## 2 Materials and Methods
### Ground-Based Spectrum Measurements  
Seasonal canopy reflectance and SIF observations were conducted using an automated ground-based continuous observation system known as AS-SpecFOM, (Agri-SIF Envi \begin{table}
\begin{tabular}{c c c} \hline \hline
**Crop** & **Growth Stages** & **Definition** \\ \hline \multirow{8}{*}{Winter wheat} & Regreen (RG) & The plant turns green again after the winter. \\  & **Jointing (JT)** & From 1 st node detectable to last leaf visible. \\  & **Heading (HD)** & Head is fully exposed to frost, hail and pests. Plant attains final height. \\  & **Milk (MK)** & Starch and protein content determination starts, or namely ‘grain filling’. \\  & Ripening (RP) & Kernel hard, difficult to divide by thumbnail. The plant is \\  & Harvest (HV) & completely yellow. \\ \hline \multirow{8}{*}{Summer maize} & Sowing (SOW) & Seeds have been planted in the soil. \\  & Emergence (VE) & Shoot (coloeptile) has emerged from the soil. \\  & **5 th leaf (VS)** & The 5 th leaf collars present. \\  & **Jointing (JT)** & Between V6 and V9, the first stem of maize grows to the height \\  & **Jointing (JT)** & approximately 2 cm. \\  & **Tasseling** & Lowest branch of the tassel is visible. \\  & **Silking** & One or more silks extends outside of husk leaves. \\  & **Milk (MK)** & Kernels filled with ‘milky’ fluid, or namely ‘grain filling’. \\  & **Maturity (MT)** & Kernels at maximum dry matter accumulation; a ‘black layer’ will form \\  & **at kernel base (2–3 days after physiological maturity).** \\  & **Harvest (HV)** & The plant has been harvested. \\ \hline \hline \end{tabular} Note: The bolded growth stages in this table are key growth stages (KGSs).  
\end{table} 
Table 1: Ground-observed crop growing stages. ronmental Technology Co., Ltd., in Nanjing, China). This system bears a resemblance to FluoSpec2 and comprises two essential components [59]. The core of the system features a QEPRO spectrometer (Ocean Optics, Dunedin, FL, USA) with a spectral range spanning from 730 to 785 nm, a full-width half-maximum (FWHM) of 0.17 nm, and an impressive signal-to-noise ratio (SNR) of approximately 1000. This QEPRO spectrometer is primarily utilized for the retrieval of SIF data. In conjunction with the QEPRO, the system incorporates an HR2000+ spectrometer (Ocean Optics, Orlando, FL, USA) with a broader spectral range, covering wavelengths from 350 to 1000 nm, an FWHM of 1.1 nm. The HR2000+ is dedicated to capturing canopy reflectance measurements and vegetation indices (VIs). Specifically, the enhanced vegetation index (EVI) was computed using Equation (1) [60]. In Equation (1), \(\rho_{NIR}\), \(\rho_{red}\) and \(\rho_{blue}\) are reflectance at near-infrared, red and blue bands after the atmospheric correction. The entire system boasts a 25\({}^{\circ}\) field of view (FOV) and is positioned at an approximate height of 10 m above the canopy. \[EVI=2.5\times\frac{\rho_{NIR}-\rho_{red}}{\rho_{NIR}+6\times\rho_{red}-7.5\times \rho_{blue}+1} \tag{1}\]  
Table 2 provides an overview of the in situ SIF and EVI data collected for winter wheat and maize during specific years. In the case of winter wheat, data were systematically gathered in 2019, 2021 and 2022. The absence of data for winter wheat in 2018 and 2020 was attributed to the construction and the relocation of the observation system, respectively. For maize, in situ SIF measurements were consistently acquired from 2018 to 2022, while in situ EVI data were obtained from 2019 to 2022. The lack of in situ EVI data in 2018 resulted from data loss. To mitigate the impact of cloud cover on in situ SIF measurements and the photosynthetic activity of crops, measurements conducted during periods with a clearness index (defined as actual shortwave radiation divided by top-of-atmosphere shortwave radiation) lower than 0.5 were identified as observations made under cloudy conditions and were consequently excluded from the dataset [61].