Diachronic drought assessment of Greek water-supply reservoirs using open EO and reanalysis data

Conventional drought monitoring in Greece has largely relied on in-situ measurements (rain gauges, reservoir records) to infer meteorological and hydrological indices. Despite the fact that the gauge measurements are valuable, most of the mountains basins lack of them. Moreover, reservoir information is not always frequent or openly available and meteorological indicators alone do not always reflect the evolving situation in major water-supply reservoirs. For this reason, Satellite Earth observation in combination with reanalysis data provide a strong complement. Satellite imagery allows reservoir water extent to be mapped directly and repeatedly, while meteorological data capture the spatial variability across entire river basins, supporting both situational awareness and longer-term analysis.

In this study, an open-data, long-term monitoring pipeline was implemented in Google Earth Engine, combining freely available satellite and reanalysis datasets. Monthly reservoir surface-water extent (2017–2025) was derived from Sentinel-2 optical imagery using multiple water indices (NDWI, MNDWI, AWEI) with consistent cloud/shadow masking and monthly compositing. A key element for “long-memory” drought assessment was added through the JRC Global Surface Water Monthly Recurrence dataset (1984–2021) from post-processed satellite retrievals, which provided an historical baseline. ERA5-Land reanalysis data were used to characterize climate conditions, including precipitation for the calculation of Precipitation Index SPI (3/6/12 months), temperature anomalies, a heat-ratio metric (share of days with daily Tmax above the historical 90^th percentile) and snow cover fraction for relevant mountainous headwaters.

The above methodology was applied for two water-supply systems under clear “emergency” pressure: the Attica system, where Mornos is the main source and Evinos supports it via transfer, and the Aposelemis system in Crete, which also depends on inflows linked to the Lasithi area. During 2024-2025 Attica experienced persistently low reservoir levels, with 2025 being among the lowest conditions since the Evinos reservoir was integrated and broadly comparable to the 2007–2008 major drought. In 2025, the Mornos reservoir declined from ~65% of its historical maximum extent in May to ~51% by September, marking the lowest levels recorded in the past two decades, despite limited meteorological relief during winter 2024/25. Evinos showed stronger monthly fluctuations, with values in the most stressed months commonly around ~60% of seasonal maxima. In Crete, Aposelemis shifted from high reservoir capacity during 2019–2022 (often ~80–90% of maximum extent) to a prolonged decline after 2023, reaching approximately one-third of maximum reservoir coverageduring 2025. This evolution is consistent with persistent precipitation deficits and increased heat stress across the region.

The integrated EO–reanalysis assessment showed that drops in reservoir levels often follow meteorological drought indicators with a delay of months to even years, highlighting the need for continuous monitoring. Using Google Earth Engine and open satellite and reanalysis data, a scalable open-data pipeline was developed for near-real-time drought tracking and water-resource awareness, supporting proactive drought management in Greece and other Mediterranean basins.