## 5 Discussion  
### Evaluation of SPPs against Ground Rainfall Observations  
This study employs an early evaluation of the near- and post-real-time versions of the TRMM-era SPPs (TMPA) and the GPM-era SPPs (IMERG and GSMaP) on sub-daily time scales in contrast with the ground rainfall observations in the Chindwin River basin, Myanmar. This study found that among all near-real-time SPPs, TMPA 3B42 RT has the best overall performance, followed by IMERG-L and IMERG-E, whereas GSMaP-NRT demonstrates inferior capability in capturing precipitation dynamics at grid and basin scales (Figure 6 and Table 1). IMERG-F presents the highest accuracy, slightly superior to 3B42V7 among the four post-real-time SPPs. GSMaP-MVK is ranked as third, and the performance of GSMaP-GAUGE is unsatisfactory (Figure 6 and Table 1). [PERSON] et al. [47] carried out the first comparative study on the quality of the 3B42V7 and IMERG-F version 03D (V03D) data sets in the same region and found that 3B42V7 generally outperforms IMERG-F at daily and monthly time scales. Our study demonstrates that the rainfall retrieval algorithms of IMERG-F were remarkably enhanced, and the quality of IMERG-F V05B was greatly improved in comparison with its previous version (V03D) and even surpassed its predecessor, 3B42V7, in the Chindwin River basin. This finding implies that the current release of IMERG-F can be considered the suitable replacement for 3B42V7 in the study area. Many previous studies [24; 25; 50; 72; 73; 74] found that post-real-time SPPs generally present higher quality than their near-real-time versions in many regions of the world because of gauge-based adjustment. In this study, IMERG-F evidently outperforms the near-real-time SPPs (IMERG-E and IMERG-L) in the Chindwin River basin. However, this case is different from those of GSMaP and TMPA SPPs. Although calibrated with the CPC precipitation data, the post-real-time product GSMaP-GAUGE greatly underestimates precipitation with higher magnitudes of underestimation than the near-real-time product GSMaP-NRT (Figure 4a,b), and GSMaP-GAUGE has the lowest overall score among all the three GSMaP SPPs (Figure 6). The precipitation estimates from GSMaP-GAUGE largely depend on the CPC data set. If the CPC data set has good precipitation estimates in a region, then GSMaP-GAUGE will also show remarkable scores in this region [75]. The present study checked the CPC data set [76] to explore the possible error sources for GSMaP-GAUGE and found that the CPC data do not include any gauges within the Chindwin River basin but mainly adopt the rainfall records at a few gauges located at the western slopes of the Arakan Mountains to extrapolate the rainfall situation in the investigated basin. The CPC precipitation data extrapolated from the rainfall records at the rain gauges outside of the studied basin are likely to contain large biases because precipitation in the Chindwin River basin is largely affected by the southwest monsoon activities and complex topography. Thus, this phenomenon might remarkably influence the accuracy of the GSMaP-GAUGE precipitation estimates. This study also analyzed the gauge density of the GPCC monthly gauge precipitation analysis data set [77] and found that the number of gauges is very limited and highly varies in the Chindwin River basin, usually two to five stations in the dry season (October-April) and zero to two stations in the rainy season (May-September). Bias-corrected using the GPCC data set, 3B42V7 considerably reduces the systematic errors in 3B42 RT (Figure 4a,b). However, the GPCC-basedbias-correction does not evidently improve other diagnostic statistics (Figure 4), and even 3B42 RT has better overall performance than 3B42V7 (Figure 6). In addition to the errors in the GPCC data set, other error sources of 3B42V7 should be analyzed as well. This work will be conducted in the near future. As parallel SPPs in the GPM-era, IMERG and GSMaP products in several regions of the world were evaluated and quantitatively compared [29; 72; 74; 78; 79; 80]. [PERSON] et al.