## 2 Materials and Methods
### MEGAN-MOHYCAN: Biogenic VOC Emission Modelling
The estimation of \(\gamma_{SM}^{*}\) follows:  
\[\gamma_{SM}^{*}=\left\{ \begin{array}{cc}1&\beta_{t}>0.6\\ V_{CMAX}/\alpha&0<\beta_{t}<0.6\\ 0&\beta_{t}=0\end{array} \right. \tag{3}\]  
The empirical parameter \(\alpha\) (=37) was calibrated by [69] based on field measurements at the MOFLUX site. Two evaluation approaches are considered to assess the response of biogenic isoprene emissions to drought in the MEGAN biogenic emission model. Single-point estimations of isoprene fluxes using algorithms from Equations (2) and (3) are evaluated against measurements conducted in 2011 and 2012 presented hereinabove. Global and regional (over North America) approaches are also conducted through the assessment of tropospheric HCHO columns using satellite-based observations against simulations from a chemistry transport model that accounts for \(\gamma_{SM}\) (Equation (2)) with different values of \(\Delta\theta\). The MEGAN-MOHYCAN model is set up for the MOFLUX site as follows. Single-point model simulations of daily isoprene fluxes for years 2011 and 2012 are performed using local meteorological conditions provided by the AmeriFlux website ([[https://ameriflux.lbl.gov/sites/siteinfo/US-MOz](https://ameriflux.lbl.gov/sites/siteinfo/US-MOz)]([https://ameriflux.lbl.gov/sites/siteinfo/US-MOz](https://ameriflux.lbl.gov/sites/siteinfo/US-MOz)), accessed on 23 June 2021). Hourly measurements of air temperature and photosynthetic photon flux density from the tower are used. Daily LAI measurements for the 2012 campaign were obtained from [32]. Since direct local measurements of LAI were not available to us for 2011, we use the daily 2012 values, corrected for interannual variation based on monthly averaged Moderate Resolution Imaging Spectroradiometer (MODIS) LAI (MODIS 15A2H collection 6 available at [[https://lpdaac.usgs.gov](https://lpdaac.usgs.gov)]([https://lpdaac.usgs.gov](https://lpdaac.usgs.gov)), accessed on 15 June 2021) at 0.5\({}^{\circ}\) resolution in the vicinity of the MOFLUX site. More precisely, the 2012 local values are multiplied by the ratio of MODIS LAI between the two years (Figure S1). The canopy emission factor \(\varepsilon\) was set to 8.6 mg m\({}^{-2}\) h\({}^{-1}\). The value is chosen such that the model without any drought stress correction (\(\gamma_{SM}=1\)) matches the average measured isoprene fluxes of the 2011 campaign, when the effect of drought was of lesser magnitude. As will be seen in Section 3, the \(\gamma_{SM}\) parameterization has an almost negligible impact in 2011. In the following, the simulation without stress (\(\gamma_{SM}=1\)) will be referred to as the baseline. The impact of drought stress is accounted for by multiplying the baseline emissions by either daily \(\gamma_{SM}\) (Equation (2)) or \(\gamma_{SM}^{*}\) (Equation (3)). The latter was obtained using CLM5 (instead of CLM4.5 as in [PERSON] et al., 2018) and local environmental conditions observed at the MOFLUX site. The daily \(\gamma_{SM}\) is obtained using daily satellite-based root-zone \(\theta\) provided by the GLEAM model at 0.25\({}^{\circ}\times\) 0.25\({}^{\circ}\) spatial resolution, along with the \(\theta_{W}\) distribution taken from the static soil properties defined in GLEAM (see Section 2.3). For reasons explained in Section 3.1, the empirical parameter \(\Delta\theta\) is re-evaluated and set to \(\Delta\theta=0.12\); the adjusted factor using GLEAM is referred to as \(\gamma_{SM}^{OPT}\). The drought factor using the default \(\Delta\theta=0.04\) is denoted \(\gamma_{SM}^{0.04}\).