## 4 Results  
### Comparison of IMERG Series Products with Rain Gauge Data  
In this study, the IMERG series products over the XRRB from 1 April 2014 to 31 December 2017 were statistically analyzed in terms of three spatial scales: grid cumulative scale, basin scale, and grid scale. The accuracy of the IMERG series products at daily and monthly scales is evaluated in the XRRB. As shown in Table 2, at the daily scale, the FR has better CORR (0.61/0.71) than the ER (0.59/0.69) and LR (0.59/0.69) on both the grid accumulation scale and the basin scale, and these CORR results are statistically significant at the \(p<0.001\) level. However, the RMSE value does not show a significant advantage, while the BIAS (0.01) is significantly better than that of the ER (\(-0.14\)) and LR (\(-0.16\)). On the monthly scale, the FR product also has a higher CORR (0.94/0.99) than the ER (0.88/0.93) and LR (0.88/0.92) products on both the grid accumulation scale and the basin scale. The RMSEs of the FR (48.03/18.99 mm) are also lower than those of the ER (71.50/54.88 mm) and LR (73.13/58.34 mm) products. For the systematic error, the ER and LR products generally tend to underestimate the precipitation, with BIAS of \(-0.14\) and \(-0.16\), respectively, while the FR tends to overestimate the precipitation, with relatively low BIAS of 0.01. Moreover, the statistical significance of the difference among the products was also tested using the \(t\)-test at the 95% significance level, and the results are shown in Table 3. The test results reveal that there are significant differences among the ER, LR, and FR products, with all their \(p\)-values less than 0.05. For the detection effect at the grid accumulation scale (Table 2), the POD values of all three IMERG products are above 0.8 on the daily and monthly scales, the FAR values are below 0.37, and the HSS values are above 0.56. FR shows better POD (0.81, 1.00) and HSS values (0.57, 0.98) than ER and LR at the grid accumulation scale. The results of three indexes (POD, FAR, and HSS) demonstrate its good ability in detecting precipitation. The detection performance at the average scale of the watershed is also satisfactory. The POD values of all three IMERG products are above 0.83, FAR values are below 0.22 and HSS values are above 0.67 at both daily and monthly scales. FR yields better POD (0.84, 1.00) and HSS values (0.67, 1.00) than ER and LR at the basin scale. The reason lies in the improvement of the performance of dual-frequency precipitation radar (DPR) and the GPM Microwave Imager (GMI); thus, the spatial structure and physical features of different precipitation particles in the clouds can be monitored from various angles, and, therefore,the capability to discover different-level precipitation has been significantly improved [41]. In our previous study, we proved that the GPM IMERG products have an advantage in capturing rainfall events and estimating instantaneous precipitation, especially for light rain events [19]. Combining the RMSE, Bias, CORR, and significance test results, we can conclude that the FR product is more accurate than the ER and LR products. These results are not unexpected because the FR products have been corrected using the Global Precipitation Analysis Products of the Global Precipitation Climatology Centre (GPC) (on the monthly scale). Therefore, their grid cells around the GPCC stations will be effectively reduced. In general, the three IMERG products have reasonably good performances in detecting precipitation, with a slight advantage for the FR. The IMERG series products exhibit significantly better accuracy results on the monthly scale than on the daily scale. Analyses of the accuracy of the IMERG series of products on the grid scale and all the metrics are included in Figures 3 and 4. The box plots of the accuracy metrics on the daily scale (Figure 3a-c) show that the CORR and BIAS of the FR product (median: CORR of 0.62, BIAS of 0.02) are superior to the ER (median: CORR of 0.60, BIAS of 0.15) and LR (median: CORR of 0.60, BIAS of 0.17).