## 2 Materials and Methods  
### Ground Measurements  
A dataset comprising the ground spectral, canopy structure, and vegetation physiological measurements of winter wheat, as well as meteorological and SM measurements, was acquired at the Fangshan Comprehensive Experimental Station (39\({}^{\circ}\)35\({}^{\prime}\)N, 115\({}^{\circ}\)42.5\({}^{\prime}\)E) of Beijing Normal University, following the experimental settings of different drought levels for four plots. It was used to evaluate the performance of F760 tot for drought monitoring. Winter wheat was sown in October 2016 in rows in four plots (4 m \(\times\) 4 m) and harvested in June 2017. To protect the wheat from freezing, the same irrigation was conducted on four plots before the green-up stage. After the green-up, the four different irrigation treatments in Table 1 were applied in order to generate four drought levels: well-watered treatment, moderate drought, severe drought, and extreme drought, corresponding to the order of P1, P2, P3, and P4. Among the four plots, only the irrigations of P1 met the needs of the winter wheat during growth. The winter wheat in other plots suffered different levels of water stress. During the study period, there was no rainfall, and therefore the experiment was not affected by precipitation. Eventually, the prolonged drought levels of four plots were generated, and the period between 28 March 2017 and 19 May 2017 was selected as the time window for field measurements. Details on the measurements including the parameters, methods or devices, and intervals are summarized in Table 2 as follows. The spectral measurements were conducted using an automated field spectroscopy system [14], which was improved based on the FluoSpec system [25]. As displayed in Figure 1, the core components of this system were an Ocean Optics HR2000+ spectrometer (wavelength: 680-775 nm; FWHM: \(\sim\)0.13 nm) and a fiber switch (OceanOptics, Inc., Dunedin, FL, USA) to measure the up- and downwelling radiation fluxes in sequence (nadir view, \(\sim\)4.5 m height, hemispherical-conical configurations, footprint: 0.45 m). One of the fibers which pointed toward the wheat canopy to collect upwelling radiance was a bare fiber (FOV: 25\({}^{\circ}\)), and the other fiber attached to a cosine corrector (CC-3, OceanOptics, Inc., Dunedin, FL, USA) was pointed toward the sky to collect incident light (FOV: 180\({}^{\circ}\)). In order to reduce the influence of dark current, the HR2000+ spectrometer was placed in an incubator with a fixed temperature of 23 \({}^{\circ}\)C. Another key component was the stepping motor with a round trip that could be rotated over the 4 plots in 6-min intervals. Moreover, the surface soil moisture at the 20-cm soil layer, incident photosynthetically active radiation (PAR, 400-700 nm), and meteorological parameters were collected automatically using different devices. Additionally, some critical growth parameters of wheat such as the leaf area index (LAI), chlorophyll content (Chl), and relative water content of leaf (RWC) were manually measured in a coarser temporal resolution. Specifically, the growth parameters were measured six times during the field campaign. \begin{table}
\begin{tabular}{c c c c c} \hline
**Plot** & **P1** & **P2** & **P3** & **P4** \\ \hline
03/28 & 1 m\({}^{3}\) & 1 m\({}^{3}\) & 1 m\({}^{3}\) & 0 \\
04/22 & 1 m\({}^{3}\) & 0.6 m\({}^{3}\) & 0.4 m\({}^{3}\) & 0 \\
05/02 & 1 m\({}^{3}\) & 0.6 m\({}^{3}\) & 0.4 m\({}^{3}\) & 0 \\
05/12 & 1 m\({}^{3}\) & 0.6 m\({}^{3}\) & 0.4 m\({}^{3}\) & 0 \\ \hline \end{tabular}
\end{table} 
Table 1: Irrigation treatments for four plots.