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Figure 2.6 further demonstrates parameter variations among methods of parameter extraction. The average travel distance-based slope method produces the smallest slope because it accounts for the flatter slopes in the lower part of the subcatchment area, thus emphasizing areas that are more subject to higher discharges. The terrain slope method results in the steepest slope values as this method equally emphasizes each maximum local slope value at each cell. The average flow path slope method is steeper than the average travel distance-based slope method because there are fewer divides in the lower part of the catchment. The global slope and manual methods resemble each other in that the models used to calculate these slope values are similar (Garbrecht et al., 1999b).

Garbrecht et al. (1999b) conclude that each method is equally valid, and the user should select a method that is most appropriate for the user’s application. For instance, if the user is most interested in calculating runoff, the average travel distance based slope method and the average flow path slope method are better suited for this calculation than the terrain slope method (Garbrecht et al., 1999b).

2.6

MODELING URBAN AREAS AND FUTURE CONDITIONS

Although digital data in the form of DEMs is readily available and easy to

work with, it does not accurately describe terrain in urban areas. Barrett (2000)

suggests in larger areas, where it is not feasible to digitize these drainage systems,

to delineate the watershed under undeveloped conditions. The errors associated

from water entering and leaving the watershed should cancel out, at least with

larger areas.

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