both automated and traditional methods from 177 subcatchments located in the USDA-ARS Walnut Gulch Experimental Watershed in Tombstone, Arizona.
For the manual method in Garbrecht’s study, length is measured subjectively as the distance between the upslope subcatchment drainage boundary and downslope channel. Slope is calculated as a lumped parameter by converting variable slope into a straight-line profile (Gray’s method) while maintaining the horizontal distance and area under the profile. For the automated methods, DEMs were processed using the TOPAZ software, producing 183 subcatchments. Length and slope values were extracted using data reduction (DR) models. (Garbrecht et al., 1999b)
Subcatchment length is important in hydrologic modeling applications because it is used to estimate runoff travel distance or flow routing distance. Garbrecht et al. (1999b) describe two methods for calculating this length using data reduction (DR) models. One method is the average travel distance, and the other is the average flow path length, or the distance of overland flow within a subcatchment.
The average travel distance traveled by surface runoff is calculated as the average distance from every point in the subcatchment to the first downstream channel that the flow reaches at this point, or the arithmetic mean of all travel distances within a subcatchment. For subcatchments that are rectangular in shape the average travel distance is about half the subcatchment length, and twice the average travel distance corresponds to length from the drainage divide at the upstream boundary to the downstream channel, as can be seen in Equation 2.1.