-B8, 2012
yvided by local
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of their area. In
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ad no gullies or
lapping on areas
oping (Figure 3)
ig on Quickbird
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km?. These data
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s classified as
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calculating the
les an effective
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a higher than
(AUC 60-70%,
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del combining
'ed results. The
ully occurrence
n AUC of 80%
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B8, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
(Figure 4). We therefore based the predictive model on
probability values based on elevation above drainage line in
areas that were initially classified as gully-sensitive areas.
100 -
90 -
80 -
70 -
60 -
50 4
40 -
Cumulative mapped gullies (%)
30 4
20 #20 cm > 6 A Elevation
s above drainage
Model prediction
= Random prediction
10 $10 om^
0 4e” i ;
0 20 40 60 80 100
Cumulative gully sensitive area (%)
Figure 4. Area Under Curve (AUC) representing the
relationship between gully occurrence and elevation above
drainage line in gully sensitive areas. Triangles indicate
elevation above drainage line. A random prediction (dashed
line) is expected to be linear throughout the examined area and
therefore would have an AUC of 50%. The model (solid line)
shows an AUC of 80%. This means that elevation above
drainage line is explaining the distribution of most of the
mapped gullies in a smaller area providing improved prediction
ability. For example, the model locates 60% of the gullies
within only 15% of the gully sensitive area closest to drainage
line (<75cm).
A final, comprehensive broad-scale semi-quantitative gully
presence map gully was developed. The Burdekin was divided
into 5521 cells of 5 km x 5 km. Each cell was assigned with one
of seven gully presence values ranging from very low to very
high. The gully presence values were determined by several
methods. Where available, values were assigned based on
observations from high resolution imagery. At locations without
high resolution imagery, the gully presence values were
assigned based on results of statistical analysis, which examined
the values in the already assigned cells against (i) the extent of
no-gully area within each cell; or (ii) the association of a cell
with a sub-bioregion (DSEWPC 2011) where most observations
were assigned as low gully value; or (iii) information from the
predictive model. The final map provides a gully presence value
for each 5 km x 5 km grid cell in the Burdekin with various
confidence levels relating to the above methods used to
determine these values (Figure 5).
3. RESULTS
Initial results showed that areas at a distance greater than 300 m
from a drainage line, or with high tree cover (FPC above 30%),
or with high slopes (above 10° or on basaltic geology had
extremely low probability of gully occurrence (Table 1). Based
on these relationships, the Burdekin was divided into two areas:
A no-gully area, which covers 47% of the Burdekin and where
less than 7% of the gullies occur; and a gully sensitive area,
which covers 53% of the Burdekin and where more than 93% of
gullies occur (Figure 6). By combining the no-gully area with
the elevation above drainage line layer a 30m resolution gully
predictive model was produced that has predicted more than
90% of the mapped gullies within less than 20% of the total
Burdekin area (Figure 7).
The gully presence map provides a regional view of gully extent
(Figure 8). Overall, several areas were identified as gully ‘hot-
spots’. The most affected areas are in the south of the Upper
Burdekin subcatchment, the northern part of the Suttor
subcatchment and the Bowen Broken Bogie subcatchment.
These results allow for better targeting of gully research in later
stages of this research, where gully expansion rates and
sediment volumes will be examined.
LL] Major subcatchment
Classification method
High resolution imagery
Extent of no-gully area within cell
Bioregion -
SESS Predictive model | 0
Figure 5. Classification methods for gully values for the gully
presence map.
Tondsoape Gullied Gullied % of
Class Seap training | validation | total
description ; :
sites sites area
Gully | 4 image ine,
ur. , 0 0, 0
Em. or FPC<30%, 93% 100% 53%
or slope >10°
>600 m from
drainage line,
EA 0
No-gully | or FPC>30 %, 7% 0% 47%
area or slope >10°,
or basaltic
soil
Table 1. Classification of the Burdekin into no-gully and gully
sensitive areas according to gully occurrence at various
landscapes.
