X-B8, 2012
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, using NDVI
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1 types evident
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- Mudd, 1998;
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The temporal
urements allow
e observations
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
However, the dynamics of green wetland areas are also
controlled by rainfall events on the GAB, natural intra-annual
variability of species present within the wetlands, and by
historical events at particular sites. Thus, multi-annual
variations in vegetation extent are clearly related to the total
rain measured at the Hamilton Station (Figure 3c). Ephemeral
wetland and Phragmites phenologies explain the mean
monthly variations observed in the area index calculated
(Figure 3d). Anthropogenic actions, like uncontrolled fire,
also play a crucial part in temporal dynamics of spring
wetland vegetation (Figure 3a).
Therefore, short-term variations observed in vegetated
wetland extent must be interpreted with care and placed in a
longer context if they are to be used as indicators of spring
flow modification. Long-term medium resolution satellite
studies provide a good understanding of variability and a
strong baseline for assessment of changes, and give temporal
context for less frequent high resolution studies. For example,
White and Lewis (2011) record a substantial increase in
wetland area at DCS using three epochs of high resolution
satellite imagery between 2006 and 2010). The MODIS index
of wetland area (AreaNDVI) also shows an increase of about
50% (Figure 3, AreaNDVI = 13.90, 16.04 and 20.93 km?)
over these three dates (December 2006, May 2009 and May
2010), but in addition provides a fuller temporal context for
these records: 2006 follows six years of declining wetland
area, there was further decline in 2007 and 2008, with some
increase in wetland area in 2010.
While the MODIS NDVI spatial resolution is not sufficient to
give precise measurements of wetland area; it does give
objective, repeatable measurements of relative changes in
area, and provides new understanding of intra and inter-
annual variability. Variations in the DSC wetland extent
Observed over the ten years are the result of the combined
phenological, natural and anthropogenic influences at
different springs throughout the complex. Nevertheless, the
trends documented in Figure 10b are quite alarming as they
suggest decreases of a third in the area index since 2002 and
a return of wetland vegetation since only 2010.
MODIS photosynthetic activity monitoring in association
with higher resolution temporally targeted studies must be
extended to give a fuller picture of the long term dynamics
and trends for DSC. Temporal analyses such as those
implemented in this paper help define the range of natural
variation in the spring-fed wetlands, and could form the basis
for thresholds of acceptable change for environmental
management purposes. Change in wetland area beyond such
thresholds could provide timely warning of responses to
changing aquifer pressures or climate change. Further
research at smaller groups of GAB springs is testing the
limits imposed by the MODIS NDVI spatial resolution, and
will extend our understanding of the spring wetland
dynamics.
s. CONCLUSIONS
The aim of this study was to test the potential of MODIS
NDVI data as long-term tracer of wetland vegetation in the
Dalhousie Spring Complex of the Great Artesian Basin.
Results obtained proved that photosynthetic activity within
DSC wetlands can be discriminated from surrounding land
responses in this medium resolution imagery. Up to ten years
of wetland vegetation extent were documented. The study
reaffirmed the correlation existing between vegetated areas
and groundwater flow, but also demonstrated the important
influence of rainfall, natural species phenologies, and human
activity on the observed seasonal and inter-annual vegetation
dynamics. Over the whole Dalhousie Spring discharge area,
declining trends in the extent of wetland areas were observed
over the 2000-2009 period followed by a return of wetland
vegetation since 2010. This study underlined the need to
continue long-term medium resolution satellite studies of the
GAB to fully understand variability and trends in the spring-
fed wetlands. Complemented by high resolution studies, use
of MODIS imagery over several decades could become a
powerful tool for monitoring potential impacts of aquifer
drawdown and climate changes in the Great Artesian Basin.
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