Satellite image 10-day composites of the MERIS Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) over Ireland

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Resource or Project Abstract

The aim of this study was to develop a methodology, based on satellite remote sensing, to estimate the vegetation Start of Season (SOS) across the whole island of Ireland on an annual basis. This growing body of research is known as Land Surface Phenology (LSP) monitoring.
The SOS was estimated for each year from a 7-year time series of 10-day composited, 1.2 km reduced resolution MERIS Global Vegetation Index (MGVI) data from 2003 to 2009, using the time series analysis software, TIMESAT. The selection of a 10-day composite period was guided by in-situ observations of leaf unfolding and cloud cover at representative point locations on the island. The MGVI time series was smoothed and the SOS metric extracted at a point corresponding to 20% of the seasonal MGVI amplitude. The SOS metric was extracted on a per pixel basis and gridded for national scale coverage. There were consistent spatial patterns in the SOS grids which were replicated on an annual basis and were qualitatively linked to variation in landcover. Analysis revealed that three statistically separable groups of CORINE Land Cover (CLC) classes could be derived from differences in the SOS, namely agricultural and forest land cover types, peat bogs, and natural and semi-natural vegetation types. These groups demonstrated that managed vegetation, e.g. pastures has a significantly earlier SOS than in unmanaged vegetation e.g. natural grasslands. There was also interannual spatio-temporal variability in the SOS. Such variability was highlighted in a series of anomaly grids showing variation from the 7-year mean SOS. An initial climate analysis indicated that an anomalously cold winter and spring in 2005/2006, linked to a negative North Atlantic Oscillation index value, delayed the 2006 SOS countrywide, while in other years the SOS anomalies showed more complex variation. A correlation study using air temperature as a climate variable revealed the spatial complexity of the air temperature-SOS relationship across the Republic of Ireland as the timing of maximum correlation varied from November to April depending on location. The SOS was found to occur earlier due to warmer winters in the Southeast while it was later with warmer winters in the Northwest. The inverse pattern emerged in the spatial patterns of the spring correlates. This contrasting pattern would appear to be linked to vegetation management as arable cropping is typically practiced in the southeast while there is mixed agriculture and mostly pastures to the west. Therefore, land use as well as air temperature appears to be an important determinant of national scale patterns in the SOS.
The TIMESAT tool formed a crucial component of the estimation of SOS across the country in all seven years as it minimised the negative impact of noise and data dropouts in the MGVI time series by applying a smoothing algorithm. The extracted SOS metric was sensitive to temporal and spatial variation in land surface vegetation seasonality while the spatial patterns in the gridded SOS estimates aligned with those in landcover type. The methodology can be extended for a longer time series of FAPAR as MERIS will be replaced by the ESA Sentinel mission in 2013, while the availability of full resolution (300m) MERIS FAPAR and equivalent sensor products holds the possibility of monitoring finer scale seasonality variation.

This study has shown the utility of the SOS metric as an indicator of spatio-temporal variability in vegetation phenology, as well as a correlate of other environmental variables such as air temperature. However, the satellite-based method is not seen as a replacement of ground-based observations, but rather as a complementary approach to studying vegetation phenology at the national scale. In future, the method can be extended to extract other metrics of the seasonal cycle in order to gain a more comprehensive view of seasonal vegetation development.

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Contact Information for This Resource

Dr. Brian O'Connor
University College Cork

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Data, Files, Information Objects Related To This Project Resource

This resource has SEMI-PUBLIC availability. This means that the metadata for the resource is available to the public but the data files and information objects connected to the resource are not. There are currently 9 data files and/or information objects connected to this resource. You will need to contact the owners of this resourceto seek their consent to access the files. Contact information for the owners of this resource can be found in the Responsible Parties information section of the metadata.


