Comparison of the temporal distribution pattern of precipitation and the cumulative effect of daily droughts on the local scale in Iran

Abstract

The environmental effects of drought appear to be substantial. At local scales, even small areas can have diverse climates due to their geographical location. This study compares the cumulative effect of daily droughts from 1989 to 2019 using daily precipitation data from nine synoptic stations in the Tehran Province, Iran. The Effective Drought Index (EDI), the Standardized Precipitation Index (SPI), and the China Z-Index (CZI) have been applied to calculate droughts. Initially, we have determined drought characteristics (severity, frequency, and duration) using the run theory. Later, the copula models (Frank, Gaussian, and Gumbel) have been used to find dependence between drought characteristics at different time scales as a cumulative effect. The results show that a pattern of the number and order of temporal distribution variables of precipitation play a major role in intensifying droughts. Also, the findings indicate that the frequency-duration of daily droughts in semi-arid and mountainous stations located in the north of the province has a cumulative effect on monthly and annual droughts. The severity frequency of daily droughts affects long-term droughts in the stations located in the middle and south of the province characterized by a drier climate. The variability in the intensity and shift of daily droughts in the mountainous stations is higher compared to other areas but it decreases on monthly and annual scales. This study found that the copula model is an ideal tool to demonstrate regional differences of drought in the regions with climatic and geographical diversity.