## 4 Discussion
### Winter Oil Seed Rape (WOSR)  
In situ data were also available for WOSR at Gowran, Co. Kilkenny. The crop was sown in September 2015 and harvested the following year in August 2016 [40]. A benefit of this site was the availability of both ecosystem and heterotrophic respiration. Ref. [56] assimilated optical and SAR data into the SAFY model for the estimation of GAI and biomass for rapeseed over three years in France, the model making use of the in-season remote sensing data derived GAI from Sentinel-1, Sentinel-2 and Landsat-8 simulated GAI with R\({}^{2}\) ranging from 0.84-0.92 and RRMSE from 28-41%. Our modeling approach represents an improvement in simulating biomass but not GAI, the reason maybe as described above, local calibration of partitioning coefficients (Supplementary Materials, Table S2)that partitions biomass into leaf and non-leaf biomass. Daily GAI growth and senescence is derived by multiplying daily biomass with the leaf partitioning coefficients (PL\({}_{\text{a}}\) and PL\({}_{\text{b}}\)) and specific leaf area parameter. In the assimilation approach these parameters and the other variable parameters (Supplementary Material) can be optimized with in-season remote sensing data. Ref. [57] estimated rapeseed LAI using the empirical equations on UAV multispectral images at different growth stages, the study concludes that LAI estimation at seedling and flowering stages have high errors in comparison to the elongation stage and it may also be a probable reason for the underestimation of GAI single point observation in our work (Figure 6a). The carbon fluxes module of the model simulated the components of the carbon budgets. Due to lack of observed data, we have compared the modelled simulated ecosystem respiration that includes aboveground autotrophic and soil respiration with the observed ecosystem soil respiration. The soil chambers used in the study were inserted into the interrow spacing between the plants but did not include plants, therefore CO\({}_{2}\) flux consisted predominantly of heterotrophic respiration with a minor contribution of autotrophic root respiration. The large difference in CO\({}_{2}\) emissions between the modelled respiration and measured soil respiration values thus can be attributed to the aboveground autotrophic respiration not captured by the soil chambers (Figure 7a,c). Soil respiration was also measured in bare soil without vegetation to quantify CO\({}_{2}\) emissions only of heterotrophic origin. In contrast, the model simulated heterotrophic respiration was compared with the observed heterotrophic respiration and a low error with improved correlation was found compared to the ecosystem respiration model (Figure 7b,d). Ref. [25] simulated the components of the carbon budget for sunflower in France, the study shows ecosystem respiration was simulated with R\({}^{2}\) = 0.83 and RMSE = 0.96 g\(\cdot\)C\(\cdot\)m\({}^{2}\cdot\)day\({}^{-1}\), higher than that found in this study. Additional evaluation data are required to confirm if the model is capable of producing more reliable estimates of ecosystem respiration. In a previous study [58], the CERES model was evaluated at several European flux sites, the model estimated components of carbon budget with RMSE = 30 g\(\cdot\)C\(\cdot\)m\({}^{2}\cdot\)day\({}^{-1}\). In the present study, heterotrophic respiration is somewhat underestimated in comparison to the observed heterotrophic respiration as it will be limited by temperature, moisture and C substrate (Figure 7b). Overall, the model was found to estimate biomass across all crops and sites. Biomass is a function of APAR, ELUE and is constrained by both temperature (F\({}_{\text{T}}\)Ta) and water (WST) stress. However, there are some uncertainties in simulating GAI and yield. For the simulation of GAI, the best model estimates were associated with SB, likely due to the availability of an optimal suite of model parameters for this crop.