Estimation of Discharge for Ungauged Catchments Using Rainfall-Runoff Model in Didessa Sub-Basin: The Case of Blue Nile River Basin, Ethiopia

Abstract

Runoff estimation in ungauged catchments is probably one of the most basic and oldest tasks of hydrologists. This long-standing issue has received increased attention recently due to the prediction in ungauged basin initiative by the International Associations of Hydrological Science. In developing countries like Ethiopia most of the Rivers are ungauged. Therefore, applying regionalization techniques for ungauged or poorly gauged river basin is crucial. This paper deals with predicting discharge at ungauged catchments using conceptual lumped rainfall-runoff model HBV-96 in Didessa sub-basin. Four regionalization methods (multiple linear regression, spatial proximity, area ratio and sub-basin mean) were applied to transfer model parameter values from the gauged to the ungauged catchments. In regional model, gauged catchments model parameters and physical catchment characteristics of ungauged catchments were used to develop the equations in order to predict discharge from ungauged catchments. To have better understanding of model parameter performance, the sensitivity analysis of eight model parameters were performed manually by trial and error. The evaluation shows that runoff coefficient (Beta), recession coefficient of upper reservoir zone (Khq), limit for evapotranspiration (LP), field capacity (Fc), percolation (Perc) and capillary rise coefficient (Cflux) are more sensitive than others. The model performance was evaluated using Nash Sutcliff efficiency and relative volume error. The result shows that from seven gauging river four of them have good agreement and distribution since Nash Sutcliff efficiency greater than 0.60 and relative volume error lies between +10% and -10%. In predicting model parameters from ungauged catchments in regional model method p-value ≤ 0.1 and determination coefficient (R 2 ) ≥ 0.94 were obtained. The model parameters in ungauged catchments were determined by using regional model, sub-basin mean, area ratio and spatial proximity methods and simulated the discharge by using HBV-96 model. River flow from ungauged catchments simulated by regional model method was the best. Therefore, regional model method is recommended in predicting discharge for ungauged catchments.