EUMETSAT’s H SAF snow water equivalent maps supporting water management in Italy

Northern Italy, dominated by the Po River basin and the Alpine headwaters, is one of Europe’s most climate sensitive hydrological systems. Recent analyses, including climate models and long term meteorological observations, reveal several coherent trends in water availability, drought severity, snow dynamics, and streamflow behaviour. Since the early 2000s, the Po River basin has entered a period of recurring drought, amplified by global warming (Faranda et al., 2023). Warmer temperatures are also driving a decrease in snow depth and snow covered area, reducing meltwater contributions to spring and summer streamflow. Six decades of meteorological data from the Po River basin show increasing drought severity, with temperature emerging as a key intensifier. The Po is then transitioning from a reliable perennial river into a system prone to seasonal and interannual water shortages.

Italy as a whole is a topographically and climatically complex region including some of the highest mountain peaks in Europe (the Alps and the Apennines). The central question is: how much snow is accumulated across the landscape at any given time? Determining the most accurate approach for quantifying snow amount and its spatial distribution remains an open question, as several options exist—including in situ measurements, remote sensing data, and models—each with distinct advantages and limitations. For other geographical areas, a successful approach has been the definition of models that assimilate in situ and remote sensing data to obtain reanalysis products that provide reliable estimates of the snow water resources.

Water management in Italy operates at multiple levels, with responsibilities shared between the national government and local authorities. Regions, watershed districts, and municipalities all play a role, especially during emergencies. For example, local mayors can issue urgent water-use restrictions in response to extreme events such as droughts or water shortages. These local measures reflect how the hydroclimatic conditions directly affect communities.

Snow is a key part of this system, as it acts as a natural water reserve that feeds rivers and supports water availability during spring and summer. Thus, monitoring how much water is stored in the form of snow in the mountains is essential for effective water management.

To support this need, the CIMA (International Centre for Environmental Monitoring) Research Foundation developed S3M Italy, an operational system used in civil protection activities. It combines data from weather stations, satellites (Sentinel-2/MIS, TERRA/MODIS (up to September 2024); Suomi NPP/VIIRS (since September 2024), and METEOSAT/SEVIRI), and ground-based snow sensors to provide up-to-date information on snow conditions. This allows local to national authorities to monitor snow amount in near real-time and make informed decisions, especially during critical periods.

Building on the integration of satellite and ground-based observations, we developed a set of operational products designed to support decision-making. A key component is a consistent, high-resolution snow dataset for Italy (IT-SNOW), which provides daily estimates of snow water equivalent (SWE) across the country from 2010 to present. This long-term consistency allows not only real-time monitoring, but also the construction of a robust climatology of snow conditions.

Using this reference baseline, we generated anomaly maps and further indicators that show how current snow conditions compare to historical averages. These products make it possible to quickly assess whether a given season is characterized by snow deficit or snow excess, and to quantify the magnitude of events such as the 2023–2024 snow deluge in terms of stored water. This type of information is particularly relevant for water management, as it translates snow conditions into a metric directly linked to water availability.

To ensure accessibility and impact beyond the scientific community, these products are disseminated through dedicated dashboards and visual tools, designed for both technical users and the general public (https://www.cimafoundation.org/en/italy-snow-updates/). In parallel, we actively engaged with national and international media outlets to communicate the evolving snow conditions and their impacts. This combination of near real-time information, historical context, and targeted communication helped to turn Earth observation data into actionable data for decision-makers and to inform society.

To better understand how much water is stored in snow across Italy, we relied on a combination of satellite data, ground measurements, and modeling. Key Earth Observation (EO) inputs included snow-cover maps from Sentinel-2/MSI and MODIS/VIIRS satellites, as well as products from METEOSAT/SEVIRI (provided by EUMETSAT’s H SAF). Sentinel-2 and MODIS provide high spatial resolution but at rather low frequency and METEOSAT provides measurements at high frequency but rather low spatial resolution. Combining those provides frequent wide area observations of snow conditions.

These satellite data were combined with information from thousands of weather stations and hundreds of snow sensors on the ground. By integrating all these sources in the S3M Italy system, we produced a dataset called IT-SNOW, which delivers daily maps of key variables such as snow depth and snow water equivalent at high spatial resolution (about 500 meters), from 2010 to now.

This approach helps overcome the limitations of using any single data source alone. Satellite observations provide broad spatial coverage, while ground measurements improve accuracy, and modelling ensures consistency over time. The resulting information allows users to track how snow accumulates and melts across the country, offering a reliable and up-to-date picture of snow water resources to support water management decisions.

The 2023–2024 winter in Italy provides a clear example of how Earth observation–based snow information can support real-world decision-making. After months of severe snow deficit (down to –53% compared to historical conditions by mid-February), northern Italy experienced an abrupt transition in early March 2024, when two weeks of intense snowfall led to a surplus of +13% in snow water resources by early April. This rapid shift, effectively ending a multi-year snow drought in the Alpine region, had immediate implications for water availability, hydropower production, and drought mitigation.

Through S3M Italy, this evolution was tracked in near real-time, allowing authorities and stakeholders to quantify not only the magnitude of the recovery, but also its spatial distribution. Crucially, the system revealed that most of the surplus was concentrated above 2000 m, while lower elevations remained in deficit. This information is essential for water management, as it indicates potential mismatches between total snow availability and its timing and usability for downstream needs.

The ability to place current conditions in a historical context, through anomaly maps and climatological comparisons, provided an intuitive way to communicate the severity of both the initial deficit and the subsequent recovery. These insights supported local to national authorities, watershed management, and civil protection agencies in assessing water resources and planning accordingly during a highly variable season.

Beyond institutional users, the work also had a strong societal impact. Snow conditions and their impacts were widely communicated through dedicated dashboards and extensive media coverage at both national and international levels. This helped to raise public awareness of the links between climate variability, mountain snow, and water security.

Overall, the 2024 snow deluge illustrates the challenges posed by increasing climate variability: while extreme snowfall events can temporarily alleviate drought conditions, their uneven distribution and dependence on temperature highlight the ongoing vulnerability of Italy’s water system. The integration of Earth observation data into operational tools such as S3M Italy is therefore key to supporting informed, timely, and transparent decision-making.

The S3M Italy output is also operational on the MyDewetra platform, which is used by the Italian Civil Protection System at both regional and national levels. In particular, Snow Water Equivalent (SWE) information is employed by regional authorities to support hydrological assessments and hydraulic alert procedures. On 1 April 2024, a snowmelt-driven event affecting SWE dynamics impacted the Po River basin, resulting in the issuance of national yellow and orange hydraulic alerts for 2 and 3 April 2024.

An example of the operational use of S3M in Italy is provided by the Eastern Alps River Basin District, which has integrated S3M outputs into its routine water management activities. According to Nico Dalla Libera, "the integration of S3M into our operational workflow has enabled a step change in how snow water resources are monitored. Previously, assessments relied mainly on point-based in-situ observations, which could not fully capture the spatial heterogeneity of snow across basins and elevation ranges. With S3M, we now have access to spatially distributed, near-real-time estimates of snow water equivalent (SWE), which we incorporate into our monthly bulletins and drought severity assessments. This has strengthened the evidence base for water management decisions and improved our capacity to monitor water availability, particularly during critical periods of water scarcity."