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Access Information For This Resource

SAFER-Data Display URL
Resource KeywordsSatellite imagery, vegetation, photosynthesis, Envisat MERIS, vegetation seasonality, land surface phenology, vegetation monitoring
EPA/ERTDI/STRIVE Project Code2009-PhD-ET-14
EPA/ERTDI/STRIVE Project ThemeEnvironmental Technologies
Resource Availability: Non Owner-Users Cannot Download Files from This Resource
Limitations on the use of this ResourceAwaiting confirmation that the data are free to access for third party users from the data providers (G-POD service of the European Space Agency). Will update usage status when notified.
Number of Attached Files (Publicly and Openly Available for Download): 0
Project Start Date Sunday 30th September 2007 (30-09-2007)
Earliest Recorded Date within any attached datasets or digital objects Tuesday 21st May 2002 (21-05-2002)
Most Recent Recorded Date within any attached datasets or digital objects Saturday 1st January 2011 (01-01-2011)
Published on SAFERMonday 22nd August 2011 (22-08-2011)
Date of Last EditFriday 26th August 2011 at 18:13:33 (26-08-2011)
Datasets or Files Updated On Friday 26th August 2011 at 18:09:09 (26-08-2011)

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Geographical and Spatial Information Related To This Resource

Description of Geographical Characteristics of This Project or Dataset
Island of Ireland bounded by 51°-56°N latitude and 5°-11°W longitude

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Supplementary Information About This Resource

In this section some supplementary information about this resource is outlined. Lineage information helps us to understand why this project was carried out, what policy or research requirements did it fulfil, etc. Lineage is important in understanding the rationale behind the carrying out of a project or the collection of a specific dataset etc. Links to web sites, applications, papers, etc are outlined to provide you with additional information or supplementary reading about the project or dataset

Lineage information about this project or dataset
The project arose from evidence from the Irish phenological gardens suggesting that spring is occurring earlier and therefore the growing season is being extended. the goal of the work was to monitor the timing of spring across Ireland from remotely-sensed satellite data. Satellite sensors such as MERIS collect data in the visible and near-infrared spectral bands over the whole Earth on a regular basis. From these data, vegetation indices may be constructed and the fraction of photosynthetically absorbed radiation (FAPAR) - a measure of vegetation growth- may be calculated. Analysis of such measurements has allowed a characterization of the seasonality of vegetation across the island.
Supplementary Information
A multiannualtime series of 10-day composited, 1.2 km reduced resolution Medium Resolution Imaging Spectrometer (MERIS) Global Vegetation Index (MGVI) data from May,2002 to January,2011. The MGVI has been optimised to estimate the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR), a measure of plant vigour and recognised as a terrestrial Essential Climate Variable by the Global Climate Observing System(GCOS). The products are MERIS Level 3 aggregated products.These data are operationally processed and produced at the Grid Processing-on Demand (G-POD) of European Space Research Institute (ESRIN) using the European Commission ? DG Joint Research Centre (JRC) algorithm and software. The MERIS Level-3 Aggregated Products (MLAP) are produced from MERIS Level-3 Time Composite Products (MTCP) at reduced resolution (1.2 km). The MLAP are produced by spatially aggregating MTCP over a low spatial resolution window (the same or a different geographical window). All information concerning the spatial resolution of the aggregated products as well as the type of projection are given in the header of the file.The MLAP including datasets and metadata are stored in Hierarchical Data Format (HDF4.2) files. The datasets are multidimensional arrays containing scientific data values.Therefore they represent not just the FAPAR measure but consist of several datasets of scientific data as well as metadata. Data products included in the file
contain information about the MGVI/FAPAR values inside each grid cell and correspond to the spatial average values, the standard deviation and the number of samples used to make the statistic. Additional products concern several statistics on the number of pixels detected by several flags, the spatial averaged value of the normalized reflectance in the blue, green, red and near-infrared domain and the two rectified bands, and four median values with standard deviation of both illumination and viewing angles. Metadata exist in the HDF file to provide comprehensive information about the data and can be associated with each field. They are stored as HDF Attributes.
Links To Other Related Resources
Home page of the Joint Research Centre (JRC)FAPAR project (Opens in a new window)

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Other Similiar Projects on SAFER

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