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Time of concentration is a primary basin parameter which represents response time of a rainfall runoff system. The accuracy of estimation of peak discharge or flood hydrograph is sensitive to the accuracy of the estimated time of concentration. Existing commonly-used empirical models for estimating time of concentration all include surface slope in the denominator, that is, the greater the surface slope, the less the time of concentration and vice versa. The problem occurs when such empirical models are to be used for estimating time of concentration on very flat terrains because as the surface slope approaches zero, the time of concentration approaches infinity. Based on engineering judgment such long times of concentration are not representative of reality.

In this study, two types of laboratory experiments were developed to measure overland flow times on flat surfaces. One is the rainfall test using a mobile artificial rainfall simulator, and the other the impulse runoff test. Test plots were 6 feet wide by 30 feet long with slopes ranging from 0.24 to 0.48 percent. Surface types tested include bare clay, lawn (short grass), pasture (tall grass), asphalt and concrete. The experiment results were compared with those predicted by other empirical models. It was found that most of the empirical models under-estimate the time of concentration parameter. The antecedent soil moisture not included in most empirical models played an important role in affecting the time of appearance of runoff at the outlet, which affected the time of concentration.