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: