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.