A Systematic Evaluation of Time of Concentration (TC) Models in a Predominantly Agrarian Catchment of Southeastern Nigeria

Time of concentration (Tc) is a parameter in runoff estimation, used to study and design different types of hydraulic structures in watersheds. Any error in Tc calculation leads to an inaccurate estimation of the discharge, which can lead to over-sizing or under-sizing of designed facilities that can have great economic and environmental consequences. Therefore, choosing the correct method to estimate Tc for a specific catchment is of great importance. This study was aimed at comparing the applicability of fifteen carefully selected equations of time of concentration to Ifite Ogwari Catchment in Anambra State Nigeria. The catchment was first delineated in QGIS and all the requisite parameters for estimating the time of concentration as per the selected equations determined. Afterwards the selected equations were employed to obtain the T c values for each sub-catchment. The calculated values of T c were then subjected to exploratory statistical analysis and sensitivity analysis. Results showed that the Temez, Haktanir & Sezen and Chow equations gave the most representative of values of times of concentration of the sub-catchments with 100% of all T c values obtained using these equations fell within the inclusion criteria of median ± 0.5 SD while returning the median T c three times each out of eleven catchments. The Pasini and Sheridan models were the worst performing of all the fifteen equations evaluated with all the T c values calculated using these equations fall outside median ± 0.5 SD and median ± SD. The Pasini and Sheridan models generally showed signs of gross T c overestimation such that the values of T c obtained using these two equations are about 3 to 4 times higher than the median values. Sensitivity analyses indicate that 82% of the equations consider the longest flow path as the most important catchment parameter while the equations of Dooge and Pilgrim & Mc Dermont consider catchment area as the most important catchment parameter. Slope was the least important parameter in all fifteen equations investigated for application in the study area.