P = 1-f(x) when P < 0.5; P = 1-P, and the SPEI’s sign is
inverted when P > 0.5. The values of the constants are d1 =
1.432788, d2 = 0.189269, d3 = 0.001308, c0 = 2.515517, c1 = 0.802853, c2
= 0.010328 c3 = 0.010328. SPEI values also range from -2.0 to +2.0, with
negative values indicating drought and positive values indicating wet
conditions. The SPEI time series displays positive and negative values
corresponding to wet and dry periods. The drought state was determined
using a threshold of -1 (SPEI ≤ -1).
Palmer Drought Severity Index
(PDSI)
PDSI measures long-term drought using local temperature and moisture
data (Palmer, 1965). While helpful in assessing prolonged droughts, PDSI
has limitations in reflecting short-term droughts due to its lagging
nature. Monthly observations from 126 meteorological stations were used
to calculate PDSI values.
PDSI aims to measure the duration and intensity of long-term droughts
using local temperature and moisture data. The index estimates the
amount of water stored in the soil using an equation considering
precipitation and the soil’s water balance. The anomaly index (z-index)
was calculated using cumulative monthly precipitation data. The z-index
was determined each month by calculating the difference between the
climatically suitable for existing conditions (CAFEC) and actual
precipitation. The z-index was incrementally computed using a recursive
method. Its value ranges from -4.0 (extreme drought) to +4.0 (incredibly
moist conditions), making it helpful in tracking prolonged drought or
wet spells.