Are estimates of catchment response time inconsistent as used in current flood hydrology practice in South Africa?

Abstract

Catchment response time parameters are one of the primary inputs required when design floods, especially in ungauged catchments, need to be estimated. The time of concentration (TC) is the most frequently used time parameter in flood hydrology practice, and continues to find application in both event-based methods and continuous hydrological models. Despite the widespread use of the TC, a unique working definition and equation(s) are currently lacking in South Africa. This paper presents the results of the direct and indirect TC estimation for three sets of catchments, which highlight their inherent variability and inconsistencies. These case studies demonstrate that estimates of TC, using different equations, may differ from one another by up to 800%. As a consequence of this high variability and uncertainty, we recommend that, for design hydrology and calibration purposes, observed TC values should be estimated using both the average catchment TC value, which is based on the event means, and a linear catchment response function. This approach is not only practical, but also proved to be objective and consistent in the study areas investigated in this paper.