Developing a new hydrological drought index and comparing it with meteorological drought indices in Iran's various climates

Abstract Drought is a natural phenomenon that can have an impact on many aspects of human life. Drought indices are commonly used to monitor drought levels. In this research, a fundamental change was made on structure of the SPI index and the hydrological effective rainfall was replaced by total precipitation, thus the new hydrological drought index (SPIH) was derived. The SPIH index was used to present the hydrological drought status of Iran's climatic zones over a 63-year period (1955–2018). Furthermore, it was compared to the four other drought indices of SPI, SPEI, RDI, and RDIe in six stations chosen to represent each climatic zone of Iran (coastal wet, mountain, semi-mountain, semi-desert, desert, and coastal desert). In general, changes in the correlation coefficient between all drought indices demonstrate that with decreasing precipitation and rising evapotranspiration, the correlation coefficient declines and the disparity between all the indices becomes more evident. In most climatic zones, with the exception of coastal wet, the correlation between SPI-SPIH is not as strong as the two-way correlation between other indices. The coastal wet regions have the highest correlation between SPI and SPIH (0.98), followed by mountainous (0.93), semi-mountainous (0.92) regions, and desert areas have the lowest (0.61). In general, in arid areas, hydrological drought based on the SPIH index has a low association with meteorological drought based on the SPI, SPEI, RDI, and RDIe indexes. This demonstrates that there are differences between meteorological drought (SPI, SPEI, RDI, and RDIe) and hydrological drought indices (SPIH) in different climates.