How do you analyze the trend of a drought index?
In short: the trend of a drought-index series is usually tested with the non-parametric Mann-Kendall test, which detects a monotonic increasing or decreasing trend without assuming a distribution, and its magnitude is estimated with Sen’s slope. A statistically significant negative SPI or SPEI trend indicates intensifying drought over time. DMAP V2.1 performs trend analysis on drought indices automatically.