## 1 Introduction  
Biogenic volatile organic compounds (BVOCs) are highly reactive and have large effects on the oxidizing potential of the troposphere, air quality and climate [1, 2, 3]. They are mainly emitted by terrestrial vegetation and amount to ca. 1000 Tg year\({}^{-1}\) globally, with about half of the share consisting of one compound, isoprene [4, 5, 6, 7, 8]. Isoprene influences tropospheric hydroxyl radical levels through multiple processes [9, 10, 11, 12, 13, 14], thereby altering the lifetime of methane. Under high-NO\({}_{\text{x}}\) conditions, it is a major precursor of tropospheric ozone [2, 15, 16, 17, 18] and it also affects the growth of secondary organic aerosols [19, 20, 21, 22], both of which are considered as important short-lived climate forcers. Extreme events such as droughs bring about changes in the surface emissions of biogenic hydrocarbons. Under mild to moderate drought, water stress can either have no significant impact on isoprene emission or can stimulate it, even in case of a significant inhibition of photosynthesis (e.g., [23; 24; 25]). In fact, there is evidence that isoprene improves plant thermotolerance [26; 27; 28]. This increase in isoprene emissions has been hypothesized as a possible result of the drought-induced stomatal closure, which causes a reduction in transpiration rates as the plants attempt to conserve water, and subsequently increases leaf temperature [29; 30]. However, as water stress becomes more severe, the general consensus is that isoprene emission capacity of a plant is greatly reduced [23; 24; 25; 30; 31; 32]. The effects of drought stress on isoprene emissions have been investigated in several studies, most of which were conducted in the laboratory [33; 34; 35; 36; 37; 38]. Field observations at ecosystem-scale during natural droughs are scarce [30; 32; 39; 40] and, to the authors' knowledge, only one site provides measurements of isoprene fluxes. During the 2011-2012 field campaigns in central U.S., at the Missouri Ozarks AmeriFlux (MOFLUX) site, isoprene and other BVOC fluxes were monitored for two consecutive growing seasons concomitant to two drought events [30; 32]. Moreover, the effect of drought on isoprene may also be monitored at greater spatiotemporal scales using other proxies. Formaldehyde (HCHO) is a key intermediate in the oxidation of the large majority of hydrocarbons released by vegetation, anthropogenic activities and fire. BVOCs, among which isoprene is largely dominant, are the major precursors of HCHO over vegetated areas (e.g., [41; 42; 43]), and therefore, satellite HCHO is an excellent proxy for the emissions of BVOCs. The anthropogenic signal of HCHO is low and only detectable in highly polluted regions [44; 45], and does not play a role in the isoprene-dominated MOFLUX site area. The contribution of fires to the HCHO abundances is generally important in the vicinity of fires or downwind, but only small agricultural fires occurred during the summers of 2011 and 2012 over Missouri. The BVOC emissions are therefore the main driver of the HCHO interannual variability over Eastern US [46]. [PERSON] et al. (2017) [47] have shown that remotely sensed sun-induced fluorescence (SIF) and HCHO columns qualitatively capture reductions in flux-derived gross primary productivity (GPP) and isoprene emission at the MOFLUX site, respectively, on weekly to monthly time scales, but with muted responses. They concluded that even fairly moderate reductions in satellite SIF and HCHO column could be indicative of severe soil drought. The present study aims at assessing the response of isoprene emissions to drought modelled by the Model of Emissions of Gases and Aerosols from Nature (MEGAN; [5; 48]). The current parameterization in MEGANv2.1, dependent on root-zone soil water content and permanent wilting point (denoted \(\theta_{W}\)), was derived based on a study conducted on potted plants [33].