Modelling Daily Gross Primary Productivity with Sentinel-2 Data in the Nordic Region-Comparison with Data from MODIS  
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1 Department of Physical Geography and Ecosystem Science, Lund University, Solvegatan 12, SE-223 62 Lund, Sweden; [EMAIL_ADDRESS] ([PERSON]); [EMAIL_ADDRESS] ([PERSON]); [EMAIL_ADDRESS] ([PERSON]); [EMAIL_ADDRESS] (J.A. ); [EMAIL_ADDRESS] (M.M. ); [EMAIL_ADDRESS] (R.J.); [EMAIL_ADDRESS] (M.K. ); [EMAIL_ADDRESS] (L.E.) [PERSON]  
1 Department of Physical Geography and Ecosystem Science, Lund University, Solvegatan 12, SE-223 62 Lund, Sweden; [EMAIL_ADDRESS] (S.J. ); [EMAIL_ADDRESS] ([PERSON]); [EMAIL_ADDRESS] ([PERSON]); [EMAIL_ADDRESS] ([PERSON]); [EMAIL_ADDRESS] (M.M. ); [EMAIL_ADDRESS] (R.J.); [EMAIL_ADDRESS] (M.K. ); [EMAIL_ADDRESS] (L.E.) [PERSON]  
1 Department of Physical Geography and Ecosystem Science, Lund University, Solvegatan 12, SE-223 62 Lund, Sweden; [EMAIL_ADDRESS] ([PERSON]); [EMAIL_ADDRESS] ([PERSON]); [EMAIL_ADDRESS] ([PERSON]); [EMAIL_ADDRESS] ([PERSON]); [EMAIL_ADDRESS] ([PERSON]); [EMAIL_ADDRESS] ([PERSON]); [EMAIL_ADDRESS] ([PERSON]); [EMAIL_ADDRESS] (L.E.) [PERSON]  
1 Department of Physical Geography and Ecosystem Science, Lund University, Solvegatan 12, SE-223 62 Lund, Sweden; [EMAIL_ADDRESS] (S.J. ); [EMAIL_ADDRESS] (J.H. ); [EMAIL_ADDRESS] (H.J. ); [EMAIL_ADDRESS] (J.A. ); [EMAIL_ADDRESS] (M.M. ); [EMAIL_ADDRESS] (R.J.); [EMAIL_ADDRESS] (M.K. ); [EMAIL_ADDRESS] (L.E.) [PERSON]  
2 Department of Environmental Engineering, Technical University of Denmark, Bygningstorvet 115, 2800 Lyngby, Denmark; [EMAIL_ADDRESS]  
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3 Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Skosymarksgrand, SE-901 83 Umea, Sweden; [PERSON]  
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1 Department of Physical Geography and Ecosystem Science, Lund University, Solvegatan 12, SE-223 62 Lund, Sweden; [EMAIL_ADDRESS] (S.J. ); [EMAIL_ADDRESS] (J.H. ); [EMAIL_ADDRESS] (H.J. ); [EMAIL_ADDRESS] (J.A. ); [EMAIL_ADDRESS] (M.M. ); [EMAIL_ADDRESS] (R.J.); [EMAIL_ADDRESS] (M.K. ); [EMAIL_ADDRESS] (L.E.) [PERSON]  
1 Department of Physical Geography and Ecosystem Science, Lund University, Solvegatan 12, SE-223 62 Lund, Sweden; [EMAIL_ADDRESS] (S.J. ); [EMAIL_ADDRESS] (J.H. ); [EMAIL_ADDRESS] (H.J. ); [EMAIL_ADDRESS] (J.A. ); [EMAIL_ADDRESS] (M.M. ); [EMAIL_ADDRESS] (R.J.); [EMAIL_ADDRESS] (M.K. ); [EMAIL_ADDRESS] (L.E.) [PERSON]  
1 Department of Physical Geography and Ecosystem Science, Lund University, Solvegatan 12, SE-223 62 Lund, Sweden; [EMAIL_ADDRESS] (S.J. ); [EMAIL_ADDRESS] ([PERSON]); [EMAIL_ADDRESS] ([PERSON]); [EMAIL_ADDRESS] ([PERSON]); [EMAIL_ADDRESS] (M.M. ); [EMAIL_ADDRESS] ([PERSON]); [EMAIL_ADDRESS] (M.K. ); [EMAIL_ADDRESS] (L.E.) [PERSON]  
1 Department of Physical Geography and Ecosystem Science, Lund University, Solvegatan 12, SE-223 62 Lund, Sweden; [EMAIL_ADDRESS] (S.J. ); [EMAIL_ADDRESS] ([PERSON]); [EMAIL_ADDRESS] ([PERSON]); [EMAIL_ADDRESS] (J.A. ); [EMAIL_ADDRESS] (M.M. ); [EMAIL_ADDRESS] (R.J.); [EMAIL_ADDRESS] (M.K. ); [EMAIL_ADDRESS] (L.E.) Footnote 21: email: {lars. on the light use efficiency (LUE) model [7], empirical models with site- or ecosystem-specific environmental variables [8], or mechanistic models with remotely sensed inputs [9; 10; 11]. LUE models express GPP as a product of the photosynthetic light use efficiency (\(\epsilon\)), and photosynthetically active radiation absorbed by the vegetation (APAR). Although APAR is nonlinearly correlated to GPP at the daily time step [12; 13], the relationship can be considered linear over monthly or annual periods [14; 15; 16]. APAR can be derived by multiplying PAR (photosynthetically active radiation) by fAPAR (fractional PAR absorbed by the vegetation). Several studies have shown that satellite-derived spectral vegetation indices have linear or near-linear relationships with fAPAR within different vegetation types and climatic conditions and are therefore used as proxies of fAPAR [17; 18; 19; 20]. The LUE factor, on the other hand, is more difficult to estimate.