## 1 Introduction  
Over the last 50 years, remote sensing has proven its value for large-scale drought stress monitoring. Classically, remote sensing monitors the greenness of the vegetation. These methods provide an insight on stresses that a plant has suffered during its growing season [1], but it gives little information on the plant health at the moment the measurement was taken. Other techniques focus on the soil moisture content and on the meteorological variables [2], but they do not consider the wide variety in stress responses across ecosystems. A first attempt in monitoring the instantaneous plant stress came through evapotranspiration estimates, based on thermal remote sensing. In order to link thermal measurements to evapotranspiration, it is necessary to have accurate data on various variables including the leaf and the air temperature, which is a major obstacle for interpreting thermal infrared data [3]. A more recent technique for measuring instantaneous stress uses passive microwave emission to calculate the vegetation optical depth, which is linked to the vegetation water content [4] or the turgor pressure [5]. A game changer in stress monitoring has been the introduction of the sun-induced chlorophyll fluorescence (SIF), a remote sensing signal sensitive to the photosynthetic activity of a plant [6] ([PERSON] et al., 2019). Photosynthesis is particularly sensitive to drought conditions; since these induce stomatal closure to reduce the CO\({}_{2}\) uptake, the energy demand by the dark reactions is lowered. As a result, there is a need for an alternative energy sink, which is provided for by a series of reactions, collectively known as non-photochemical quenching (NPQ), which increases when the CO\({}_{2}\) uptake lowers. This, in response, also alters the SIF emission [7]. Through the stomata that are still open, plants lose water and therefore show a decrease in their turgor pressure [8]. This leads to changes in the leaf angle [9], changing the canopy reflectance, especially in the near-infrared [10]. This makes high-temporal resolution observations of near-infrared reflectance another promising indicator for instantaneous stress monitoring. Since its discovery [11], chlorophyll fluorescence has proven its value for monitoring of photosynthetic activity, mostly through the form of pulse-amplitude modulation (PAM; [12]). Here, a leaf is illuminated with artificial light pulses, stimulating its chlorophyll molecules, after which they send out fluorescent light as a reaction. This fluorescent reaction informs the stress status. Specifically, stress lowers the actual PSII efficiency (YII), as the demand for energy in the photosynthetic electron transport chain lowers. As an alternative energy sink, NPQ increases. The increase in NPQ and decrease in Y(II) have a downstream effect on the emission of the steady-state fluorescence emission (Fs/F0). The latter variable describes the emission of fluorescence by a chlorophyll molecule under light-incubated conditions. Fs/F0 is conceptually similar to SIF, which is the steady-state emission of chlorophyll fluorescence measured by a spectrometer and uses the sun as illumination source. A drought-induced decrease in Fs/F0 has been observed over various species [13]. SIF provides a fluorescence signal from the leaf to the ecosystem scale. Since SIF only comprises of 1-2% of the absorbed energy, its retrieval is limited to solar and atmospheric absorption lines [14]. Canopy-scale SIF is typically retrieved at the O\({}_{2}\)-A and O\({}_{2}\)-B atmospheric absorption bands, situated at 760 nm and 687 nm, respectively. Their corresponding SIF\({}_{\lambda}\) observations are referred to as SIFA and SIFB. Canopy-scale SIF\({}_{\lambda}\) is expected to behave differently at these wavelengths. First, the relative contribution of photosystem I (PSI) to the SIF\({}_{\lambda}\) emission is more important for SIFA compared to SIFB. Second, 687 nm is within the absorption spectrum of chlorophyll, causing SIFB to be prone to reabsorption by another chlorophyll molecule.