March 19, 2021

Theia reveals annual mountain snow cover

After developing single-date images of snow cover, the partners of the Theia land surfaces data centre are now generating the first satellite maps providing a synoptic picture of annual snow cover in the Alps and Pyrenees.

The story begins back in 2015 with the launch of the Sentinel-2 Earth-observing satellite. Simon Gascoin, a hydrologist at the CESBIO biosphere space research centre, a French laboratory working on snow cover within the Theia land surfaces data centre, used the first images from the satellite to produce a snow cover map of his beloved Pyrenees. Since then, more than 50,000 snow cover products have been delivered for part of Europe, the Atlas Mountains and the Lebanon Mountains at unmatched resolution. These maps show the snow cover at a given date. Since this February, some of these single-data maps have been compiled to produce a picture of days of snow cover over a full year. The maps can be viewed for free on CNES’s website, providing a fresh perspective of the data in single-date products.

Snow cover duration in the Northern Alps between 1 September 2017 and 30 August 2018 derived from Theia snow cover products.
Credits : Simon Gascoin

Operationalizing production with Magellium

To scale up production of single-date snow cover maps, the process had to be transferred from the laboratory to industry. A consortium headed by Toulouse-based firm Magellium was selected in 2017 after a competitive tender from the European Environment Agency and is today running the processing systems and algorithms developed by CESBIO.

The maps produced by Magellium also indicate the presence of ice in lakes and rivers thanks to an algorithm designed by Polish firm Astri Polska. These maps are viewable for free, in the spirit of the European Copernicus programme that aims to ease access to data from Earth-observing satellites.

Freed from the constraints of generating single-date maps, Simon Gascoin and his colleagues devoted themselves to conceiving new products. The first of these, a synoptic snow cover map of the French Alps, is on line since February.

New Theia snow products in development

20-metre-resolution image of snow cover in the Mont Blanc and Vanoise ranges and the Val d'Aosta region in Italy between 1 September 2019 and 31 August 2020. Crédits: Theia

Since 2018, Theia snow products have been incorporated in a map-based interface of the Durance river basin to monitor snowpack there. Under the direction of the Durance Valley Syndicat Mixte joint planning authority, which encompasses the South region, four departments and 13 inter-borough councils, this interface helps to anticipate availability of water, improve flood forecasting and track the dynamics of artificial snow. Armed with a history of snow cover for a whole winter season or even a full year, planners could mine more information or analyse satellite data in different ways. This is the thinking behind the annual snow cover map products.

By combining snow cover data from more than 70 single-date maps within one annual map, Simon Gascoin and CNES engineer Aurore Dupuis aim to make such data easier to interpret and use not only for water planners in mountain areas but also nature parks tasked with creating hiking trails and monitoring wild flora and fauna, as well as livestock farmers looking to determine the best paths to pastures for their cattle and sheep.

Sentinel-2’s ability to acquire imagery every five days or less makes retracing variations in snow cover even more useful. And with the satellite’s 20-metre resolution, it all adds up to an annual map unmatched anywhere in the world.
Theia’s annual snow cover maps are an opportunity to uncover previously unsuspected relationships, causes and effects, aiding Copernicus’ objective of enabling new discoveries through free access to data.