A hybrid drought Index for assessing agricultural drought in arid and semi-arid coastal areas of Southern Iran

Precipitation and seasonal streamflow are the two major sources of water for vegetation cover in arid and semi-arid regions. A simple and novel hybrid drought index is presented in this study to assess the joint effects of precipitation and streamflow on vegetation cover in seven coastal sub-basins in southern Iran. To this end, the Standardized Precipitation Index (SPI) and the Streamflow Drought Index (SDI) were used to track meteorological and hydrological droughts, respectively. The suggested index in this study, called the Entropy-based Drought Index (EnDI), combines SPI and SDI utilizing the entropy weighing approach. The capability of the entropy method to construct hybrid index and capture various drought features was compared to the widely used copula function approach. The hybrid index in the latter case was called the Copula-based Drought Index (CoDI). The results indicated that the rank correlation (r(s)) between SPI and SDI varied between 0.327 and 0.726 in the studied sub-basins. However, the two hybrid indices EnDI and CoDI exhibited extremely high correlations (r(s) >= 0.9). The SPI benefited both hybrid drought indices more than the SDI, and the contribution of SPI to EnDI was higher than its contribution to CoDI. EnDI had stronger associations with the Normalized Vegetation Difference Index in nearly all the sub-basins, possibly because precipitation had a larger effect of on EnDI than CoDI. As a result, EnDI was recommended as a superior index for assessing vegetation droughts across the research region.