Spatiotemporal Characterization of Drought and Flood Extremes Across Assam Using Multi-Scale Standardized Precipitation Index (1901–2024)

Abstract

Abstract Significant challenges in water resources, agriculture, and disaster risk management arise from droughts and floods—the most common hydroclimatic extremes affecting people in Northeast India. Despite the traditional view of Assam as a flood-prone area, emerging evidence indicates a rising risk of droughts, highlighting the need to study both extremes together. The present study offers an integrated, spatiotemporal analysis of drought and flood events across 21 districts of Assam, using 124 years (1901–2024) of monthly rainfall data. It employs a three-temporal analysis based on the Standardized Precipitation Index (SPI) (SPI-3, SPI-6, SPI-12). A non-parametric SPI approach, based on empirical cumulative distribution functions, was used to identify and characterize drought and flood events through run theory, assessing frequency, duration, magnitude, intensity, and severity. Events were grouped into moderate, severe, and extreme categories. Results reveal strong scale dependence of hydroclimatic extremes, with short-term events occurring more frequently but lasting less, while long-term events are rarer yet more severe and persistent. Spatial disparities are evident: droughts are more frequent in western districts, flooding dominates in the central Brahmaputra valley districts, and hill districts experience less frequent but more intense events. In some districts, droughts and floods pose double risks. Moderate events are most common across all scales, whereas severe and extreme events significantly impact long-term risk due to their extended duration and large magnitude. This research provides a robust century-scale baseline for drought and flood risk assessmen t in Assam, offering district-level insights crucial for disaster preparedness, water resources management, agricultural planning, and responses to climate change.