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<br />001342 <br /> <br />13 <br /> <br />processes of a watershed is sufficiently different from the philosophy <br />of conceptual modeling to warrant separate classifications. <br />Conceptual rainfall-runoff models - Conceptual rainfall-runoff <br />models are single transform functions that convert rainfall events into <br />watershed runoff responses. The watershed runoff phenomenon is simply <br />viewed as a "black-box" response ignoring all of the complex inter- <br />dependent mechanisms of stormwater flow. These models can be classified <br />according to the form of the transform function as linear, quasi-linear, <br />and non-linear. Measured success has been attained with the first two <br />models but at the present time the non-linear models do not appear to <br />provide significantly impt'oved accuracy of runoff prediction for urbani- <br />zing basins to warrant their added computational difficulties. <br />The application of linear conceptual rainfall-runoff models is <br />perhaps best illustrated by Espey's unit hydrograph studies (35, 36). <br />He developed regression equations that relate the characteristics of <br />unit hydrographs for watersheds with different percentages of develop- <br />ment to the physiographic features of those watersheds (percent imper- <br />viousness, channelization character, drainage area, and length and <br />slope of main channel). Runoff hydrographs of various return inter- <br />vals for urbanizing watersheds (changing percentages of imperviousness <br />and channelization) can then be calculated by applying a rainfall <br />event with the same return interval to the unit hydrograph derived <br />from these equations. The major assumptions of this linear transfor- <br />mation is that the derived watershed unit hydrograph is unique for all <br />rainfall events and the recurrence interval of the rainfall event is <br />exactly equal to the recurrence interval of the runoff event. <br />