Hydrological regime shifts and their implications for drought monitoring in a Mediterranean river basin

Hydrological non-stationarity complicates drought assessment and water-resources management. This study analyses monthly streamflow records from five gauging stations in the Seyhan River Basin to identify persistent changes in flow regimes and evaluate their implications for hydrological drought monitoring. Change-point detection, regime-aware trend analysis, and distribution-based modelling are applied to distinguish abrupt regime shifts from gradual trends and to quantify associated changes in streamflow characteristics. Results reveal a coherent basin-wide regime shift between 1999 and 2000. This transition is marked by substantial reductions in mean discharge and, at several stations, increased variability, indicating a shift towards a lower-yield and more variable flow regime. Trend analyses show that apparent long-term declines largely reflect this abrupt shift rather than persistent monotonic change. Drought indices based on fixed historical reference periods systematically overestimate post-shift drought severity. In contrast, non-stationary standardized frameworks provide a more consistent characterization of drought conditions under evolving flow regimes.