Assessing compound hydrometeorological extremes and their transformative impacts on streamflow in a changing climate

Abstract

The likelihood of compound events has increased in the past and is projected to rise further with continued global warming. This study developed a methodology for assessing compound hydrometeorological extremes under changing climate conditions. A novel index was introduced to identify compound hot-dry and cold-wet events. The index was applied in the Ellagawa Subbasin of the Kalu River Basin, Sri Lanka. The compound event index integrates the Standardized Precipitation Index (SPI) and Standardized Temperature Index (STI). The index's performance was further enhanced by comparing it with the Standardized Streamflow Index (SSI) to capture hydrological impacts. Future streamflow was projected using a calibrated and validated Hydrologic Engineering Center–Hydrologic Modeling System (HEC-HMS) model under two Shared Socioeconomic Pathways (SSPs): SSP2-4.5 and SSP5-8.5. Climate data from the CNRM-CM6-1 Global Climate Model (GCM) were downscaled for the 2031–2050 and 2051–2070 periods. Results indicate a decline in extreme hot-dry events but an increase in mild to severe hot-dry conditions, leading to more frequent low streamflow. While overall cold-wet events may decrease, extreme cases could rise slightly, with a corresponding increase in high streamflow due to intense rainfall. The 2031–2050 period is identified as particularly critical, highlighting the need for climate-hydrological analysis.