runoff hydrographs using snyder and scs synthetic unit hydrograph methods: a case study of selected rivers in south west nigeria

This paper presents the development of runoff hydrographs for selected rivers in Ogun-Osun river catchment, south west, Nigeria using Snyder and Soil Conservation Service (SCS) methods of synthetic unit hydrograph to determine the ordinates. The Soil Conservation Service (SCS) curve Number method was used to estimate the excess rainfall from storm of different return periods. The peak runoff hydrographs were determined by convoluting the unit hydrographs ordinates with the excess rainfall and the value of peak flows obtained by both Snyder and SCS methods observed to vary from one river watershed to the other. The peak runoff hydrograph flows obtained based on the unit hydrograph ordinate determined with Snyder method for 20-yr, 50-yr, 100-yr, 200-yr and 500-yr, return period varied from 112.63m3/s and 13364.30m3/s, while those based on the SCS method varied from 304.43m3/s and 6466.84m3/s for the eight watersheds. However, the percentage difference shows that for values of peak flows obtained with Snyder and SCS methods varies from 13.14% to 63.30%. However, SCS method is recommended to estimate the ordinate required for the development of peak runoff hydrograph in the river watersheds because it utilized additional morphometric parameters such as watershed slope and the curve number (CN) which is a function of the properties of the soil and vegetation cover of the watershed.