Evaluating the Effect (A Case of Central Rift Valley Basin, Ethiopia) Effect of Catchment Characteristics on River Flow (A Case of Central Rift Valley Basin, Ethiopia) River Flow (A Case of Central Rift Valley Basin, Ethiopia)

Abstract

Understanding the relationship between catchment characteristics and the resulting flow is a fundamental problem for water resource planning, operation and management.In this paper, an attempt has been made to evaluate the effect ofphysical catchment characteristics on reaction of river flowof watersheds from Gidabo, Gelana and Bilate catchments in central rift valley basin using statistical method and hydrological model. This allowed to be seen the most influential watershed characteristics that govern the runoff generation and hopes to offer useful tips for decision makersin efficient management and planning of water resources in the study area.Statistical analysis using statistical package for social science (SPSS)was used in nine watersheds and recognizedrelationships among the watershed variables, among the hydrological variables, as well as between the watershed and hydrological variables.Runoff coefficient, average annual rainfall and average slope of a watershed were themost important variables to forma good regression model to explain the response variable (mean annual stream flow) in the sub basin.Furthermore, Soil and water assessment tool (SWAT) was used todistinguish the watershedvariables that causes runoff variation inthe two comparableand neighboring watersheds; Gidabo near Miesso and Gelana near Tore.Watershed characteristics that differs significantly in the two watersheds (Rainfall, temperature, land use and Soil) were tested and evaluated in the calibrated and validated SWAT model one at a timeby assumingthat Gidabo have similar in rainfall, temperature,land use and soil as Gelana.Rainfall, temperature and land use/land cover of Gelana would have resulted in decrement of the long term annual mean flow of concurrent period at Gidabo witha magnitude of 55%, 12.6% and 10.8% respectively while replacing Gelana’s soil for Gidabo resultsa 5.4% increment of flow at Gidabo.As resulted from statistical analysis and SWAT model rainfall was the highly significance watershed variable and a direct source of water. However, it losses in terms of surface runoff in watersheds where shrubs and forest cover were substantially low and in intensively cultivated watersheds.