Introducing a Novel Standardized Precipitation Evaporation Differential Index (SPEDI) for Improved Flash Drought Detection and Assessment: A Case Study in South Korea

This study introduces the Standardized Precipitation Evaporation Differential Index (SPEDI), a new composite drought index designed to better capture flash drought conditions by accounting for both precipitation deficits and evaporative demand. The performance of SPEDI is compared with three established indices—the Standardized Precipitation Index (SPI), the Standardized Precipitation Evapotranspiration Index (SPEI), and the Evaporative Demand Drought Index (EDDI)—in detecting and characterizing flash droughts across South Korea. To evaluate their effectiveness, we analyzed spatial and temporal patterns of flash droughts and compared outputs from each index with records of agricultural damage during four representative years (2017, 2018, 2019, and 2022). Results show that while SPI and SPEI often fail to capture the rapid onset and intensity of flash droughts, EDDI and SPEDI more reliably reflect observed impacts. In particular, SPEDI demonstrated the highest agreement with actual crop damage, offering early warnings in 12 out of 14 events and accurate timing in 10 out of 14 cases. A simplified model examining the relationship between precipitation and evaporative demand further supports SPEDI’s improved ability to represent water balance conditions, especially under high evaporative stress, where SPEI tends to fall short. These results suggest that SPEDI offers a more accurate and practical approach for identifying flash droughts, with clear potential for use in early warning systems and drought risk management. This study presents SPEDI as a valuable tool for supporting drought response planning and improving understanding of flash drought behavior in a changing climate.