Understanding extreme hydroclimatic events is essential for predicting ecosystem vulnerability and guiding water resource management. This study analyzed the Standardized Precipitation Evapotranspiration Index (SPEI) at 3-, 6-, 12-, and 24-month timescales for semi-arid and sub-humid climatic zone in the Zagros forests, Western Iran, over the period 1993–2022. In the semi-arid climatic zone, severe wet events occurred in 1994–1995 with SPEI values ranging from +2.26 to +2.87 at the 3-month and reaching a maximum value of +3.02 at the 6-month, whereas extreme droughts were recorded in 2021–2022, with SPEI minima of −2.81 to −2.73 at the 3-month, and a minimum of −3.03 at the 6-month, and longterm droughts (12–24 months) with SPEI below −2.0. In the sub-humid climatic zone, short-term wet anomalies occurred in 1994–1995 (+2.13–+2.32 for 3–6 months) and 2018–2019 (+2.11–+2.39 for 12 months), whereas severe droughts in 2021–2022 reached −2.88 (3-month), −2.16 (6-month), −2.29 (12-month), and −2.16 to −2.20 (24-month). Mann–Kendall trend analysis indicated statistically significant negative trends across all timescales in the sub-humid zone (p<0.05), while in the semi-arid zone, significant trends were observed only up to the 12-month scale. Despite the relatively low frequency of extreme drought events (<4% of months), cumulative drought severity was substantial in both climatic zones. These findings highlight the growing hydroclimatic extremes and emphasize the need for proactive adaptive management of water resources and forest ecosystems. IDSPI analysis revealed a climatic gradient: while extreme droughts (D3) were primarily confined to the semi-arid zone, the sub-humid zone exhibited higher susceptibility to persistent severe droughts (D2) highlighting their vulnerability to sustained hydroclimatic stress.
