Abstract
This paper is focused on the lag time of flow confluence. Lag is described empirically by the precipitation factor in hydrological modelling, but the traditional way to establish the relationship between lag and intensity is not very satisfactory in arid and semiโarid regions. A total 215 rainfallโrunoff experiments were conducted to reveal the effects of net rain duration on lag and intensity. The results show that a correlation between lag and rain intensity exists under certain conditions. There is a critical value in net rain intensity (0ยท8โ1 mm min^โ1^) in the correlation curve of lag and net rain intensity in the given experimental watershed in the laboratory. Similarly, a critical value of net rain duration also exists. This value is the total confluence time. The features of lag time change significantly if intensity or duration is less or greater than the critical value. The study also explores the joint effects of net rain intensity and duration on lag. The formula established among lag, intensity and duration resulted in a better fit. Therefore, the twoโparameter (intensity and duration) empirical formula for lag is better than the traditional singleโparameter (intensity) method. This twoโparameter correlation formula can also be applied to a temporally and spatially uneven runoff processes. A typical field watershed is selected to test the results of the experiments. Copyright ยฉ 2006 John Wiley & Sons, Ltd.