Assessing meteorological droughts in the Çoruh River basin, Turkey: A probabilistic approach using SPI, SPEI, and copulas

Abstract

This study presents a comprehensive analysis of meteorological drought characteristics in the Çoruh River Basin (ÇRB), utilizing both the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI). While previous research has examined droughts in the region, this study marks the first application of SPEI, offering a more holistic evaluation by integrating temperature-driven evapotranspiration effects. Drought events were identified using run theory, and their characteristics were systematically analyzed through univariate and bivariate frequency analyses. The Lognormal distribution was identified as the most suitable marginal distribution for both drought duration and severity, based on goodness-of-fit tests. Dependence structures between drought characteristics were modeled using copula functions, with the Gaussian copula providing the best fit for SPI and the Frank copula for SPEI. Return period analysis revealed a strong interdependence between drought duration and severity, with SPEI demonstrating superior capability in capturing prolonged drought events compared to SPI, particularly under rising temperature conditions. The findings underscore the limitations of precipitation-only indices in drought assessment and highlight the critical importance of incorporating temperature-sensitive indices like SPEI for accurate drought characterization. By employing advanced statistical methods, including copula-based modeling and joint return period analysis, this study establishes a robust framework for evaluating meteorological droughts and their probabilistic characteristics. The results emphasize the necessity of using temperature-inclusive drought indices for reliable risk assessments and climate adaptation planning. This research provides valuable insights for policymakers, hydrologists, and water resource managers, offering a foundation for enhanced drought monitoring, early warning systems, and mitigation strategies in the ÇRB and other drought-prone regions.