctuations in a large temperature and pressure region all the way up to ambient conditions. The water fluctuates between the two liquid states and mixtures of the two as if it can’t make up its mind. It is these fluctuations that give water its unusual properties. The state beyond a critical point is called supercritical and ambient water is in that state.
Another remarkable finding of the study is that that the dynamics of the system slows down as it enters the critical point. “It looks almost that you cannot escape the critical point if you entered it, almost like a Black Hole”, says Robin Tyburski, researcher in Chemical Physics at Stockholm University.
“It’s amazing how amorphous ices, such an extensively studied state of water, happened to become our entrance to the critical region. It’s a great inspiration for my further studies and a reminder of the possibilities of making discoveries in much-studied topics such as water”, says Aigerim Karina, Postdoc in Chemical Physics at Stockholm University.
“It was a dream come true to be able to measure water under such low temperature condition without freezing” says Iason Andronis, PhD student in Chemical Physics at Stockholm University. “Many have dreamt about finding this critical point but the means have not been available before the development of the x-ray lasers”.
“I find it very exciting that water is the only supercritical liquid at ambient conditions where life exists and we also know there is no life without water. Is this a pure coincidence or is there some essential knowledge for us to gain in the future?“, says Fivos Perakis, an associate professor in Chemical Physics at Stockholm University.
“There has been an intense debate about the origin of the strange properties of water for over a century since the early work of Wolfgang Röntgen”, explains Anders Nilsson. “Researchers studying the physics of water can now settle on the model that water has a critical point in the supercooled regime. The next stage is to find the implications of these findings on waters importance in physical, chemical, biological, geological and climate related processes. A big challenge in the next few years.”
The study was done in cooperation with the POSTECH University and PAL-XFEL in South Korea, Max Planck Society and Johannes Gutenberg University in Germany, and St. Francis Xavier University in Candada. People from Stockholm University contributing to the study include Aigerim Karina, Robin Tyburski, Iason Andronis and Fivos Perakis. Other people that contributed to the study include previous members of the Chemical Physics group at Stockholm University - Kyung Hwan Kim, Marjorie Ladd-Parada and Katrin Amann-Winkel.
Further reading in Science: Experimental evidence of a liquid-liquid critical point in supercooled water by Seonju You and Marjorie Ladd Parada et al.
Science DOI: https://doi.org/10.1126/science.aec0018
Journal
Science
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
Experimental evidence of a liquid-liquid critical point in supercooled water
Article Publication Date
26-Mar-2026
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