Basin Regionalization for the Purpose of Water Resource Development in a Limited Data Situation: Case of Blue Nile River Basin, Ethiopia

It is a familiar exercise in hydrology to characterize river basins into hydrologically homogeneous regions by using parameters suited to explain typical hydrological variables such as extreme and annual flows. This paper discusses regionalization of the Blue Nile River Basin (BNRB) by using statistical techniques and describes the selection of best-fit distribution models to estimate the flood frequency of the basin; such undertakings have not been made before. The BNRB is delineated into five homogeneous regions based on statistical parameters of station data. Approximately 14 different distributions are analyzed. The generalized logistic model is the best fit for Regions I and IV. Log-Pearson Type III distribution is appropriate for Region II. Lognormal and generalized extreme value distributions are selected for Regions III and V, respectively. For all distributions, probability weighted moment parameter estimation method is most efficient, but for log-Pearson Type III, ordinary moments are chosen. For each region, a unique regional flood frequency curve is developed. These curves are important to estimate the flood quantiles of ungauged catchments in the data scarce area of the basin; hence, they meet the needs of water engineers who currently face tremendous challenges in designing small and medium hydraulic structures in the basin.