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<br />to the response time of the basin. This approach, while it has <br />many shortcomings because it ignores a number of important system <br />characteristics, continues to be used by drainage engineers because <br />of its simplicity. A conservative estimate (23) is that better than 90% <br />of the storm drain systems in the U. S, are planned using this technique. <br />Other procedures such as rainfall runoff analysis as performed <br />by the Corps, are used but to a lesser extent and are subject to the uncer- <br />tainties involved in predicting the state of the watershed prior to the <br />runoff event of interest. More recently, attention is being directed <br />towards the development and application of watershed simulation that are <br />responsive to urban watershed peculiarities (7). These models are designed <br />to account on a continuous basis for most of the moisture within the water- <br />shed and many use the simple concepts that had been used historically but <br />hecause of the computer, the accounting is done on a much grander scale. <br />In any event, the state-of-the-art in urban hydrology is probably such <br />that the skill and judgement of the analyst are far more important than <br />the technology that is used to develop the runoff hydrology; <br /> <br />Performance Criteria <br />It has been the practi ce (and s ti 11 is) to express performance cri teri a <br />in hydrologic terms and consider planning to be primarily the design of a <br />system that wi 11 accompl ish the performance cri teri aat least cost. In <br />present day planning performance standards are still adopted and in a few <br />instances performance is a variable to be determined as a result of the <br />planning study. The table on page 18 is an example of performance criteria <br />used in current planning. <br /> <br />13 <br />