## 4 Results
### Comparison of IMERG Series Products with Rain Gauge Data
The RMSE metrics results have values that are close to each other. All three IMERG products perform satisfactorily in terms of detection capability and the three metrics all have similar values (Figure 3d-f). The three \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Spatial Scale** & **Temporal Scales** & **Products** & **CORR** & **RMSE (mm)** & **BIAS** & **POD** & **FAR** & **HSS** \\ \hline \multirow{6}{*}{Grid accumulation scale} & \multirow{3}{*}{Daily} & ER & 0.59 *** & 11.78 & \(-\)0.14 & 0.80 & 0.37 & 0.56 \\  & & LR & 0.59 *** & 11.77 & \(-\)0.16 & 0.81 & 0.37 & 0.57 \\  & & FR & 0.61 *** & 12.16 & \(+\)0.01 & 0.81 & 0.37 & 0.57 \\  & & ER & 0.88 *** & 71.50 & \(-\)0.14 & 1.00 & 0.02 & 0.98 \\  & & \multirow{2}{*}{Monthly} & LR & 0.88 *** & 73.13 & \(-\)0.16 & 1.00 & 0.02 & 0.98 \\  & & FR & 0.94 *** & 48.03 & \(+\)0.01 & 1.00 & 0.02 & 0.98 \\ \hline \multirow{6}{*}{Basin scale} & \multirow{3}{*}{Daily} & ER & 0.69 *** & 9.04 & \(-\)0.14 & 0.83 & 0.22 & 0.67 \\  & & LR & 0.69 *** & 9.10 & \(-\)0.16 & 0.83 & 0.22 & 0.67 \\  & & FR & 0.71 *** & 9.56 & \(+\)0.01 & 0.84 & 0.22 & 0.67 \\  & & ER & 0.93 *** & 54.88 & \(-\)0.14 & 1.00 & 0.00 & 1.00 \\  & & LR & 0.92 *** & 58.34 & \(-\)0.16 & 1.00 & 0.00 & 1.00 \\  & & FR & 0.99 *** & 18.99 & \(+\)0.01 & 1.00 & 0.00 & 1.00 \\ \hline \hline \end{tabular}  
* ***: \(p<0.001\)  
\end{table} 
Table 2: Summary of the evaluation metrics for the three IMERG products at daily and monthly resolutions over the Xinfengjiang River reservoir basin. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **(H, \(p\)-Value)** & **ER** & **LR** & **FR** \\ \hline \multirow{6}{*}{Grid accumulation scale} & \multirow{3}{*}{Daily scale} & ER & (S, \(<\)0.05) & (S, \(<\)0.05) \\  & & LR & & (S, \(<\)0.05) \\  & & FR & & (S, \(<\)0.05) \\  & & ER & & (S, \(<\)0.05) \\  & & LR & & (S, \(<\)0.05) \\  & & FR & & (S, \(<\)0.05) \\ \hline \multirow{6}{*}{Basin scale} & \multirow{3}{*}{Daily scale} & ER & (S, \(<\)0.05) & (S, \(<\)0.05) \\  & & LR & & (S, \(<\)0.05) \\ \cline{1-1}  & & FR & & (S, \(<\)0.05) \\ \cline{1-1}  & & ER & & (S, \(<\)0.05) \\ \cline{1-1}  & & LR & & (S, \(<\)0.05) \\ \cline{1-1}  & & LR & & (S, \(<\)0.05) \\ \cline{1-1}  & & FR & & (S, \(<\)0.05) \\ \hline \hline \end{tabular}  
* \(\otimes\) Significant Difference.  
\end{table} 
Table 3: Summary of the statistical significance of differences of the products (STA, ER, LR, and FR products) at daily and monthly resolutions over the Xinfengjiang River reservoir basin. IMERG products all contain a few outliers with PODs (Figure 3d). From the box plots of the accuracy indicators of each product at the monthly scale in Figure 4, the CORR, RMSE, and BIAS results (Figure 4a-c) show that the FR values (median: CORR of 0.96, RMSE of 45.74, BIAS of 0.02) are more accurate than the ER (CORR:0.88, RMSE:67.99, BIAS:0.15) and LR (CORR:0.88, RMSE:69.856, BIAS:0.17) values. Note that the CORRs and RMSEs of FR, as shown in the box plots of (Figure 4a,b), are distributed in a narrower range than those of ER and LR, indicating the lower uncertainty of the near-real-time products. All three IMERG products perform satisfactorily in terms of detection capability (Figure 4d-f). Overall, the conventional indicators show the advantage of the post-real-time FR. The IMERG products show similar performances in terms of the detection effects. All the metrics improved as the temporal resolution changed from daily to monthly at the grid accumulation scale and basin and grid scales. Figure 4: Box plots of statistical accuracy indices (**(a)** CORR, **(b)** RMSE, **(c)** BIAS, **(d)** POD, **(e)** FAR, and **(f)** HSS) on a monthly scale of the three IMERG products versus rain gauge data at the grid scale. Figure 3: Box plots of statistical accuracy indices (**(a)** CORR, **(b)** RMSE, **(c)** BIAS, **(d)** POD, **(e)** FAR, and **(f)** HSS) on a daily scale of the three IMERG products versus rain gauge data at the grid scale.