Analysis of the rainfall-runoff pattern of a catchment with limited data to estimate the runoff potential. Case study: The Mefou sub basin in Cameroon

Abstract: Water resources management in a catchment requires the quantification of the water potential. This is usually achieved by the estimation of the hydrological and meteorological characteristics of the basin using observed data to carry out statistical estimates or for use in hydrological models. Hydrological and meteorological data for the Mefou sub basin in Cameroon were collected to analyse the rainfall-runoff pattern and estimate the potential runoff volume from the sub basin. The data included daily series of rainfall, runoff, temperature and evapotranspiration for some periods in which data was available for ten meteorological stations and one hydrological station within or near to the catchment. Data was available for only one rainfall station at Yaoundé and one runoff station at Nsimalen directly located within the sub basin. The rainfall station had data for seven years from 1976 to 1982 while the runoff station had data for five years from 1976 to 1977 and 1984 to 1986. However, a nearby rainfall station at Sangmelima had observed data for 13 years from 1975 to 1987 which could be used as a replacement station for Yaoundé. Simple statistical and graphical methods were used to analyse and compare the data from the Yaoundé and Sangmelima stations to determine a correlation between them in order to extend the Yaoundé data to 10 years at least. Similarly simple statistical and graphical methods were used to determine a monthly and annual relationship between the rainfall and runoff data of the Yaoundé station and the Nsimalen station. These relationships were used to extend the monthly and annual runoff to the intermediate years without data from 1978 to 1983 and 1987. It was necessary to extend this data to at least 10 years in order to increase the possibility of including both wet and dry periods of the sub basin in the hydrological estimates. The rainfall and runoff data was eventually extended to 12 years from 1976 to 1987. After extending the data the average annual runoff volume of the entire sub basin was estimated from the analysed data. This runoff was found to be within the range of previous estimates carried out in the sub basin though the volume is inadequate to satisfy the water resources requirements of the sub basin. The data was then used in the Swedish HBV rainfall-runoff model which is a semi distributed conceptual model to estimate catchment parameters, forecast the values for some years with missing data and compare results from the statistical and graphical methods. Some future climate scenarios were also simulated in the model and the results were also in line with other previous studies carried out in the bigger catchment within which the Mefou is a sub catchment. The difficulties in making hydrological and meteorological estimates in catchments with little or no data are highlighted by this project and recommendations are made towards facilitating the carrying out of satisfactory water resources studies in other sub catchments with similar limitations.