One Water Vision: an international coalition supported by advances in satellite technology

Pressures on water resources are reaching critical levels on a global scale. Now that prolonged droughts, failing aquatic ecosystems, ever-increasing rainfall variability and increasing competition for water between agriculture, industry and population centres are becoming commonplace, a coordinated response must be achieved. In this context, the One Water Summit, organised jointly by France and Kazakhstan, was held in Riyad on 3 December 2024, bringing together high-level political and economic players, funders and scientists. 

This vision can again be seen in the announcement of the One Water Vision initiative, an international programme of scientific cooperation involving more than 26 institutions from different countries.

What sets One Water Vision apart from other international water programmes is its emphasis on Earth observations and new satellite missions, such as SWOT and TRISHNA.  The programme will make use of the services already provided and will take them to another level, providing users – river basin management bodies, countries and farmers – with practical and operational tools.

Satellite technologies to help the water cycle

The dual challenges of full coverage and high-quality data are the sine qua non of understanding and predicting the spatial and temporal dynamics of water as a resource. In this context, the SWOT (Surface Water and Ocean Topography) and TRISHNA (Thermal infraRed Imaging Satellite for High-resolution Natural resource Assessment) missions play a central role. These next-generation satellites, which are the products of international collaboration, bring hitherto unachievable resolution and accuracy to the analysis of water cycles, including coverage of isolated areas with little instrumentation. 

SWOT: a revolution for continental hydrology

Until now, most altimetric satellites have been used for oceanography, lacking sufficient accuracy to map rivers or small lakes. Launched in 2022, SWOT is the first satellite designed primarily for inland water hydrology. “It’s a revolutionary project, both in its nature and its technical reach”, Pierre-Olivier Malaterre, a researcher at INRAE and a specialist in hydraulics tells us. Indeed, SWOT provides complete 2-D coverage of water levels at global scale with unrivalled accuracy.

“The data transmitted by SWOT are of remarkably high quality and have exceeded our most optimistic predictions. They will be providing us with research data for the next ten to twenty years”, states Pierre-Olivier Malaterre. From its orbit, 891 km above the earth’s surface, the satellite uses radar, penetrating cloud cover and outperforming visual monitoring techniques. It maps more than 6 million lakes and reservoirs on each pass and is able to measure water levels to within approximately 10 centimetres. 

While a key aspect of its mission may be to collect data beyond the Earth’s oceans, the SWOT satellite also represents a major step forward for oceanographics, measuring sea levels “to the nearest centimetre”, says Pierre-Olivier Malaterre. These data are already being used for a variety of purposes, notably in calculating sea currents and predicting wave heights. They are also integrated into advanced meteorological forecasting models, thus contributing to improvements in the understanding and prediction of ocean dynamics.

Collaboration at top international level

The outcome of a Franco-American partnership between the CNES and NASA, the development of SWOT has depended on intensive collaboration between scientific colleagues. INRAE’s expertise in hydrology and hydraulics has earned it a place in this process:

• Recognised expertise. INRAE has been working with the CNES on this project for a decade, drawing on its researchers’ specialist knowledge of hydrology and hydraulics. One of INRAE’s researchers now occupies a key role in the SWOT scientific project: Hind Oubanas, based in Montpellier, has taken on the management of the hydraulics side of the project, having spent two years on secondment as a visiting scientist at NASA.
• Calculating river flows: SWOT measures the surface elevation, width and slope of river water, but does not directly provide data on the corresponding discharge of the rivers it monitors. With his US counterparts, Pierre-Olivier Malaterre oversees one of the mission’s required products, combining the observational data provided by the satellite with other data to compute river flows. The discharge estimates are produced by using six algorithms, validated by the SWOT Science Team. Two of these algorithms were developed and tested by the French team.

A practical example: the Congo basin 

The Congo basin, the second largest river basin in the world, offers an excellent opportunity to showcase SWOT’s potential for improving transboundary water resource management. With just two or three ground-based gauging stations in operation for this basin that extends across nine countries, data on water levels and flows are very sparse. SWOT fills this gap by providing regularly updated mapping of river and lake water levels that can be freely accessed by all the countries concerned. These data make it possible to improve the forecasting of flood and drought risk and to help build collaborative water management between riparian States.

A further major benefit of these satellite projects lies in the free open data they make available. This open access helps to build international cooperation and allows the integration of data with the primary purpose of providing critical support for resource management and risk forecasting.

TRISHNA: Observing ecosystem temperature and water dynamics

Jointly developed by the CNES (France) and the ISRO (India), TRISHNA will be launched in 2026. Equipped with a high-precision infrared thermal sensor developed by the French teams, TRISHNA records surface temperatures with a spatial resolution of 60 metres and a revisit frequency of 2 to 3 days.

The temperatures it measures serve as key indicators for the mapping of vegetation evapotranspiration rates across the entire surface of the globe. A major function of the water cycle, evapotranspiration returns water directly to the atmosphere without entering groundwater or river systems.

• Higher evapotranspiration levels reduce the earth’s surface temperature, since water consumes heat as it evaporates
• Plants under water stress reduce their transpiration levels, increasing surface temperatures.

“The data we make available on evapotranspiration will provide real added value for water management – up to now, it simply hasn’t existed”, stresses INRAE researcher Albert Olioso, a specialist in remote sensing and water resources and the creator of the mathematical models needed to calculate the relationship between the temperatures measured by TRISHNA and evapotranspiration.

The previous generation of satellites are indeed unable to provide precise and regular monitoring of evapotranspiration. TRISHNA, with its frequent passes and high resolution, will fill this gap. “For agronomists, access to these data on evapotranspiration will make it possible to improve predictions and optimise irrigation management”, explains Albert Olioso, stressing the benefits this scientific achievement will bring to the strategic planning of farm water management. 

Several satellite projects have already tried and failed to incorporate these parameters. In 2008, the CNES began its current push to find a solution, drawing on the combined expertise of INRAE and the IRD. Over the years, Jean-Pierre Lagouarde (INRAE) has shaped the mission in his role as Principal Investigator for a working group established by the CNES, making a decisive contribution to its progress. More recently, Albert Olioso has joined an international advisory group on the development of the LSTM (Land Surface Temperature Monitoring) satellite mission by the European Space Agency (ESA), ensuring its coordination with TRISHNA and other future satellite missions such as the US-Italy SBG mission. 

Strategic support for water and risk management

The data provided by TRISHNA will thus enable:

• optimisation of agricultural irrigation: precision adjustments to water provision based on crop needs
• prediction of environmental risks: monitoring of early signs of droughts or wildfires, often linked to reduced evapotranspiration
• modelling of climate impacts: integration of temperature and water data to refine weather and climate forecasting
• study of water dynamics: improving resource management for lakes, rivers and coastal zones.

The SWOT and TRISHNA missions are game changers in the observation and management of global water dynamics. With the One Water Vision initiative, the analysis of these data will make it possible to transform these satellites’ observations into practical actions that meet strategic challenges: