## 4 Discussion and Conclusions
Therefore, since the \(\theta\) measured at 10 cm always exceeds the threshold of 0.13 + 0.04 = 0.17 m\({}^{3}\) m\({}^{-3}\), no drought stress is predicted by MEGANv2.1 based on 10 cm \(\theta\) data. The level at which the \(\theta\) is defined is important due to the general increase of \(\theta\) with depth and to changes in the temporal evolution due to water percolation. Regarding the assessment of drought impacts using formaldehyde columns, the following shortcomings and limitations are raised. OMI HCHO observations lack the precision required to conduct an evaluation on isoprene emission estimates at short temporal scales (daily and weekly), which would be most relevant for drought impact studies. With its much higher resolution and increased sensitivity to day-to-day variability [102], the TROPOMI sensor on Sentinel-5p offers promising perspectives for future research. As we have seen, the exclusion of OMI data characterized by high AOD or high pyrogenic emissions degrades the statistics while not fundamentally affecting the conclusions of the study. Modelled tropospheric HCHO columns over the MOFLUx site pointed to an overall better agreement with the adjusted parameterization and the radius of 20 km was chosen as the best for representing the site given that satellite overpasses within a smaller radius were too infrequent (<17/month for 10 km). At greater coverage (30 km), however, the MAGRITTE runs with the least soil moisture stress (as in MAGRITTE, \(\gamma_{SM}=1\) and MAGRITTE, \(\gamma_{SM}^{0.04}\)) are in best overall agreement with OMI observations (Table S1). At the global scale, high values of \(\Delta\theta\) are not supported by observations. Indeed, multi-year global simulations (2005-2016) with the CTM IMAGES indicated that the use of \(\Delta\theta\) = 0.12 m\({}^{3}\) m\({}^{-3}\) does not improve significantly the comparison with OMI HCHO over dry regions, and it worsens the comparison over most forested areas. The best agreement with OMI in terms of interannual variability is found when the drought activity factor is not accounted for (not shown here). The reason for such disparity between the MOFLUx study and the large-scale evaluation using HCHO could lie in the spatial resolution. Disparities in soil water content between local and grid cell scales can be huge given the great heterogeneity in soil properties like porosity and subsequently for the 'absolute' values of soil moisture and the empirical parameter. Another issue with the current parameterization is that it is independent of various environmental factors other than the soil moisture characteristics. Furthermore, the response of isoprene emissions depends on the drought characteristics and plant species. The parametrization of drought stress should account for different effects across biomes and species [104; 105; 106]. For instance, [PERSON] et al. (2018) [39] suggested an exponential reduction function of the SWC for drought-adapted species. In addition, longer and recurrent drought (e.g., 10 years) could have progressive and cumulative effects over time in forested ecosystems dominated by long-lived species [18]. Besides the complexity of the drought response, the lack of ecosystem-scale, long-term monitoring of soil moisture and isoprene emissions is another important issue. Direct comparison between MEGAN and flux measurements in various drought conditions has been only possible for the MOFLUx site. Other campaigns including isoprene concentration measurements in drought-impacted ecosystems ([39; 40]) provide qualitative insights but no direct constraint on the fluxes. A possible avenue for drought stress parameterizations could be the development of empirical relationships making use of remotely sensed photosynthetic parameters or vegetation indices such as SIF [47], Emissivity Difference Vegetation Index (EDVI; [107]) or Photochemical Reflectance Index (PRI; [108]). So far, however, progress has been limited in that direction.