## 3 Results
### Daily Time Series and Diel Cycle  
During the mild drought year, the daily drought stress factors have virtually no impact on isoprene fluxes except when adopting the optimized factor \(\gamma_{SM}^{OPT}\), which entails a flux reduction of about 10% during the month of August (Figure 3a). The bias-corrected MEGAN simulations explain 93% of the variability of daily isoprene fluxes during summer 2011 for the simulations using \(\gamma_{SM}^{OPT}\) or using \(\gamma_{SM}=1\) (Figure 3b). Measurements are missing during 10-17 August due to an equipment failure. The reasons for the model overestimation in the second half of August are unclear, but could be partly due to an underestimation of the effect of leaf age as previously noted in MEGAN comparisons with flux measurements at a temperate forest [49].  
In the severe stress year (2012), the model performance is lowest in the no-stress configuration (\(\gamma_{SM}=1\)) both in terms of correlation (r = 0.76) and root-mean square error (RMSE = -5 mg m\({}^{-2}\) h\({}^{-1}\), Figure 3d). The \(\gamma_{SM}^{*}\) factor improves only slightly the comparison (r = 0.79 and RMSE = -3 mg m\({}^{-2}\) h\({}^{-1}\)), whereas the application of \(\gamma_{SM}^{OPT}\) improves the fit in 2012 to r = 0.90 and RMSE = -2 mg m\({}^{-2}\) h\({}^{-1}\). The stress factor \(\gamma_{SM}^{OPT}\) decreases the isoprene fluxes from early June to late October, with a maximum reduction of about 70% at the end of August, and a significant decrease of 40-50% in late summer (Figure 3c). In the measurement dataset, the highest daily flux occurs on June 24, at \(\sim\)18 mg m\({}^{-2}\) h\({}^{-1}\). Without the stress factor, modelled isoprene fluxes peak 12 days later, on 6 July. Those fluxes overestimate the observations by 6.00 mg m\({}^{-2}\) h\({}^{-1}\) in July August (a percentage bias of about 98%). This bias is strongly reduced (to 0.42 mg m\({}^{-2}\) h\({}^{-1}\)) due to the application of \(\gamma_{SM}^{OPT}\) i.e., \(\sim\)7%. The drop in emissions is sharper with \(\gamma_{SM}^{*}\), up to 90% in July August (Figure 3c), leading to a model underestimation reaching 3.13 mg m\({}^{-2}\) h\({}^{-1}\) (about \(-\)58%) for that same period (Figure 3d). The improvement of estimated isoprene fluxes using \(\gamma_{SM}^{OPT}\) is also seen in the drop in the RMSE from \(\sim\)5 mg m\({}^{-2}\) h\({}^{-1}\) to roughly 2 mg m\({}^{-2}\) h\({}^{-1}\). Overall, the comparison shown in Figure 3 points to a better agreement with the use of \(\gamma_{SM}^{OPT}\) compared to \(\gamma_{SM}^{*}\). Finally, the model succeeds at reproducing the normalized diel pattern of emission fluxes (Figure 4), independent of the application of the drought stress factor. Nevertheless, the diurnal peak occurs sooner in 2011, by one hour in June, and as much as 2 h in late summer.  
Figure 3: (**a**) Time series of the daily soil moisture stress activity factor in 2011 from \(\gamma_{SM}^{OPT}\) (orange) and \(\gamma_{SM}^{*}\) (blue); (**b**) Daily isoprene fluxes averaged over 7–17 h (CST) for 2011. Model results are shown for the cases \(\gamma_{SM}=1\) (grey line), \(\gamma_{SM}^{OPT}\) (orange) and \(\gamma_{SM}^{*}\) (blue). The latter overlays the grey line since \(\gamma_{SM}^{*}=1\) over the entire period in 2011. (**c,d**): as (**a,b**), for 2012. Isoprene measurements (green shading) are from campaigns conducted in 2011 by [30] and in 2012 by [32].