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.