Evaluation of lag time and time of concentration estimation methods in small tropical watersheds in Ethiopia

Study region: Six small agricultural watersheds in three tropical climatic regions of Ethiopia. Three regions of contrasting climate: highland, midland, and lowland regions with respectively high, middle, and low elevations and rainfalls. Study focus: Lag time (TL) and time of concentration (TC) are two measures of how quickly a stream responds to runoff-producing rainfall. These parameters are the main inputs used to estimate peak flow under flood conditions in ungauged watersheds. Many empirical methods have been proposed to estimate TL and TC, but the validity of none of them has been tested. This study compared 10 commonly used methods by using measured TL and TC. New hydrological insights: Measured median values of TL and TC for 176 rainfall–runoff events were used to evaluate the performance of empirical methods. For individual watersheds, the estimates of TL and TC differed by up to 2.6 h and 4.4 h, respectively. Most of the empirical methods tended to substantially underestimate TL and TC, which would lead to overestimation of runoff volume. TL and TC computed by two methods that consider both overland and channel flow were closest to the measured values of TL and TC, because such mixed flow is typical of tropical climate regions. Our results show the need for caution when empirical methods developed in regions with a particular climatic and geomorphological conditions are applied elsewhere.