The use of a derived regional flow duration curve to assess the low flow characteristics in ungauged catchment: a case Study of Rwegura Catchment –Burundi

Rwegura power plant is a major hydropower plant in Burundi .The water level in the reservoir has decreased and this has resulted in a reduction in generated power, creating shortfalls on the demand. Rwegura reservoir is situated in an un-gauged catchment and therefore, there is a fundamental need to develop a method that can be used to assess the hydrology during times of low flow. In this study, a flow duration curve for Rwegura ungauged site is developed through Regionalization approach. Based on spatial rainfall pattern, a hydrological homogeneous region in which belongs Rwegura catchment was derived. For each gauged site, a dimensionless monthly duration flow duration curve was derived and a regional dimensionless flow curve was found by averaging the dimensionless flow duration curves of all gauged sites. Multiple regression equation was developed to find relationship between mean monthly flow (index flow) for the gauged sites and catchment characteristics. The derived regression equation was used to find the index flow at Rwegura ungauged site and thereafter the flow duration curve was found as a product of dimensionless regional flow duration curve and index flow. The validation was based on the comparison of the few available observed stream flow data (1980 – 1983) and the Nash efficiency criterion was found to be 91%. The derived flow duration curve was used to characterize the low flow in the range from 70% and 90% of time flow is equalled or exceeded. The range can be used to assess the reliability of Rwegura Power generation and environmental flow requirement in Rwegura ungauged Catchment.