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the downstream channel. length value since the this (Garbrecht et al., 1999b).

The travel distance consistently gives the smallest method accounts for all the cells in the watershed

Moglen et al. (2001) show that DEMs at a 30 meter resolution are not sufficiently dense for analyzing flat areas; thus a higher resolution grid must be used regardless of the quality of the 30 meter grid. Garbrecht et al. (1999a) note the reason that the lower resolution grid will not work is due to the fact that as some DEMs (with the exception of NED DEMs, as NED is in floating point meters) are reported in meters or feet, the computed slope can only take on a limited number of values. For example, a 30 meter DEM in meters could have a slope value of zero, or a multiple of 0.033 (for a 1 meter change in elevation). These increments may be suitable to model terrain in mountainous terrain with large slopes, but insufficient to provide accurate values in flat areas (Garbrecht et al; 1999a).

Subcatchment slope, similar to subcatchment length, is an

variable

for

runoff

calculations.

Garbrecht

et

al.

(1999b)

present

important four DR

methods of slope calculation. These travel distance slope, the average flow

are the average terrain slope, the path slope, and the global slope.

average

Equation 2.3 shows the calculation for average terrain slope is the average of the local slope value at every point in the subcatchment.

St =

1 ns

ns

s

i =1

t i

Equation 2.3

21

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