## 2 Materials and Methods
### Spectral and Flux Measurements  
The FluoSpec2 consists of two subsystems: a directional-hemispherical system for SIF retrieval and a hyperspectral observational system for calculating vegetation indices. Each subsystem consists of a spectrometer, a splitting fiber, an inline shutter, and two fibers. These fibers are utilized to capture downwelling irradiance and upwelling canopy reflected radiance, respectively. The cosine corrector (CC3, Ocean Optics) attached to the irradiance fiber is used to collect downwelling solar energy at a field of view (FOV) of 180\({}^{\circ}\), while the bare fiber is designed as a FOV of 25\({}^{\circ}\) to collect the canopy radiance at the nadir. The end of the fibers was positioned at a 5 m tower above the ground, with a 2.2 m diameter sample area on the ground. The directional-hemispherical system collects data by the QEPRO spectrometer, which has a Full Width Half Maximum (FWHM) of 0.15 nm and covers a wavelength range from 730 nm to 780 nm. The hyperspectral observational system measures data by the HR2000+ spectrometer, which has a wavelength coverage from 350 nm to 1100 nm and has a FWHM of 1.1 nm. A laptop coordinates two subsystems to execute data collection automatically.  
The carbon dioxide (CO\({}_{2}\)) exchange between the atmosphere and crops was measured by the eddy covariance (EC) method. The CO\({}_{2}\) flux measured system consists of a CO\({}_{2}\)/H\({}_{2}\)O open-path infrared gas analyzer (IRGA) and a three-dimensional ultrasonic anemometer. The IRGA continually collected CO\({}_{2}\) and water vapor turbulence data with a frequency of around 10 Hz. The collected raw EC data can be used to extract the latent heat (LE), soil heat flux beneath the canopy (G), sensible heat (H), Obukhov length (L), and friction velocity (u*). The measured variables can be utilized to characterize the plant stress status,  
Figure 1: Information of the two field sites deployed with FluoSpec2 systems. (**a**) The specific location for US-Ne2 and US-Ne3 sites; (**b**) a simple schematic diagram of a FluoSpec2 system consisted of a QEPRO spectrometer and a HR2000+ spectrometer; (**c**) a conceptual field deployment for spectral measurements.  
such as LE which can be a potential indicator for water stress. The measuring height of the EC instruments for soybean at the US-Ne2 and US-Ne3 sites was maintained at 3 m for the duration of the growing season; for corn, it was at 3 m when the canopy height was less than 1 m and it rose to 6 m as the canopy height grew until the end of the growing season. According to [PERSON] et al. [27], the SIF-GPP relationships were not substantially changed after upscaling nadir SIF to GPP footprint at the crop sites planted with corn and soybean. In addition, the half-hourly mean and daily mean data were processed using the averaging approach proposed by [PERSON] et al. [28].  
Moreover, an automatic weather station (AWS) is capable of continually measuring climatic variables like air temperature (Ta), the photosynthetically active radiation (PAR), relative humidity (RH), precipitation, and air pressure, among others. [PERSON] and [PERSON] [29] provide comprehensive information on the EC and meteorological measurements for the US-Ne2 and US-Ne3 sites. To match the flux observations, we calculated the 30 min average of PAR and all other meteorological variables.