Abstract:
Despite uncertainties and errors in measurement, observed peak discharges are the best estimate of the true peak discharge from a
catchment. However, in ungauged catchments, the catchment response time is a fundamental input to all methods of estimating
peak discharges; hence, errors in estimated catchment response time directly impact on estimated peak discharges. In South
Africa, this is particularly the case in ungauged medium to large catchments where practitioners are limited to use empirical
methods that were calibrated on small catchments not located in South Africa. The time to peak (TP), time of concentration (TC)
and lag time (TL) are internationally the most frequently used catchment response time parameters and are normally estimated
using either hydraulic or empirical methods. Almost 95% of all the time parameter estimation methods developed internationally
are empirically based. This paper presents the derivation and verification of empirical TP equations in a pilot scale study using 74
catchments located in four climatologically different regions of South Africa, with catchment areas ranging from 20 km2 to
35 000 km2. The objective is to develop unique relationships between observed TP values and key climatological and
geomorphological catchment predictor variables in order to estimate catchment TP values at ungauged catchments. The results
show that the derived empirical TP equation(s) meet the requirement of consistency and ease of application. Independent
verification tests confirmed the consistency, while the statistically significant independent predictor variables included in the
regressions provide a good estimation of catchment response times and are also easy to determine by practitioners when required
for future applications in ungauged catchments. It is recommended that the methodology used in this study should be expanded
to other catchments to enable the development of a regional approach to improve estimation of time parameters on a
national-scale. However, such a national-scale application would not only increase the confidence in using the suggested
methodology and equation(s) in South Africa, but also highlights that a similar approach could be adopted internationally.
