Developing reservoir drought index and conducting copula-based frequency analysis for Lake Urmia basin in Iran

This study investigates the hydrological drought applied to five reservoirs in the Lake Urmia basin in Iran, a region suffering from unsustainable water resource management. The study develops the Standardized Reservoir Index (SRI) for analyzing reservoir droughts using the standardization technique. Also, the copula analysis was conducted to calculate joint probabilities and return periods of drought characteristics, i.e., duration and severity. The Standardized Precipitation Index (SPI) and SRI time series were analyzed with the Sustainability Index (SI) to capture the anthropogenic impact on drought exacerbation. The results reveal a 7- to 9-month lag time to propagate from meteorological drought (SPI) to hydrological drought (SRI). A comparison of SPI and SRI shows a significant SI decrease between 2 % and 13 %, highlighting anthropogenic impacts on drought exacerbation. Examination of the precipitation trend using the modified Mann-Kendall test also shows an insignificant trend at the 5 % significance level. The copulas were fitted with high accuracy, with Nash Sutcliffe efficiency values between 0.9 and 1, and root mean square error below 0.16. The results indicate that, from the perspective of the OR state, exacerbated hydrological droughts are detectable in the investigated reservoirs. The average probability of occurrence was 42 % for meteorological droughts and 46 % for hydrological droughts, for different severity and duration values. • Standardized Reservoir Index (SRI) was developed to investigate hydrological drought. • Copula calculated joint probabilities and return periods for SPI and SRI. • The methodology was implemented in the distressed Lake Urmia basin. • A 7- to 9-month lag was detected from meteorological to hydrological drought in the basin. • Results show reservoirs worsened hydrological drought under the OR perspective.