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DERIVING GPP BY MEANS OF THE fAPAR MERIS PRODUCT IN AN AGRICULTURAL ECOSYSTEM OF CASTILE AND LEON (SPAIN)

María Luisa Sánchez(1), Beatriz de Torre(1), Isidro Pérez(1,1), Vanessa Paredes(1) and Angeles García(1)

(1) University of Valladolid, Prado de la Magdalena s/n, 47005, Spain

Abstract

This paper presents the results of the GPP 8-d estimated values using the fAPAR product supplied by MERIS in an agricultural ecosystem located in the geographical centre of the Spanish plateau over selected periods (mainly the growing season) in 2003, 2004, 2005 and 2006. It is worth mentioning that, except for 2003, which was a “rainy year”, the period of study was dominated by drought, with 2005 being particularly severe.

Estimates of GPP-8d were determined as follows: where PAR is the incident photosynthetically active radiation, fAPAR, the fraction of PAR absorbed by green vegetation, ε max the potential light use efficiency, LUE, and f a scalar varying between 0 and 1 to take into account the reduction of the potential LUE under limiting environmental conditions. ε max depends on global ecosystem classes and typically ranges from 0.00060 to 0.00129 kgC MJ-1. Here the optimal value was parameterised through a comparison of the estimated GPP-8 d and those measured in an eddy-covariance tower installed at the measuring site. The f values were assumed to be equal to EF, the evaporative fraction, defined as: , where LE and H represent the measured latent heat and sensible heat fluxes, respectively.

Access to the MERIS images was performed by means of the MERCI interface and processed using BEAM VISAT software, version 3.7, from ESA (TOC Vegetation Processor). The overall number of images processed was 482. The fAPAR values considered in this paper correspond to the means of the individual MERIS images available daily during each of the 8-d concurrent observational data in the 1040 m x1160 m pixel centred on the measuring site.

The observational GPP-8d concurrent values were determined by subtracting the CO2 net fluxes, NEE, to the respiration term, RE. RE was parameterised on air temperature, T, and EF, the latter included as a proxy to characterise soil water availability.

The linear fit of GPP 8-d estimates versus the concurrent observed ones yielded a correlation coefficient of 0.94, proving the goodness of the fit. The slope, εmax, 0.00060 kgC MJ-1, was consistent with the LUE optimal values for crops reported in the literature.

 

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry