Zero-flow and hydrotype estimation with karst-SWAT and Sentinel-2 data in the Keritis Basin, Crete

Study region The Keritis Basin is located on the Island of Crete (Greece). Study focus This study presents a novel methodology integrating satellite imagery and hydrological modeling to provide daily forecasts of flow conditions in two non-perennial reaches of the Keritis River (Greece) during 2019–2023. A Karst-SWAT model was used to estimate daily streamflow and classified Sentinel-2 images were used to derive flow condition (flowing, ponding or dry). The combined dataset enabled the definition of critical thresholds to distinguish transitions from flowing to zero-flow events and between ponding and dry conditions. New hydrological insights for the region The integrated approach significantly improved the understanding of flow intermittency patterns at the daily scale by quantifying zero-flow events and estimating the hydrotype of the river reaches. Both studied reaches of the Keritis River were classified as Intermittent–Fluent. However, they exhibited pronounced interannual variability, largely driven by hydrological droughts. Water balance analysis indicates that deep aquifer recharge, due to the complex karst system, plays a primary role in controlling the occurrence of annual drying events, while evapotranspiration primarily modulates the frequency and magnitude of zero-flow peaks.