Leaf Area Index (LAI) is defined as total one- sided leaf area per unit horizontal ground surface area . LAI is a key variable for vegetation monitoring and important for modelling energy and matter fluxes between plants and their environment, since it quantifies the green plant area that constitutes the biosphere-atmosphere interface . Photosynthesis takes place in leaves, which directly drives gas exchange of oxygen and carbon oxide, biogenic emissions, and biomass production. Being a proxy for vegetation biomass, LAI is also suited for characterizing vegetation abundance and spatial distribution. Furthermore, LAI has an important effect on the hydrological cycle by quantifying the water exchange through canopy interception and transpiration, thus influencing evapotranspiration, infiltration and runoff generation. Spatially and temporally explicit information on LAI, which is needed to account for the natural variability of ecosystems, can however only be derived using remote sensing.
The Sentinel-2 grassland Leaf Area Index maps are derived based on the inverted radiation transfer model PROSAIL (PROSPECT+SAIL ). The level 1C Sentinel-2A data are calibrated and atmospherically corrected using the sen2cor processor, as well as resampled, gap filled, and masked. The local geometries are calculated based on the EEA EU-DEM. Grassland areas are defined according to the Copernicus Land Monitoring Services Natural Grasslands layer and the CORINE 2012 Pastures layer. During the inversion, the reflectance values are compared against pre-calculated Look-up tables (LUTs) using a cost function. The procedure is optimized to the specific conditions of alpine grasslands as well as to the LUT and sensor characteristics (noise levels, spectral resolution). As result of a multiple solutions approach, a spatially explicit inversion uncertainty is provided together with the LAI values.
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