## 2 Materials and Methods
### Satellite Imagery  
To gather cropland planting information, we utilized Landsat and Sentinel time series data spanning from 2019 to 2023. On a globe scale, the dual Sentinel satellite constellations (S2A and S2B) revisit every 3.8 days, and incorporating Landsat-8 data will further reduce the interval to 2.9 days on average [18]. The surface reflectance products of Landsat-8 Operational Land Imager (OLI) and Sentinel-2 MultiSpectral Instrument (MSI) were processed through GEE [19]. Low-quality observations were identified and excluded from further processing. We used the quality assurance layers QA60 and QA_PIXEL for Sentinel-2 and Landsat-8, respectively, to remove clouds and other low-quality observations.  
Imagery harmonization was conducted to merge Landsat-8 and Sentinel-2 time series. We used the red, blue, and near-infrared bands from two sources: OLI bands 2, 4, and 5, and MSI bands 2, 4, and 8A [26]. First, given the spectral response similarities of OLI and MSI sensors, as shown in Table 1 and Figure 3, we employed linear models described by [27] to transform MSI data into OLI-like data. Then, to reconcile the difference in spatial resolutions, bands with 20 m or 30 m resolution were downscaled to 10 m. Finally, two normalized datasets with comparable spectral ranges and spatial resolutions were merged into a single dense dataset.  
We counted the total number of observations and the number of good-quality observations per pixel in Shaanxi throughout 2021 as an example (Figure 4). On average, the Sentinel-2 constellations and Landsat-8 exhibited revisit cycles of approximately 2.9 days  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{\begin{tabular}{c} **Spectral** \\ **Band** \\ \end{tabular} } & \multicolumn{2}{c}{**OLI**} & \multicolumn{2}{c}{**MSI-2A**} & \multicolumn{2}{c}{**MSI-2B**} & \multirow{2}{*}{\begin{tabular}{c} **Transformation** \\ **Equation** \\ \end{tabular} } \\ \cline{2-3} \cline{5-8}  & \begin{tabular}{c} **Central** \\ **Wavelength** \\ **(nm)** \\ \end{tabular} & \begin{tabular}{c} **Bandwidth** \\ **(nm)** \\ \end{tabular} & \begin{tabular}{c} **Central** \\ **Wavelength** \\ **(nm)** \\ \end{tabular} & \begin{tabular}{c} **Bandwidth** \\ **(nm)** \\ \end{tabular} & \begin{tabular}{c} **Central** \\ **Wavelength** \\ **(nm)** \\ \end{tabular} & \begin{tabular}{c} **Bandwidth** \\ **(nm)** \\ \end{tabular} & \begin{tabular}{c} **Bandwidth** \\ **(nm)** \\ \end{tabular} & \
\begin{tabular}{c} **Station** \\ **(nm)** \\ \end{tabular} \\ \hline Blue & 482.0 & 60 & 492.7 & 65 & 492.3 & 65 &
\begin{tabular}{c} OLI = 0.0003 + \\ 0.9570 MSI \\ 0.041 + \\ 0.9533 MSI \\ 0.0077 + \\ 0.9644 MSI \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 1: The spectral band characteristics of Landsat-8 OLI and Sentinel-2 MSI sensors and the transformation equations.  
Figure 3: Spectral response functions of Landsat-8 OLI and Sentinel-2 MSI for the approximately equivalent bands utilized in this study [28; 29].  
and 13 days, respectively. Their combination resulted in a more frequent observation interval, reduced to 2.4 days. After the removal of cloud-affected data, the frequency of good-quality observations remained reasonably high. To provide a comparative perspective, the revisit cycles for Landsat-8, Sentinel-2, and the combined Landsat-8 and Sentinel-2 datasets were approximately 21.2 days, 4.8 days, and 3.9 days, respectively.