Spatiotemporal trends and clustering of environmental flow components in western Lake Urmia Basin using IHA and multivariate analysis

Abstract Despite increasing concerns over hydrological degradation and ecosystem decline in the Lake Urmia Basin, comprehensive basin‑scale assessments of long‑term environmental flow alterations across multiple tributary rivers remain limited. The present study was therefore undertaken to quantify spatiotemporal trends in environmental flow components across the Lake Urmia Basin and to identify the key hydrological indicators responsible for differentiating the hydrological regimes of its tributary rivers. Daily discharge data from 44 hydrometric stations, covering periods between 1949 and 2023, were analyzed using the Indicators of Hydrologic Alteration (IHA) framework. The derived 22 environmental flow indicators were subsequently subjected to hierarchical clustering and Principal Component Analysis (PCA) to group stations based on hydrological similarity and to identify the most influential discriminating variables. We found considerable spatial heterogeneity in hydrological trends across the basin. Low‑flow persistence indicators, particularly the frequency and duration of extreme low flows, exhibited significant increasing trends at many stations (approximately 17 out of 44 for frequency and 11 out of 44 for duration). Notably, extreme low‑flow frequency contributed 22% to station differentiation, and the first two principal components together explained over 50% of the total variance. Cluster analysis identified two distinct hydrological regimes, a flood‑dominated cluster characterized by high peak flows and rapid rise rates, and a low‑flow cluster marked by prolonged dry periods. Stations located downstream of major storage dams experienced the most pronounced alterations, suggesting a possible influence of anthropogenic interventions on natural flow regimes. We aimed to provide a comprehensive basin-scale assessment of spatiotemporal environmental flow alterations in the western Lake Urmia Basin and present an integrated analytical framework for identifying dominant hydrological change patterns, which may be adapted by water managers in other arid and semi-arid basins facing similar challenges.