## 3 Results  
### Soil Water Content and Soil Moisture Stress Factor  
The \(\gamma_{SM}\) factor (Equation (2)) depends explicitly on the plant water status. Figure 2 shows the daily \(\theta\) as measured at a depth of 100 cm and as represented by the root-zone (0-250 cm) GLEAM product at the grid cell encompassing the MOFLUX site in years 2011 and 2012. The exceptional conditions of 2012 are shown by the decrease in the observed \(\theta\), from 67% at the beginning of May to a minimum of 34% by the end of August. The drought condition was relieved with the return of precipitation illustrated by the sharp increase in the \(\theta\) at the end of August 2012. The mild drought of 2011 had a lesser impact on \(\theta\), which decreased from 70% in July to 56% in August.  
The root-zone \(\theta\) of GLEAM correlates well with in situ observations in both years, with Pearson's coefficients of 0.81 and 0.91. While part of the bias stems from differences in soil depths and spatial horizontal resolution, model biases are expected due the ill-representation of certain soil properties (field capacity, critical and residual soil moisture, etc.); nonetheless, absolute values are expected to differ due to the lack in regional representativeness of soil moisture point in situ measurements, mainly due to the high spatial variability in porosity. The local mean porosity varies between 0.55 m\({}^{3}\) m\({}^{-3}\) at 50 cm and  
Figure 2: Time series of the volumetric soil moisture (in m\({}^{3}\) m\({}^{-3}\)) at the MOFLUX site as obtained from measurements at 100 cm (orange) and as estimated by the GLEAM model at the root-zone level at the 0.25\({}^{\circ}\)\(\times\) 0.25\({}^{\circ}\) grid cell including the site (blue) in (**a**) 2011 and (**b**) 2012. The dashed horizontal lines mark the threshold values (\(\theta_{1}=\theta_{W}+0.04\)) for each dataset. The blue shading indicates the transition between the GLEAM wilting point and the threshold value (\(\theta_{W}+0.12\)) obtained by model optimization against isoprene MOFLUX data.  
about 0.70 m\({}^{3}\) m\({}^{-3}\) at the surface ([PERSON], personal communication), based on measurements at four locations around the Missouri site; the IGBP-DIS porosity used in GLEAM is 0.43 m\({}^{3}\) m\({}^{-3}\) at the site, and vertically constant.  
Following the \(\gamma_{SM}\) parameterization of MEGANv2.1, both the locally measured \(\theta\) (orange line on Figure 2) and the GLEAM \(\theta\) (blue line) always exceed the default threshold value, \(\theta_{1}=\theta_{W}+0.04\), represented by dashed lines in Figure 2. The threshold is estimated at 0.27 m\({}^{3}\) m\({}^{-3}\) based on local \(\theta_{W}\), and at 0.21 m\({}^{3}\) m\({}^{-3}\) in GLEAM based on the IGBP-DIS \(\theta_{W}\) value. This implies that the isoprene fluxes are not impacted by the drought stress activity factor (i.e., \(\gamma_{SM}=1\)) according to the MEGANv2.1 parameterization.  
Therefore, the current parameterization fails to capture the drought effect due to the low value of the empirical parameter \(\Delta\theta\), which was based on only one study [33]. Therefore the \(\Delta\theta\) value is re-calibrated using the MOFLUX measurements, in order to derive an optimal value of the parameter \(\Delta\theta\) that provides the best match with the MOFLUX isoprene dataset. To that aim, we vary the \(\Delta\theta\) parameter in the model and select the value which minimizes the root-mean square error (RMSE) between modelled and in situ daily fluxes in both years. Using the soil moisture and \(\theta_{W}\) from GLEAM, the optimized \(\Delta\theta\) is 0.12. Using this value, the GLEAM \(\theta\) falls below the threshold \(\theta_{1}\) of 0.29 (= 0.12 + 0.17) during several weeks in 2011 (after 28 July) and the entire summer and early fall in 2012 (after 5 June).