## 2 Materials and Methods
### Field Data Collection  
Spectral measurements at the canopy level were carried out under clear-sky conditions in July and August 2015, when the maize plants were at the late big trumpet period and the ripening stage, respectively. The vegetation fraction during our campaign is greater than 0.8. A calibrated HR4000 high-resolution spectroradiometer (Ocean Optics Inc., Dunedin, FL, USA) was installed at approximately 20 m from the eddy flux tower to measure the diurnal cycles of the canopy radiometric response (Figure 2a). It records reflected radiation within the spectral domain of 647-1087 nm with a full width at half maximum (FWHM) bandwidth of 0.13 nm. During our experiment, the spectroradiometer was housed in a thermally regulated box (SteadiQ, Ocean Optics Inc., Dunedin, FL, USA) and the internal temperature was maintained at 25 \({}^{\circ}\)C to reduce dark current  
Figure 1: Location of the study site. drift. Spectralon reflectance standards (WS-1-SL, Ocean Optics Inc., Dunedin, FL, USA) were used to measure the incident irradiance. For the canopy measurements, the instrument's fiber optic was attached to a swiveling mount that was elevated approximately 1 m above the canopy and adjusted for nadir view. The daily cycles of the reflectance spectra under clear sky conditions at four different canopy locations were collected in our campaign. As the FOV (Field of View) of the instrument is 25\({}^{\circ}\), each measurement can acquire a circle area of 0.5 m in diameter (Figure 2b). During the measurement, the swiveling mount was manually controlled to acquire sequential observations of the standard reference panel and the canopy targets. Specifically, each acquisition session consisted of the consecutive collection of spectra of instrument dark current, radiance of the standard reference panel, canopy radiance and radiance of the standard reference panel. After acquiring the radiance of the standard reference panel, the fiber optic was immediately switched to the canopy target to acquire canopy radiance. Then the fiber optic was switched to the standard reference panel to acquire its radiance again. The radiance of the standard reference panel at the time of the canopy measurement was determined using the averaged spectrum of the two measurements. To ensure high-quality spectral data, ten consecutive spectra were averaged as a single spectrum at each measurement. The integration time was automatically optimized during the day to maximize the signal-to-noise ratio (SNR). In the experiment, the spectral acquisitions were conducted at 0.5 h intervals. Eventually, five diurnal courses of reflectance spectra were acquired (Table 1). In addition to the spectral measurements, the absorbed photosynthetically active radiation (APAR) was collected simultaneously within 10 m away the FOV of the spectrometer using an LI-190 SA quantum sensor (LI-COR Biosciences, Lincoln, NE, USA) that measures PAR in the range of 400-700 nm. APAR acquisition is based on the measurements of four radiation components: PAR reaching the canopy, PAR reflected from the canopy, PAR transmitted through the canopy, and PAR reflected from the ground. APAR was determined by referring to Gallo and Daughtry [54]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Period** & **Date** & **Time Window (hh:mm)** & **Growth Stage** \\ \hline
1 & 10 July & 11:30–19:00 & Late big trumpet period \\
2 & 17 July & 8:00–18:30 & Late big trumpet period \\
3 & 18 July & 9:00–19:00 & Late big trumpet period \\
4 & 21 August & 8:00–18:30 & Ripening stage \\
5 & 22 August & 8:00–18:00 & Ripening stage \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the available day courses of radiometric measurements. Figure 2: Overview of the spectral measurements: (**a**) spectral measurement system; and (**b**) positions of the standard reflectance panels (S) and the four canopy targets (T1–T4). During our campaign, LAI was measured twice at 18 July and 22 August, respectively.