Streamflow Estimation in the Pra River Basin of Ghana

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The mean monthly stream flow in a river basin is one salient characteristic which relates to the storage and use of stream flow as a resource or source of supply. Mean monthly stream flow estimates are thus required for the planning, assessment and management of water resources projects, including water supply, irrigation and hydropower generation. Using stream flow data recorded at nine gauging stations in the Pra River Basin of Ghana, four sets of mathematical equations have been developed which relate the mean monthly stream flow in the basin for each month of the year to the basin climatic and physiographic characteristics. Each set of equations reflects a certain degree of sophistication in terms of data input requirement, accuracy and applicability. Of the basin parameters examined, basin area alone accounted for about 90% of the spatial variability of mean monthly stream flows in a multiple regression analysis. The inclusion of rainfall, basin relief, main channel length and stream order increased the predictability (measured by the R-squared statistic) of the equations to between 95% to 99.9%, and achieved relatively reduced standard errors of estimate. Geological, as well as climatic, homogeneity in time and space are the main assumptions of the models. It is further assumed that rainfall over the basin is temporally and already uniform. Finally, the effect of urbanization, if any, on stream flow is considered insignificant. The results suggest that the mean monthly stream flow at an ungauged site in the Pra River Basin could be well estimated from basin climatic and physiographic characteristics using the models developed. The standard errors of estimate associated with these models were 30% to 50% of the observed mean and were lowest (5 % < SE <10%) for the relatively more sophisticated of these models.
A thesis submitted to the Board of Postgraduate Studies, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfilment of the requirements for the award of the Degree of Master of Philosophy in Water Resources Engineering, 1990