Characteristics and Estimation of the Time of Concentration for Small Forested Catchments in Steep Mountainous Terrain

Accurate modeling of flood flow hydrographs for small forested catchments in steep mountainous terrain is challenging because of large errors in the estimation of response time using existing empirical equations. The time of concentration (T-C) for a catchment is a widely used time parameter for estimating peak discharges in hydrological designs. In this study, we developed an estimated T-C using readily available mountain catchment variables, a small catchment, steep slope, and narrow valley, using empirical equations. For our approach, we used directly measured data from 39 forested catchments (area: 0.02-9.69 km(2)) during 3648 observed rainfall events over a 10-year observation period. Based on the uncertainties inherent in the empirical equation, the estimated T-C values were compared and analyzed through multiple regression and two different modified empirical modelling equations using our observed catchment parameters. The mean T-C was significantly correlated with catchment size and stream length but negatively correlated with stream slope (p < 0.01). As a result, the mean T-C estimated using the three modelling equations with catchment variables was qualitatively similar and had relative differences ranging from -12.5 to 15.5 min (-49 to 56%). Therefore, the models (particularly modeling equations with multiple regression, a modified empirical formula, and modified SCS Lag) can efficiently determine the T-C and can be used in any small forested catchment in steep mountainous terrain.