A copula-based framework for assessing compound hydrological drought and water quality degradation risks in semi-arid regions

This study develops and applies a copula-based framework for the probabilistic assessment of compound hydrological drought and water quality degradation risks, using semi-arid northern Iran as a case study. Based on the analysis of 30 years of data, the framework establishes critical relationships between Standardized Streamflow (SSI) and Standardized Water Quality (SQI) indices, revealing previously unquantified non-linear dependencies. Our approach effectively identifies critical thresholds for compound events (e.g. joint return periods >20 years) within the study area. The framework’s reliance on standardized indices and minimal data requirements suggests it is well-suited for application in other semi-arid zones facing similar water scarcity challenges, such as the Mediterranean and Central Asia, though this requires future validation. Key findings show inland areas experience high-intensity droughts with high salinity (TDS>1000 mg/L), while coastal regions exhibit frequent, moderate fluctuations. This work advances drought assessment by integrating water quality impacts, providing policymakers with a practical tool for climate adaptation strategies.