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4.6 Comparison of the EM readings to the Topographic Indices at the Oak Creek site:
4.6.1 The wetness Index:
For the calculation of the wetness index several parameters could be set (compare Paragraph 3.3.1). A
boundary value for the cells threshold area was one of them. A cell with a bigger contribution area than
the threshold was regarded and treated as a creek with overland flow. We observed a small creek
forming under wet conditions therefore a threshold area, which was a little smaller area than that of the
study site was used. All other parameters were set to the default value, which means exponent h=1, the
portion area of a creek cell flowing was set to 0 and the maximum distance was set to 20m.
0
40m
Fig.13: Wetness Index calculated for the Oak Creek catchment and the kriged f1 readings from the EM survey
The correlation between the median values of all EM surveys and the calculated topographic index was
very weak (Fig.14).
22
30
25
R2 = 0.3321
EM [mS/m]
20
15
10
5
0
0
1
2
3
4
5
6
7
Topographic Index
Fig. 14: Correlation between the wetness index and the EM readings for the Oak Creek catchment
We used the median value of all surveys to reduce the influence of measure errors and slightly different
orientations of the instrument. The red points show a weak correlation to both indices as well as to the
TDR measurements. The ‘stacking’ of the points suggests that some deeper lying object (e.g. clay lens,
perched water table) that is not related to the surface topography may have influenced the EM readings.
These points were excluded from the regression. The green point was measured in the small stream
with temporary overland flow. The location of the small creek that was observed in the catchment does
not show up in the wetness index very well.
0
40 m
Fig. 15: Location of the ‘red points’ in the catchment
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4.6.2 The downslope Index:
0
35 m
Fig. 16: Downslope index calculated for the Oak Creek catchment
The correlation between the downslope index and the EM readings was weaker than for the wetness
index (Fig.17).
24
Downslopeindex vs. EM readings
30
25
EM [mS/m]
20
15
10
R2 = 0.0718
5
0
0
1
2
3
4
5
6
7
8
9
Dow nslope Index
Fig. 17: The correlation between the downslope index and the EM readings
One reason for that is the topography of the study site. The downslope index has advantages if one is
working in a terrain with terraces that cause a tailback of the down floating water. The Oak Creek study
site did not show any of these features strongly developed in its topography. Again the marked red
points fell out of the general pattern.
Both of these index approaches are limited in their ability to predict the flow paths and soil moisture
patterns by their simplicity. All other flow controlling factors besides the topography such as for
example macro pores in the soil or the bedrock topography are not taken into account.
25
4.7 The Correlation between TDR readings and EM for the Oak Creek
The correlation between the Oak Creek EM readings and the TDR measurements was weak (Fig.18).
TDR vs. EM
30
25
R2 = 0.3598
EM [mS/m]
20
15
10
5
0
0
10
20
30
40
50
60
TDR vol. Moisture content [%]
Fig. 18: Correlation between the TDR measurements and the EM readings
The highest frequency EM readings showed the best correlation. Some possible reasons for this
discrepancy are already mentioned in the section 4.3. Additional reasons for the discrepancy between
TDR and EM readings in this catchment could be:
(1) The soil might show a strong lateral variability in the clay content that influences the bulk
conductivity.
(2) The TDR integrates only over the top twenty centimeters whereas the EM penetrates up to
several meters into the ground.