Where i is the dry spell of a specific month, di represents the difference between the original rainfall and the CAFEC one, and Ki is the factor of weight. The primary factors utilized in the PDSI are the air temperature, rainfall, and the Thornthwaite method-based potential evapotranspiration (PET) described by Thornthwaite (1948).
Evaporative Demand Drought Index (EDDI)
EDDI evaluates atmospheric potential for evaporation, offering early warnings for drought onset (Hobbins et al., 2016). It is susceptible to atmospheric dryness and complements other indices by providing insights into potential drought development.
EDDI measures the atmospheric potential to evaporate water, calculated like the SPI and SPEI, which range widely around zero, with higher values indicating higher evaporative demand and potential drought conditions. The viability of applying the two-variable Gamma distribution, specifically for SPI, may be limited when the application area is extensive due to the dependence on parameter-based probability distribution types (Heim Jr et al., 2023). The probability of exceeding the set period, Eo, denoted as P (Eoi), is calculated using the following formula: