Analysis of Return Periods for Extreme Drought and Precipitation Events in Derna City, Libya, Using the China Z-Index (CZI)

Abstract

This study aims to evaluate the long-term characteristics and conditions of drought and precipitation in the city of Derna, located in northeastern Libya within the Mediterranean and semi-arid climatic region. The China Z Index (CZI) was employed as the primary statistical tool due to its suitability for non-normally distributed data, using monthly time series data covering the period from 1946 to 2010. Naturally dry summer months (June, July, and August) were excluded from the analysis. The methodology involved calculating the statistical properties of the index, analyzing the monthly frequencies of drought and wetness categories, and estimating exceedance probabilities and return periods of extreme precipitation events.

The results indicate that Derna’s climatic pattern is characterized by relative stability, with near-normal conditions prevailing in approximately 78% of the months. Extreme precipitation events account for only about 2.9% of the total observations; however, they pose a significant hazard due to their short return period of approximately 3.8 years. Furthermore, the findings reveal that the risk of extreme precipitation is substantially higher than that of extreme drought, which exhibits a return period of about 21 years. This highlights the city’s vulnerability to flash floods resulting from its topographic characteristics and coastal location. The analysis also confirms that January and February represent the peak of precipitation variability, while September is identified as the month most susceptible to rare and extreme rainfall events