Development of synthetic peak runoff hydrographs for the Asa river, Ilorin, Nigeria using Snyder and SCS methods

Abstract

Hydrological design in ungauged tropical basins is challenging due to limited streamflow data. This study developed synthetic unit and storm hydrographs for the Asa River watershed in Ilorin, Nigeria, using the Snyder and Soil Conservation Service (SCS) methods. Rainfall depths for 25-, 50-, 75-, and 100-year return periods were estimated using Gumbel’s Extreme Value Type I distribution. Rainfall excess was computed with the SCS Curve Number method using a CN value of 75 based on watershed soil and land-use characteristics. Design storm hydrographs were generated through discrete convolution. Peak discharges from the SCS method ranged from 780.75 to 1043.60 m³/s, while Snyder’s method produced values between 627.63 and 848.69 m³/s. Analysis of variance (ANOVA) showed no significant difference between the methods at the 5% level (p = 0.0546). Although the SCS method produced higher peak discharges, both approaches are suitable for estimating design floods in medium-sized ungauged tropical watersheds