###### Abstract
[28] performed a preliminary comparison of the monthly precipitation products of IMERG and TMPA at a global scale and found that noticeable systematic differences exist between these two products in various regions of the world. [PERSON] et al. [29] conducted a global assessment of timing of extreme rainfall from TRMM and GPM, and found that the IMERG Day-1 final run products achieve a slightly higher match rate than 3B42V7 in detecting extreme rainfall. Additionally, the accuracy and error characteristics of IMERG and TMPA products in comparison with gauge measurements were evaluated in many regions, such as in Xinjiang [20], Qinghai-Tibetan Plateau [30; 31], and Poyang Lake [32] regions in China; Mainland China [33; 34]; South Korea [35]; Japan [35]; India [36]; and in four regions with distinct climate and topography in Iran [37]. These studies reported that IMERG products are generally superior to TMPA [30; 31; 33; 34; 35; 36; 37] or at least comparable to TMPA products [32]. Moreover, [PERSON] et al. [38] found that IMERG products have a better bias ratio and correlation coefficient at hourly and daily scales as compared to CMORPH in the upper Blue Nile basin of Ethiopia. Evaluated against a dense gauging network in the US mid-Atlantic region, IMERG exhibits some misses and false alarms for rain detection, and overestimates drizzle and underestimates heavy rain with considerable random errors [39]. [PERSON] et al. [40] assessed IMERG against gauge-adjusted radar rainfall in the Netherlands and found a tendency for IMERG to slightly underestimate (2%) countrywide rainfall depths. NASA's Goddard Space Flight Center used IMERG to effectively estimate the historic amount of rain that fell in the Carolinas and from hurricane Joaquin over the Bahamas [41]. However, only a few investigations were performed on the hydrological utilities of IMERG products at basin scales. [PERSON] et al. [32] performed an early hydrological assessment of IMERG and found that with the Coupled Routing and Excess Storage hydrological model, the Day-1 IMERG product performs comparably to gauge reference data in the Poyang Lake basin in China and outperforms TMPA standard products in many cases. Given that topography and climate significantly influence the quality of satellite-derived precipitation, the accuracy of IMERG and TMPA products, as well as their feasibility in hydrological applications in other regions of the world, must be assessed and compared, particularly in ungauged or data-sparse regions with complex terrains and frequent flood and drought disasters. In this study, the IMERG final run and TMPA 3B42V7 precipitation products covering a data-sparse mountainous watershed in Myanmar (Figure 1) were validated and compared against a few gauge observations. In addition, streamflow simulations using these two products were evaluated and compared at daily and monthly scales. Given that GPM is still in the start phase and its calibration is not yet complete, the main objectives of this study are therefore to: (1) statistically evaluate the quality of GPM-era IMERG product compared with TRMM-era 3B42V7 estimates in the study area; and (2) comprehensively explore the feasibility of IMERG data in streamflow simulations in comparison with that of 3B42V7 in ungauged basins. This initial and early evaluation can provide useful guidelines for hydrological applications of IMERG data in river basins in Myanmar and other ungauged basins. It can also facilitate the further improvement of IMERG algorithms in the future.