## 2 Materials and Methods
### Experimental Design  
The experimental design is illustrated through the use of a general flow chart (Figure 2), which organizes the main components in this study. This design includes four main parts: (1) retrieval of global CI data from different product versions, (2) spatial distribution comparison based on global monthly CI data, (3) seasonal variation comparison between global C5 8-day CI data and C6 daily CI data, and (4) validation and comparison based on globally distributed field-measured CI data.  
For data processing, the C5 CI data were retrieved in the local server by downloading the MODIS data to a local database, while C6 CI data were directly retrieved on the Google Earth Engine cloud-based platform without downloading large amounts of MODIS data input to the local machine. After the retrieved CI data were obtained, we compared the spatial distribution and seasonal variation of these two versions of CI data on a global scale. As a major improvement in the C6 BRDF product is the daily inversion rather than an 8-day inversion of the C5 product, the different temporal resolutions of these two versions of CI data are compared for temporal variation. Moreover, these two versions of CI data are synthesized into monthly data for a global distribution comparison in terms of CI value and retrieval quality. Finally, the MODIS CI data retrieved from different product versions are validated and compared based on the collected field-measured CI data. The monthly CI data are validated by comparison with 48 spatially representative field-measured CI data,  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Site** & **Latitude** & **Longitude** & **IGBP** & **Species** & **Method** & \begin{tabular}{c} **Measurement** \\ **Dates** \\ \end{tabular} & **References** \\ \hline Tonzi & 38.43 & \(-120.96\) & 8 & \
\begin{tabular}{c} Oak savanna \\ woodland \\ \end{tabular} & DP 1 & 2009–2010 & [PERSON] et al. [80] \\ RAMI & 58.311 & 27.297 & 1 & Scots Pine & DHP 2 & 2011 & [PERSON], et al. [81] \\ TP39 & 42.710 & \(-80.357\) & 1 & White Pine & TRAC 3 & 2011, 2012 & [PERSON], et al. [82] \\ TP74 & 42.707 & \(-80.348\) & 1 & White Pine & TRAC & 2011, 2012 & [PERSON], et al. [82] \\ Yatir & 31.35 & 35.03 & 1 & Aleppo pine & TRAC & 2005, 2012, 2013 & [PERSON], et al. [83] \\ Honghe & 47.652 & 133.522 & 12 & Rice & DHP & 2012, 2013 & [PERSON], et al. [86] \\ Hailun & 47.415 & 126.818 & 12 &
\begin{tabular}{c} Maize, soybean, \\ sorghum \\ \end{tabular} & DHP & 2016 & [PERSON], et al. [87] \\ \hline \hline \end{tabular}  
* \({}^{1}\) DP: digital photography. 2 HIP: digital hemispherical photography. 3 TRAC: tracing radiation and architecture of canopies instrument.  
\end{table}
Table 2: Characteristics of validation sites with seasonal profiles of field-measured CIs.  
Figure 2: Flow chart of the comparison of CI retrievals using two versions of MODIS BRDF products between C5 (red rectangles) and C6 (blue rectangles).  
and the seasonal variabilities in the 8-day and daily CI data are evaluated using 7 field data sites with seasonal profiles of field-measured CIs. These different versions of MODIS CI retrievals are geolocated at the surface in terms of latitude and longitude.