The operational open water extent mapping application displays the extent of open water at the moment of the last satellite overpass along the coastline, in lakes (the 90 largest), and in river floodplain areas. The mapping is based on Sentinel-1 satellite images, with a spatial resolution of approximately 5×20 m, thus details smaller than 10 meters cannot be assessed from the satellite images. Daily mapping covers the flight trajectory of the Sentinel-1. Since the width of the Sentinel-1 viewing area is 250 km, the entire region of Estonia cannot be mapped with a single satellite pass. The web map’s menu allows users to select a date by moving a slider, which visualizes the mapping of that overpass.

The statistics on the frequency of water occurrence are displayed as percentages, indicating the proportion of Sentinel-1 satellite overpasses that detected water during the chosen period. This information can be accessed by selecting a comparison period from the dropdown menu located in the upper right corner of the map. For instance, areas marked in dark blue indicate that the respective area was covered with water in 90-100% of the overpasses during the selected period. In contrast, light green areas show that water was present only in 10-20% of the satellite overpasses. The dropdown menu allows for the selection of the year and month whose statistics are visualized on the web map. All statistical maps are also downloadable (select the period and click “download”). The statistical maps are presented only for the snow-free period (April-October).

Sentinel-1 is equipped with synthetic aperture radar, which allows for monitoring regardless of light and weather conditions. Nonetheless, the reliability of water mapping is affected by 1) frozen ground, 2) wet snow, and 3) fields with low vegetation, where the radar’s registered backscattered signal resembles that of water-covered areas.

For more information on the methodology of water surface extent mapping from Sentinel-1 data: L. Sipelgas, A. Aavaste, and R. Uiboupin, “Mapping Flood Extent and Frequency from Sentinel-1 Imagery during the Extremely Warm Winter of 2020 in Boreal Floodplains and Forests,” Remote Sens.,vol. 13, no. 23, 2021, doi: 10.3390/rs13234949

The mediation and processing of satellite data are carried out at the national satellite data center ESTHub.