pan-European CORINE landcover classification system has limitations for
national-scale applications in Ireland.
CORINE is derived from satellite images acquired a maximum of twice within any
one year, thereby failing to fully indicate
seasonal changes in vegetation. This has been shown to provide useful information
to improve landcover characterisation. A lack of information on seasonal
variability within a single class, for example different pasture management
practices, can limit the use of the data as well as contribute to potential
misclassification. This research explored an alternative approach to deriving
information about Irish landcovers using the seasonal cycle of vegetation
growth retrieved from medium resolution satellite imagery.
study used the Enhanced Vegetation Index (EVI) product for 2006 from the
MODIS-TERRA satellite sensor. A time-series of 250m spatial resolution, 16-day
composite EVI images was processed, using time series analysis methods. The
seasonal profile per pixel was first modelled to reduce the effect of cloud
interference. Modelled pixels were then clustered using a divergence-guided ISODATA
clustering algorithm, grouping pixels of similar seasonality together. A final Jeffries-Mathusita
threshold analysis determined cluster distinctiveness.
patterns in vegetation seasonality were detected across the island, with 5 clusters
each having a distinct seasonality pattern, and 5 cluster groupings containing
up to 20 similar patterns. Certain areas, predominantly within the CORINE
“pastures” class, exhibited two seasonal peaks in growth, with a marked decrease
in vegetation activity in June/July. Visual analysis of these areas on higher
spatial resolution satellite imagery also acquired during 2006, confirmed their
location within pasture areas. Land use such as silage cutting, practiced in
intensively-managed grasslands, is suggested as the most likely cause of the detected
double-season cycle. This finding has important implications for carbon
accounting under Ireland’s
commitments to the Kyoto protocol and can help
improve our knowledge of grassland management in Ireland.