## 4 Results  
### Meteorological Assessment  
Point-to-pixel approach was used to compare the precipitation between gauges and SPPs. As shown in Figure 1, most of the IMERG pixels are covered by only one precipitation gauge, so the point-to-pixel is an optimal choice in this case. This is because the pixel-to-pixel approach commonly requires a high density of precipitation gauges or ground-based precipitation radar. Figure 2 shows that the three IMERG products well captured the temporal variability of monthly precipitation. For example, a high precipitation amount was found in December 2014, while a low precipitation amount was found in February 2015 and March 2016. Table 1 indicates that all the IMERG products had a good correlation with precipitation gauges in monthly precipitation measurement. Only the IMERG_F was classified as a reliable source, as the CC (0.86) and RB (7.14%) values are beyond the good performance thresholds [31]. The IMERG_E and IMERG_L underestimated the monthly precipitation, with RB values of 10.42% and 17.92%, respectively. The results showed the gauge-corrected IMERG_F performed slightly better than the near real-time products in term of systematic bias.  
\begin{table}
\begin{tabular}{l c c c} \hline \hline  & **IMERG\_E** & **IMERG\_L** & **IMERG\_F** \\ \hline Daily (1) January 2010–31 December 2016 & & & \\ \hline CC & 0.60 & 0.63 & 0.61 \\ RMSE (mm/day) & 1.341 & 12.93 & 13.69 \\ RB (\%) & \(-10.42\) & \(-17.92\) & 7.14 \\ MAD (mm/mm) & 6.38 & 6.02 & 6.64 \\ POD & 0.81 & 0.82 & 0.84 \\ FAR & 0.4 & 0.39 & 0.41 \\ CSI & 0.52 & 0.53 & 0.53 \\ ETS & 0.27 & 0.28 & 0.26 \\ ACC & 0.70 & 0.71 & 0.70 \\ \hline Monthly (January 2010–December 2016) & & & \\ \hline CC & 0.86 & 0.87 & 0.86 \\ RMSE (mm/month) & 106.26 & 108.81 & 103.54 \\ RB (\%) & \(-10.42\) & \(-17.92\) & 7.14 \\ MAD (mm/month) & 76.31 & 75.62 & 74.08 \\ \hline The 2014–2015 flood (14 December 2014–3 January 2015) & & & \\ \hline CC & 0.77 & 0.79 & 0.79 \\ RMSE (mm/day) & 38.71 & 37.73 & 37.41 \\ RB (\%) & \(-4.02\) & \(-12.39\) & \(-8.46\) \\ MAD (mm/day) & 24.89 & 23.73 & 23.69 \\ POD & 0.95 & 0.94 & 0.95 \\ FAR & 0.06 & 0.05 & 0.06 \\ CSI & 0.89 & 0.90 & 0.89 \\ ETS & 0.64 & 0.67 & 0.65 \\ ACC & 0.91 & 0.92 & 0.91 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Statistical analysis of the IMERG products.  
Figure 2: Temporal pattern of monthly precipitation estimated by gauge and the Global Precipitation Measurement Integrated Multi-Satellite Retrievals (GPM IMERG) products from March 2014 to December 2016.  
For the daily scale assessment, the three IMERG products had a moderate correlation with precipitation gauges, with CC values ranging between 0.6 and 0.63. The IMERG_L presents a slightly smaller RMSE and MAD values compared to the other two products. Similarly, the IMERG_F presents a slightly better performance than the IMERG_E and IMERG_L in term of systematic bias, with a lower RB (Figure 3 and Table 1). These results are consistent with [PERSON] et al. (2012), who also found that the IMERG_E and IMERG_L underestimated daily precipitation, but the IMERG_F showed an overestimation over the Beijing River Basin, China. The GPM IMERG products accurately detected the precipitation days and non-precipitation days as the ACC values varying from 0.7 to 0.71, indicating about 70% of correct detections.