Multi-site validation of daily SCOPE-model-simulated carbon and energy fluxes

The estimation of carbon and energy fluxes gives us insights into ecosystem-climate interaction and helps make prognoses for fluxes in a future climate. For this purpose, physically-based models provide a robust method for upscaling measurements from the flux tower scale to a larger area. Currently, Earth Observation satellites deliver data to constraint algorithms of vegetation structure parameters retrieval and climate models for meteorological variables estimation. However, vegetation functioning parameters, such as the maximum carboxylation capacity parameter (Vcmax) and the parameters for the stomatal response are still taken from look-up tables of plant functional types (PFT). This study aimed at quantifying the accuracy of simulations with the Soil Canopy Observation of Photosynthesis and Energy fluxes (SCOPE, Yang et al. 2021) model of gross primary productivity (GPP) and energy fluxes (net radiation, latent, sensible, ground heat flux) across Europe, constrained by weather, remote sensing of vegetation structure, and PFT specific vegetation functioning parameters.
Vegetation structure parameter leaf area index (LAI) was retrieved from Sentinel-3 top of atmosphere radiance measured with Ocean and Land Colour Instrument (OLCI). Meteorological data were taken from in situ observations and the ERA5-Land dataset of the European Centre for Medium-Range Weather Forecasts (ECMWF) distributed along with the validation data: a dataset of flux tower measurements in the so-called Drought-2018 ecosystem eddy covariance flux product provided by ICOS (Drought 2018 Team). The values of Vcmax taken from Groenendijk et al. (2011), Kattge et al. (2009) and Norton et al. (2019) were evaluated against the default SCOPE value of 60 µmol m-2 s-1. In addition, seasonally dynamic Vcmax (as a function of LAI) was used.
The results demonstrate high uncertainty of flux simulations: from 16% to 36% of the mean annual values for GPP and from 36% to 46% for evapotranspiration. The default seasonally static Vcmax outperformed all PFT-specific cases for GPP with RMSE of 283 g C m-2 yr-1 (R2 0.75), for ET the best performance (RMSE 139 mm yr-1, R2 0.46) was achieved on Groenendijk et al. (2011) mean per PFT values of Vcmax and BallBerrySlope. Ecosystems in the Mediterranean climatic zone, savannahs in Spain and evergreen needleleaf forest in Southern Italy, remain challenging for SCOPE due to the absence of the available water constraint on the energy balance.
Overall, this work validates carbon and energy balance parts of SCOPE model across European eddy covariance sites. The results pave the way to the operational usage of SCOPE for ecosystem flux mapping, especially in view of the release of the faster SCOPE2.0 version.

The project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 721995.