## 2 Materials and Methods
### Remote Sensing Data
#### 2.4.1 Selection of Base Data and Generation of NDVI  
In this study, 10 years (2010-2019) of satellite data from the Moderate Resolution Imaging Spectrometer (MODIS) instrument aboard the Terra and Aqua satellites were used. The MODIS data products used for this study are MOD9Q1 and MYD09Q1. These data were selected as they fulfil two study criteria: (1) they cover a 10-year study period; and (2) they offer high frequency temporal data. These data also deliver a better trade-off between spatial and temporal resolution at 250 m and 1-day revisit intervals compared to other remote sensing data and their availability. In this study, low spatial resolution pixels were appropriate given the size of the rubber plantation and absence of confounding land cover uses. High-frequency temporal data at 1-day intervals were required to capture the  
Figure 4: The summary flowchart of the overall processes performed in this study are divided into three sections: (**a**) phenology; (**b**) climate; (**c**) phenology and climate.  
relatively small window of the rubber wintering period, as well as to mitigate against the impacts of frequent cloud cover associated with the study region.  
Although MODIS offers several other products, they were not suitable for this study due to: (1) low data availability, as at least three clear observations within the composite period are needed to process the BRDF inverted observation [46]; (2) overestimation of clouds, particularly in Southeast Asia [47]; (3) the MOD13 products are at a coarse resolution; and (4) the product shortens the growing season compared to other commonly used vegetation indices (VIs) for deriving phenological metrics [48].  
Hence, we utilised the surface reflectance composite products of MOD09Q1 and MYD09Q1. The value in each pixel of these products was selected from all data acquisition during an 8-day period by considering low view angle, high observation coverage, the absence of clouds or cloud shadow, and aerosol loading. The data were corrected for atmospheric conditions such as gasses, aerosols, and Rayleigh scattering. The data offer two bands: red (0.620-0.670 \(\upmu\)m) and near infrared (0.841-0.876 \(\upmu\)m). To further mitigate against persistent cloud cover, MOD09Q1 and MYD09Q1 data were combined to increase data availability for interpolation and smoothing processes while addressing issues associated with cloud cover. Both data sets showed high compatibility [49; 50], and it has been previously shown that they can be used in combination [51].  
In total, 920 images of MODIS tile h28v09 were downloaded from the USGS website ([[https://earthexplorer.usgs.gov/](https://earthexplorer.usgs.gov/)]([https://earthexplorer.usgs.gov/](https://earthexplorer.usgs.gov/)) (accessed on 1 February 2020)). The NDVI time-series data were used to represent the rubber growth cycle. Previously, NDVI values have shown utility in reflecting the greenness and consequently health status of vegetation, and have been recently employed in several phenological studies [34; 52]. It is the most commonly applied VI in optical remote sensing observations to estimate key phenological events [6; 11; 30; 53]. The NDVI is sufficiently stable to facilitate comparisons of inter-annual variations of vegetation while reducing noise [48]. In this study, the NDVI [54] values were calculated using Equation (1) where Band 1 is the red band and Band 2 is the near infrared band:  
\[\text{NDVI}=(\text{Band 2 -- Band 1})/(\text{Band 2 + Band 1}). \tag{1}\]