## 2 Materials and Methods
### Field Data Measurements  
The field values of LAI and biomass of winter wheat used in the study was measured from 4 March to 18 May 2014. Table 1 shows the date and number of samples collected during the growing season of winter wheat. In addition, to present the weather conditions during the in-situ field investigation, the  
Figure 1: The location map of the study area in Shaanxi, China (False color composite HJ image, acquired on 28 April 2014, R/G/B _vs._ band4/band3/band2). corresponding maximum temperature, minimum temperature, precipitation, relative humidity, and wind speed were recorded at nearby weather stations (Table 1). These data were obtained from the China Meteorological Data Sharing Service System (CMDSSS, [[http://cdc.cma.gov.cn](http://cdc.cma.gov.cn)]([http://cdc.cma.gov.cn](http://cdc.cma.gov.cn))). In this study, a sample plot area of \(>\)20,000 m\({}^{2}\) stands for one plot. The latitude and longitude of each plot were obtained with Global Positioning System (GPS, Trimble GeoExplorer 2008 Series GeoXH, Trimble Navigation Limited, USA) measurements. Measurements for the determination of LAI were taken from 1.5 m \(\times\) 1.5 m using the LAI-2000 Plant Canopy Analyzer (LI-COR Inc., Lincoln, NE, USA) with five replicates using the five-point sampling method from each plot in the central 30 m \(\times\) 30 m field (Figure 2). One reference reading above the canopy and four readings beneath it were made in order to determine LAI in winter wheat. Below canopy readings were made along diagonal transects between two rows and averaged. Three view caps were used: 90\({}^{\circ}\) masks normal (90\({}^{\circ}\) n), parallel to the row (90\({}^{\circ}\) p), and a 180\({}^{\circ}\) mask normal to the row. The purpose was to hide the operator from the sensor's view, providing, at the same time, the largest possible sampling area, and to determine the effect of canopy heterogeneity on the LAI-2000 sensor readings. The sensor with 90\({}^{\circ}\) n and 180\({}^{\circ}\) masks was always oriented southwards. Using 90\({}^{\circ}\) p, the sensor was oriented to the west in the morning and east in the afternoon. The portion of the view that contained the sun was thereby masked out and the effect of direct sunlight avoided. The aboveground biomass was determined from a 0.20 m\({}^{2}\) area (105 plants) by randomly cutting the representative plants with five replicates using five-point sampling method from each plot in the central 30 m \(\times\) 30 m field. All the plant samples were heated to 105 \({}^{\circ}\)C and oven dried at 70 \({}^{\circ}\)C until a constant weight was achieved, and the final dry weight (DW) of the samples was recorded. The DW was divided by sample area, and then the DW is converted to g/m\({}^{2}\). The LAI and biomass measurements during different growing stages of winter wheat were randomly divided into two parts using SPSS software (16.0, SPSS, Chicago, IBM, USA): a calibration set with 80 samples and a validation set with 40 samples. The statistics of each subset for LAI and biomass in winter wheat were summarized in Table 2. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Date of** & **Maximum** & **Minimum** & **Precipitation per** & **Relative Humidity** & **Wind Speed** & **Number of** \\
**Samples** & **Temperature (\({}^{\circ}\)C)** & **Temperature (\({}^{\circ}\)C)** & **24 h (mm)** & **(\%)** & **(m/s)** & **Samples** \\ \hline
4 March 2014 & 13.8 & 5.0 & 0 & 52 & 1.7 & 30 \\
5 March 2014 & 7.4 & 3.1 & 1.1 & 66 & 1.6 & \\ \hline
28 March 2014 & 20.9 & 9.6 & 0 & 68 & 1.3 & \\
29 March 2014 & 23.0 & 9.7 & 0 & 56 & 1.6 & \\ \hline
27 April 2014 & 21.6 & 7.9 & 0 & 60 & 1.0 & 30 \\
28 April 2014 & 23.8 & 8.9 & 0 & 64 & 1.5 & \\ \hline
14 May 2014 & 20.6 & 14.4 & 0 & 67 & 1.5 & \\
15 May 2014 & 24.5 & 12.2 & 0 & 54 & 1.4 & \\
16 May 2014 & 21.3 & 13.8 & 0 & 67 & 1.0 & 30 \\
17 May 2014 & 25.5 & 12.3 & 0 & 64 & 0.7 & \\
18 May 2014 & 27.6 & 14.3 & 0 & 63 & 1.3 & \\ \hline \hline \end{tabular}
\end{table}