# Assimilation of Earth Observation Data Over Cropland and Grassland Sites into a Simple GPP Model
## Results  
### Site Examples  
The ability of the Sim model to simulate the actual dynamics of GPP is first illustrated on several crops and grassland sites where the model has shown different degrees of realism. Examples of simulated GPP and NDVI compared to the observed time series are reported in Figures 2 and 3 for crop sites and Figures 4-6 for grassland sites. For both crops and grassland sites we selected example sites with contrasting performances of the model (i.e., nice vs. poor fit). Figure 2: Observed and modelled GPP and NDVI for the crop site DE-Kli. **Left** panels: (top) time series of GPP from eddy covariance measurements (EC), Sim model (Sim), and MODIS GPP (MOD17); (bottom) observed (MODIS) and simulated NDVI (Sim). **Right** panels: scatter plots of EC GPP versus GPP simulated by the Sim model (top panel) and MOD17 GPP (bottom panel). For the **right** panels: dashed black line is the 1:1 line, blue continues line is the linear regression; root mean square error (RMSE) between modelled and observed GPP and the coefficient of determination (R\({}^{2}\)) of the linear regression are reported. Figure 4: Observed and modelled GPP and NDVI for the grassland site HU-Bug. Description of plots as Figure 2.  
Figure 5: Observed and modelled GPP and NDVI for the grassland site At-Neu. Description of plots as Figure 2.  
Figure 3: Observed and modelled GPP and NDVI for the cropland site IT-BCi. Description of plots as Figure 2.  
Figures 2 and 3 show the observed NDVI and GPP along with the same variables modelled by the Sim model for two EC crop sites. For comparison, the MODIS GPP estimates are also reported. In the cropland site DE-Kli (Germany, 50.89288\({}^{\circ}\)N, 13.52251\({}^{\circ}\)E; [57]), rapeseed, winter wheat, maize, and spring barley were cultivated in rotation (Figure 2). The NDVI signal captures the crop growth between sowing and harvest as well as the unmanaged vegetation growth occurring in the fallow land between crop cycles. This is possible thanks to the preliminary application of phenology algorithm that detects two growing seasons per year. The Sim model fits well the observed NDVI and is also able to track GPP temporal evolution with good accuracy (R\({}^{2}\) = 0.85, RMSE = 1.66 gC m\({}^{-2}\) d\({}^{-1}\)). Although some under- and over-estimation occurs in some years (e.g., 2009 and 2012, respectively) the Sim model GPP estimates are not affected by systematic underestimation as in the case of the MOD17 product. In the cropland site IT-BCi (Italy, 40.52375\({}^{\circ}\)N, 14.95744\({}^{\circ}\)E; [58]) shown in Figure 3 both the Sim model and MOD17 exhibit significant GPP underestimation at high EC GPP levels. Although the Sim model is able to differentiate the two seasons (maize in summer and fennel or rye-grass in winter) depicted by the observed NDVI, it largely underestimates the GPP of the summer crop (i.e., maize). Underestimation of the summer crop occur even though the Sim model does not select water limitation in most of the years (i.e., because the observed NDVI is better matched by model runs without water limitation), which is in agreement with reported irrigation practices. The scatter plots of Figure 3 show that underestimation of the Sim model is nonetheless less severe than the MODIS one. A plausible explanation for this difference may be related to an underestimate of the maximum light use efficiency (\(\epsilon_{\rm m}\)) that is not able to reproduce the photosynthesis level attained by the C4 crop under a Mediterranean climate and served by irrigation. In fact, both the Sim model and MOD17 use a constant \(\epsilon_{\rm m}\) of all crops that is likely not representative for this site because the \(\epsilon_{\rm m}\) of C4 crops is typically larger than that of C3 crops [47]. However, it is noted that in different climatic conditions the model does not underestimate C4 GPP, as for example in the cropland site DE-Kli of Figure 2 where maize was grown in year 2007 and 2012.