# Crop Growth Stage GPP-Driven Spectral Model for Evaluation of Cultivated Land Quality Using GA-BPNN
## 2 Materials and Methods
### Data  
In this study, to match the 500 m spatial resolution of MODIS GPP products, field sampling plots of 500 m by 500 m were designed and within each plot, biomass was taken at five locations with one placed at the plot center and the other four at the middle points from the plot center to the corners  
Figure 1: The study area location in Guangzhou City **(a)**, and Conghua and Zengcheng District **(b)**, respectively, with a total of 420 sample plots for cultivated land quality (CLQ) (training sample plots are in yellow and validation sample plots for the model in purple) and another set of 240 sample plots in red for validating multi-scale Moderate-resolution Imaging Spectroradiometer (MODIS) gross primary production (GPP) products; **(c,d)** the validation areas for mapping at Adou Town of Conghua District and Zhongxin Town of Zengcheng District.  
along the diagonal lines. The GPP was estimated with an empirical regression model [23]: \(\text{GPP}=\frac{N\text{PP}}{0.524}\), where NPP was acquired from the following equation.  
\[NPP=\frac{B\times\beta}{\alpha} \tag{1}\]  
where \(B\) is the dry biomass obtained from each of the sampled plots at the heading stage and dried in a constant temperature drying oven at 110 \({}^{\circ}\)C. The \(\alpha\) is the ratio of aboveground biomass to total biomass, and \(\beta\) is the percentage of carbon in the biomass. For cultivated land, the value of \(\alpha\) is usually 0.8 [24], and \(\beta\) is 45% [25]. The rice samples were first destructively collected in the field sample plots and oven-dried at 110 \({}^{\circ}\)C for 50 min in the laboratory, and the temperature was then adjusted to 85 \({}^{\circ}\)C for 10 h until the sample weights did not change. The samples were finally taken out and weighed.  
Moreover, a total of 660 sample data were extracted from the CLQ map obtained from the Guangzhou National Land Department and the CLQ values were derived using gradation regulations on farmland quality in China (Regulation for gradation on agriculture land quality GB/T 28407-2012) [26]. The plot sampled area is 30 m \(\times\) 30 m, matching the 30 m spatial resolution of the downscaled MODIS products. The 660 samples were randomly divided into three groups: 294 samples in yellow for modeling (Figure 1b), 126 sample in purple for validation of the estimated CLQ (Figure 1b), and another dataset of 240 sample plots in red for assessing the accuracy of mapping CLQ at the regional scale (Figure 1c,d). In addition, 2011-2015 MODIS/Terra 8-day GPP products (MOD17A2H Version 6) at a spatial resolution of 500 m \(\times\) 500 m were acquired from the Land Processes Distributed Active Archive Center (LP DAAC/NASA). The MODIS Re-projection Tool (MRT) was employed to convert the sinusoidal projection into the Albers Equal Area projection for the MODIS GPP products. The scaling factor of 0.1 was utilized to obtain the standard MODIS-GPP products [27].  
According to the rice growth phases recommended by Ricepedia [28], the rice growth process can be characterized by five stages: seedling, tillering, jointing, heading, and maturity. In fact, the seedling stage was not taken into account because of the too small values of MODIS-GPPs detected and the impact of water on the spectral reflectance of rice. Thus, four growth stages were considered. Moreover, the dates of the acquired MODIS-GPP images with cloud cover less than 5% were consistent with the times of the four rice growth stages (Table 1) and the synchronous satellite-field experiment was carried out.