Daily GPP estimates in Mediterranean ecosystems by combining remote sensing and meteorological data

The accurate representation of terrestrial CO2 uptake (GPP) using Monteith's approach requires a frequent and site-specific parameterization of the model inputs. In this work, an optimization of this approach has been carried out by adjusting the inputs (f(APAR), PAR and epsilon) for the study area, peninsular Spain, a typical Mediterranean region. The daily GPP images have been calculated for 2008 and 2011 with a 1-km spatial resolution and validated by comparison with in situ GPP estimates from the eddy covariance data (direct validation) and by inter-comparison with the MODIS GPP product. The direct validation has evidenced an excellent agreement with correlations up to 0.98 in 2008 and 0.92 in 2011 in some sites. The inter-comparison has shown that the two GPP products are consistent temporally. However, a slightly decrease of the correlation has been observed in some areas. The validation has also shown that our optimized GPP product accounts better for the water stress than the MODIS product. The analysis of the explanatory power of the model in terms of its inputs shows, as expected, that PAR and f(APAR) are the most relevant inputs. The f(APAR) plays a major role on GPP estimation when the vegetation phenology maximum is not reached during solar solstice. Finally, it has been shown that the influence of the water stress, associated with the water shortage typical of Mediterranean landscapes, has to be evaluated accurately in order to explain the GPP inter-annual variability. (C) 2015 International Society for Photogrammetry and Remote Sensing, Inc. (ISPRS). Published by Elsevier B.V. All rights reserved.