Drought Risk Patterns in Southeast Asia 2000-2025: Province-Level Analysis Using ERA5 Reanalysis and a Multi-Index Composite Methodology

Southeast Asia faces escalating drought risk driven by the El Nino-Southern Oscillation (ENSO) and longer-term climatic change, yet much of the regional drought literature operates at country or river-basin scale, limiting its applicability to province-level agricultural and insurance risk assessment. This study characterises drought patterns at the province level across Thailand (77 provinces), Vietnam (63 provinces) and Malaysia (16 states/federal territories) (156 administrative units in total) over the 25-year period 2000-2025. We combine three complementary, openly published drought indicators: the Standardized Precipitation-Evapotranspiration Index at a three-month accumulation window (SPEI-3) derived from ERA5 reanalysis, the satellite Vegetation Condition Index (VCI) from MODIS, and the Evaporative Stress Index (ESI) from the ALEXI product. The three indicators are combined into a single composite drought-severity classification spanning six categorical levels, consistent with the U.S. Drought Monitor framework. The specific combination is calibrated against historical drought event records; the exact weights and calibration parameters are proprietary and are not disclosed. We report three principal findings. First, over the 25-year period the large majority of provinces (139 of 156, 89%) show no statistically significant monotonic trend in annual drought severity (Mann-Kendall, p > 0.05). A statistically significant drying trend (Sen's slope < 0, p < 0.05) is confined to 16 provinces, all in northern and north-eastern Thailand; the only significant wetting trend is in a single northern Vietnamese province (Phu Tho). Second, province-level drought severity is moderately and negatively correlated with the Oceanic Nino Index across all three countries (country-mean Pearson r about -0.31), strongest in Sabah (r = -0.57) and the Vietnamese central highlands (Lam Dong, r = -0.53). The 2015/16 super-El Nino produced only near-neutral region-mean conditions, whereas the most severe region-wide drought in the record (2023, region-mean SPEI-3 about -1.3) coincided with the developing 2023/24 El Nino. Third, moderate-or-worse drought (SPEI-3 <= -1) occupies 6.7%, 8.8% and 10.9% of province-months in Thailand, Vietnam and Malaysia respectively. We additionally publish a province-level data-confidence classification (42 High-, 114 Medium-confidence). The province-level composite time series (2000-2025) is released openly to enable independent validation and reuse.