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<br />OD1343 <br /> <br />20 <br /> <br />records from long term point rainfall records (a major use of hydrologic <br />model s) does not necessarily increase the accuracy of flood frequency <br />estimacion. Other shortcomings of physically-based models include the <br />difficul ties of mode"' cal ibration (63), the inadequacy of current <br />data collection methods (17), elllU as illustrated in Table II-l the need <br />for special model ing expertise (9). <br />Comparison of the Predictive Capabilities of Physically-based and <br />Conceptual Rainfall-Runoff Models <br />An urbanizing community must choose a rainfall-runoff model for <br />hydrologic prediction. There are two criteria that must be satisfied <br />by this choice. First, the model must be applicable to the intended <br />use; and second, the model must be cost effective for the community. <br />All analytical or empirical representations of real world systems <br />are developed for a specific purpose. This specificity usually <br />allows certain approximations and assumptions to be made rendering an <br />otherwise intractable problem solvabie. This model should not then <br />be used for other than its intended purpose without appropriate caution. <br />One must insure that the inherent assumptions of the model are not <br />violated. Rainfall-runoff models are no exception. The available <br />models have been developed for a variety of purposes, including: <br />1) The simulation of the quantity aspects of rural rainfall- <br />runoff phenomenon (genera 1 un it hydrograph) (15). <br />2) The simulation of the quantity aspects of urban rainfall- <br />runoff phenomenon (Colorado Urban Hydrograph Precedures, CUHP) (110). <br />3) The simulation of the quantity and quality aspects of the <br />rainfall-runoff phenomenon from combined urban sewer systems (Storm <br />Water jvjanagement Nodel, SWMI~) (b). <br />