Download TOP Index

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3.3 The Topographic Indices
3.3.1 The Wetness Index
A wetness index accounts for topographic controls of the water movement in and over the experimental
Oak Creek catchment’s slopes. The index can be derived easily from a DEM. The wetness index is
defined as follows (Beven 1979):
TI = ln (a/tan)
(9)
Where a is the contributing upslope area and per unit contour and tan is the local slope angle.
This index is a theoretical estimation of the accumulation of flow at any point in the catchment. The
spatial soil moisture distribution is assumed to be correlated to the spatial accumulation area
distribution (Western et al. 1999). The wetness index for the study site was calculated with Geasy, a
program written by Jan Seibert of Uppsala University. The program allows setting the exponential
value h, the threshold area for a creek in m2, the portion of area ‘flowing’ from a creek-cell and the
maximum distance to the downslope cell.
The exponential factor allows one to adjust the distribution of the outflow between the different
possible outflow directions of the grid cells. In the original multiple flow direction algorithm of Quinn
et al. (1991) the flow is split proportionally to the hydraulic gradient; multiplied with a factor of 0.5 for
the cardinal flow directions and 0.35 for the others.
By adding a power factor h to the equation, one can change this weighting schema to:
d i  cld (tan  ) h
(10)
where di = is the proportion in the ith downslope direction and cld is the contour length. The contour
length is the share of the cell boundary through which the flow of a certain area crosses. The default
value h is set equal to 1, which is equivalent to the original multiple flow direction (Quinn 1991 and
1995). The effect of a high h will cause a marked bias of the outflow towards the steeper angle. An h
=100 value is equivalent to a single outflow direction.
3.3.1 The Downslope Index
The down slope topography can influence the drainage at a certain point, for example downhill
damming can cause a back up of water further uphill.
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The downslope index accounts for such influences (Hjerdt et al., 2001).
tan  d 
d
Ld
(11)
where Ld [m] is the horizontal distance to the point with an elevation of d meters below the starting
point. The value of d is user defined. The downslope index can also be calculated with Geasy. The
parameters to be set by the user are the vertical distance and the maximum number of cells without
downward gradient in sequence the default value for this parameter was 100 cells.
We used a d=4 m since for larger values of elevation difference the area with a smaller elevation
difference than d to the outlet point became quite large. If on the other hand a to small value for d is
chosen and only a short distance downslope is taken into account, the index looses its meaningfulness.
The wetness and the downslope index were computed using a 2m DEM, derived from a total station
field survey of the study site on June 22nd 2001.