## 4 Discussion  
### Posterior Parameters  
In Figure 5, relative parameter changes, that is parameter changes from prior to posterior divided by their prior uncertainty, are shown. Some parameters related to phenology, water, and energy balance processes showed large changes (see parameters no. 57-75, and no. 89-97). This indicates that the observations of soil moisture and FAPAR have a major impact on the assimilation procedure and strongly constrain these processes in the model.  
A wide range of posterior values was computed for _emis_0 (Figure 5, Parameter no. 75). This parameter regulates the emissivity of the atmosphere and therefore has a strong impact on the surface water and energy fluxes through the radiation balance at the surface. At sites dominated by C3/C4 grasses (e.g., AU-DaS, ES-LMa, US-Kfs and US-Wkg),'sm+fapar' experiments resulted in a remarkable range for _emis_0 across the respective sites. C3/C4 grasses had smaller root depths (in the model the prior values amount to 0.5 m) in comparison to trees (prior values of 1.5-3.0 m). Therefore, the growth conditions of grass in the model, but also in nature, depend strongly on soil moisture in the surface layer as well as on radiation. Moreover, surface conditions also differed from site to site due to roughness and surface cover variations, which potentially led to the large range of _emis_0 values.  
Figure 6 shows the range of values for the two parameters that were spatially differentiated by soil texture. The parameters were related to the soil water retention curve, namely \(s_{b}\), scalar for water retention curve shape parameter, and \(s_{n}\), scalar for soil porosity. In Figure 6, the optimized values span a wide range across almost all soil textures. This indicates that there are large uncertainties in soil hydrology processes. In the BETHY model, soil hydrology was modeled by a bucket model with an overlying surface soil moisture layer [19]. The soil water retention curve cannot be robustly  
Figure 5: Parameter value changes relative to prior for all assimilation experiments (29 in total), with a cross symbol indicating the mean of changes, box for standard deviation, and whisker for max and min values, parameter numbers correspond to the numbers listed in Table 1.  
constrained when soil moisture or FAPAR was assimilated as demonstrated by the large range of posterior values for the respective parameters.  
When comparing the'sm' experiment and the 'fapar' experiment, a sort of a \"trade-off\" between soil moisture and FAPAR becomes evident in the assimilation. The'sm' assimilations resulted in relatively large changes in the parameter, _emis_0, that is the emissivity of the atmosphere, and the parameter, \(s_{b}\), which controls the shape of the water retention curve (see [19]). To the contrary, both parameters hardly changed when assimilating FAPAR only ('fapar'). For 'fapar' experiments, parameters related to phenology experienced the largest changes relative to their prior values, e.g., \(T_{r}\) (no. 62-63), which is the spatial range (essentially the with-in grid cell variability) of the phenology temperature trigger, \(k_{L}\) (no. 67-68), which denotes the inverse of leaf longevity, and \(\tau_{\text{W}}\) (no. 69-71), which parameterizes the length of dry spells before leaf shedding.