ATHESIS SUBMITTED TOTHE FACULTYOFMETEOROLOGYANDHYDROLOGY, WATERTECHNOLOGYINSTITUTE, SCHOOLOFGRADUATE STUDENTS

Abstract

Hydroclimatic extreme events significantly affect natural climate systems and human societies in the Abaya Chamo sub-basin. To understand these events in the Bilate catchment, this study conducted a rigorous statistical analysis utilizing various tools, including the Standardized Precipitation Evapotranspiration Index (SPEI), Streamflow Severity Index (SSI), and Soil Moisture Stress Index (SSM). Over 41 years, monthly rainfall data was analyzed to investigate normal patterns, deficits, excessive rainfall events, and seasonal variations. Through calculations of mean, standard deviation, and coefficient of variation, the variability of rainfall, streamflow, and soil moisture was assessed. Results revealed erratic rainfall patterns, varying streamflow in selected rivers, and spatial and temporal variations in soil moisture. Furthermore, the study conducted regression analysis on climate signal anomalies of El Niño-Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and North Atlantic Oscillation (NAO) to evaluate their relationship with rainfall, streamflow, soil moisture, and temperature. These climate signals were found to influence spring and summer rainfall patterns collectively. Specifically, spring rainfall in the northern part is affected by SOI_NAO, while the southwestern and eastern parts are influenced by SOI_DMI. The southern part experienced the combined effect of NAO_DMI on spring rainfall. The study also examined the impact of these climate signals on summer rainfall, streamflow characteristics, soil moisture deficit, and temperature in the catchment. Additionally, the study simulated the extreme events characterized in the watershed of the Bilate catchment using the HEC-HMS model, which showed a slight overestimation of discharge. Overall, the HEC-HMSmodel was deemed suitable for hydrological simulations in the Bilate catchment. The findings of this study provide valuable insights into the trend and time series characteristics of monsoonal patterns of rainfall, stream flow, soil moisture, and temperature in the Abaya Chamo sub-basin. They contribute to understanding the hydroclimatic dynamics of the region and have practical implications for water resource management, agriculture, and urban planning.