Linking Hydrological Drought Progression to Water Quality Recovery Using a Multi-Stage Variable-Threshold Approach

Hydrological drought, a critical phenomenon with cascading impacts on water resources, has traditionally been assessed through quantity-based indices, often neglecting its profound influence on water quality. This study addresses this gap by proposing and applying a novel Three-Stage Variable-Threshold (3S-VT) framework for high-resolution identification and characterization of hydrological drought events, followed by an analysis of water quality recovery dynamics. The framework dissects droughts into Development, Persistence, and Retreat stages using seasonally variable daily flow thresholds (25th percentile) in the Gorganroud River Basin, Iran. A comparative analysis is conducted against the widely used Standardized Streamflow Index (SSI) at multiple timescales (1-, 3-, 6-, and 12-month). The results reveal that the 3S-VT framework identifies fewer but longer-duration events, effectively capturing persistent, multi-seasonal droughts critical for long-term water resource stress, while the SSI is more sensitive to short-term, flash droughts. Spatially, an east-west hydro-climatic gradient is evident, with eastern high-altitude stations experiencing snowmelt-dependent, long-duration droughts, and western lowland stations facing intensifying, warm-season droughts driven by evapotranspiration. Furthermore, the study quantifies significant time lags in water quality recovery post-drought. Key parameters (e.g., TDS, EC, major ions) required an average of 52 days to return to baseline conditions after hydrological drought termination, with upstream areas showing slower recovery (up to 71 days) compared to downstream zones (~ 37 days), highlighting differential basin resilience. This research underscores the necessity of integrating multi-stage drought progression with water quality impact assessment for holistic water resource management, offering a refined tool for drought monitoring, long-term planning, and developing tailored adaptation strategies in climate-sensitive basins.