Witnessing ENSO with Precipitation and Flood Dynamics in the Karnali River Basin of Nepal

El Niño Southern Oscillation (ENSO), encompassing El Niño and La Niña, significantly influences river flooding patterns, prompting this study to investigate its relationship with floods in Nepal's transboundary Karnali River Basin (KRB). Focusing on extreme flood events during El Niño and La Niña years, the research aims to enhance the understanding of ENSO's impact on hydrological and hydrodynamic processes. Precipitation and discharge data spanning 1964 to 2020, sourced from the Department of Hydrology and Meteorology (DHM), Government of Nepal, were analyzed. Hydrodynamic modeling, employing HEC-HMS and HEC-RAS, simulated extreme floods of 1983, 2000, 2014 (La Niña years), and 2015 (a strong El Niño year) at the DHM hydrological station. The study examined basin characteristics, precipitation depth, river discharge, and gauge height during these events, utilizing daily data for model estimation of flood discharge and depth. Analysis of ENSO-related variability, including Sea Surface Temperature (SST), Southern Oscillation Index (SOI), and Multivariate ENSO Index (MEI), alongside pressure, temperature, and discharge data across the KRB, was conducted using a three-year running mean. The Soil Conservation Service (SCS) method was integrated within the HEC-HMS and HEC-RAS models to evaluate rainfall duration and flood response considering terrain, soil, and land use. Model simulations revealed river channel shifts, particularly along the right bank, during the 2015 ENSO event. Intensity-duration-frequency (IDF) and correlation/regression analyses further elucidated the impact of ENSO, with the lowest recorded precipitation and discharge observed during the 2015 El Niño event despite localized heavy rainfall. Comparative analysis of flood discharge, gauge height, inundation extents, depths, and velocities across ENSO years highlighted a significant relationship between observed and modeled discharge during the monsoon season. These findings offer valuable insights for water resource management and development, aiding in the anticipation of future strong ENSO and El Niño events in the region.