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where the longest flow path (L) does not define the hydraulically most distant <br />point. Occasionally, especially in mountainous areas, a point with a shorter flow <br />path may have an appreciably flatter slope (S) such that the shorter flow path <br />defines the hydraulically most distant point. For watersheds with multiple <br />choices for the hydraulically most distant point, the T~ should be calculated for <br />each point and the largest T~ should be used. <br />3. Slope (S) is the average slope calculated by dividing the difference in elevation <br />between the hydraulically most distant point and the watershed outlet by the <br />watercourse length (L). This method will usually be used to calculate S. <br />However, there are situations where special consideration should be given to <br />calculating S and to dividing the watershed into subbasins. For example, if there <br />is dramatic change in watercourse slope throughout the watershed, then the use <br />of a multiple subbasin model should be considered with change in watercourse <br />slope used in delineating the subbasins. There will also be situations where the <br />watercourse contains vertical or nearly vertical drops (mountain rims, headcuts, <br />rock outcrop, and so forth). In these situations, plotting of the watercourse <br />profile will usually identify nearly vertical changes in the watercourse. When <br />calculating the average slope, subtract the accumulative elevation differential <br />that occurs in nearly vertical drops from the overall elevation differential prior to <br />calculating S. <br />4. L~ is measured along L to a point on L that is essentially perpendicular to the <br />watershed centroid. This is a shape factor in the T~ equation. Occasionally, the <br />shape of agricultural fields or urban subbasins is nearly rectangular and this may <br />result in two different dimensions for 4a. In the case of such nearly rectangular <br />(and therefore, nearly symmetrical) watersheds or subbasins I.~ can usually be <br />satisfactorily estimated as ~/z L. <br />5. RTIMP is the effective impervious area and is used to estimate T~ for urban <br />watersheds only. RTIMP is the same value that is used to estimate rainfall losses <br />for the watershed. The calculation of T~ for urban watersheds is very sensitive to <br />RTIMP as illustrated in the following; <br />19 March 2007 9 <br />