Most of the phenomena driving the most severe impacts on territories unfold extremely rapidly. Soil saturation after hours of persistent rainfall. Snowpacks changing structure within a single day. Moisture-laden air masses fuelling intense precipitation across the Mediterranean. River basins responding almost instantaneously to extreme meteorological forcing.
Yet many of today’s Earth-observing satellite systems are still unable to follow these processes continuously as they evolve. In many cases, we observe their final impacts more effectively than the dynamics through which they emerge.
It is precisely within this still largely unresolved observational gap that Hydroterra+ was conceived: one of the four candidate missions selected by the European Space Agency within the Earth Explorer 12 programme. A mission designed to transform the way we observe the planetary water cycle, enabling near-continuous monitoring of hydrological and atmospheric processes evolving over timescales of only a few hours.
For CIMA Research Foundation, Hydroterra+ also represents something more: the opportunity to lead scientifically a future European satellite mission dedicated to understanding water dynamics and extreme events. The project will enter a decisive phase in July 2026 during the ESA Earth Explorer 12 User Consultation Meeting in Tallinn, where the four candidate missions will be presented and two selected to proceed to the subsequent Phase-A development stage.
Hydroterra+: observing the water cycle on hourly timescales
At the core of the mission lies a deceptively simple scientific question: what truly happens to the water cycle while it evolves?
Water continuously moves through the atmosphere, soils, snow, vegetation, rivers, and groundwater systems. It is a dynamic and interconnected system controlling ecosystem behaviour, agriculture, water availability, and the development of extreme events. Yet many of the processes governing these interactions evolve on sub-daily timescales – too rapidly to be systematically observed by current satellite infrastructures.
Hydroterra+ was conceived to address this observational limitation.
“With Hydroterra+, we are not simply imagining a new satellite, but a new way of observing the water cycle. Many of the processes driving extreme events evolve too rapidly to be continuously monitored today. The possibility of observing them on sub-daily timescales would open entirely new perspectives for understanding hydrometeorological phenomena and managing climate risk,” explains Antonio Parodi, Programme Director at CIMA Research Foundation.
The mission is based on the GEOSAR concept: a geostationary synthetic aperture radar1 capable of continuously observing vast regions of the Earth instead of revisiting them only every few days, as is typical of many low-Earth-orbit satellite systems. This configuration would enable hourly or sub-daily acquisitions while maintaining a persistent view over the same regions.
From soil moisture to floods: what the ESA mission would observe
From a scientific perspective, this means being able to follow processes while they are unfolding.
Soil moisture could be monitored almost in real time; snow water content observed during accumulation and melt phases; atmospheric water vapour continuously analysed to better understand the development of intense precipitation. At the same time, the mission would rapidly detect changes associated with landslides, subsidence, tectonic activity, or damage caused by extreme natural events.
This is not simply about collecting more data, but about observing relationships that today remain only partially visible. Hydrological processes are tightly interconnected: soil saturation influences surface runoff 2, , which alters basin response and can intensify the impacts of extreme precipitation. Mountain snowpack controls seasonal water availability. Atmospheric moisture feeds convective systems capable of generating high-impact events within only a few hours.
The Mediterranean and sub-Saharan Africa: climate laboratories observed by Hydroterra+
Understanding these interactions is essential for improving the prediction and management of hydrometeorological extremes, particularly in regions highly vulnerable to climate change impacts.
For this reason, Hydroterra+ focuses especially on the Mediterranean and sub-Saharan Africa.
The mission identifies these regions as natural climate laboratories: territories where highly dynamic hydrological phenomena converge and where climate change is amplifying both the frequency and intensity of extremes.
Across the Mediterranean, mountain systems, intense convective activity, rapid snowpack transformations, and increasing hydrogeological vulnerability intersect within the same regional system. From the Alps and the Pyrenees to the Eastern Mediterranean, Hydroterra+ aims to observe processes associated with extreme precipitation, snowmelt, landslides, and flash floods.
Across the Sahel and West African river basins, meanwhile, the focus shifts toward water availability and the strong sensitivity of hydrological systems to precipitation variability. In regions where water resources, agriculture, and socioeconomic stability are deeply interconnected, improving our understanding of water-cycle dynamics also means strengthening climate adaptation capacity.
Hydroterra+ could generate substantial value for water security, territorial planning, agriculture, natural resource management, and reducing vulnerability to droughts, floods, storms, and landslides.
An unprecedented scientific and technological challenge
Within the ESA selection process, the mission is currently undergoing the scientific definition and justification phase for its observational products, together with the demonstration of their technical feasibility. The scientific team has already carried out several simulation experiments and application scenarios to demonstrate the mission’s capability to achieve the Scientific Readiness Level required by ESA.
The next milestone will be the international evaluation phase in Tallinn, Estonia, during the ESA Earth Explorer 12 User Consultation Meeting scheduled from 5 to 9 July 2026. There, the four candidate missions will be presented to the international scientific community. Only two will proceed to the subsequent development phase, while the final selection of the Earth Explorer 12 mission is expected in 2028.
“For CIMA Research Foundation, leading the scientific consortium of an Earth Explorer candidate mission represents both an extraordinary scientific challenge and a responsibility toward the international community working on Earth Observation and climate adaptation. We are contributing to the definition of a vision that integrates Earth Observation, environmental modelling, and the understanding of extreme processes,” Parodi adds.
A challenge looking toward the future of satellite observation, but rooted in a very concrete necessity: finally being able to observe the water cycle while it evolves, hour after hour, before its transformations translate into extreme impacts on the ground.
- A synthetic aperture radar (SAR) is a radar instrument capable of observing the Earth’s surface through cloud cover and independently of daylight conditions. In the case of Hydroterra+, the geostationary configuration would allow continuous monitoring of the same regions of the Earth, instead of observing them only during brief orbital revisits (ESA, European Space Agency). ↩︎
- Surface runoff is the flow of water moving across the land surface when the soil can no longer rapidly absorb rainfall or meltwater. This process plays a central role in the development of flash floods and flooding events (USGS, United States Geological Survey). ↩︎