## 2 Materials and Method
### Datasets
#### 2.2.2 Satellite Datasets  
IMERG is NASA's level-3 multi-satellite GPM product. After the GPM mission, four different versions (V03, V04, V05, and V06) of the IMERG algorithm were developed. IMERG combines retrievals from PMW and microwave-calibrated infrared (IR) to produce a high-resolution global SBP product. Compared to earlier versions (V03 and V04), IMERG V05, and V06 inter-calibrate all individual PMW  
Figure 1: The study region, Nepal is divided into three different regions and the meteorological stations used are distributed from 60 m above sea level (masl) to 5050 masl. The extent of central Himalayas is adapted from [PERSON] et al. [54]. satellite estimates from Goddard PROFiling (GPROF2017) scheme [57]. Until V05B, the Lagrangian time interpolation scheme was computed from IR data, while in V06, a new model-based morphing scheme in which motion vectors are retrieved from Modern-Era Retrospective Reanalysis 2 (MERRA-2) and Goddard Earth Observing System Model (GEOS) Forward Processing (FP) data using total column water vapor (TQV) [58]. For the first time, IMERG V06 used Precipitation Retrieval and Profiling Scheme (PRPS) Sounder for Atmospheric Profiling of Humidity in the Intertropics by Radiometry (SAPHIR) only to the combined Ku-swath DPR/GMI product (CORRA) [20]. These intercalibrated estimates were then merged into a single PMW SBP estimate and used to calibrate the IR-based precipitation. IMERG uses a Kalman filter-based method to combine the observed PMW, propagated Pulse Width Modulation (PWM), and IR estimates into a single, best estimate. IMERG provides three different products, Early, Late and Final runs, with a latency of \(\sim\)4 h, \(\sim\)14 h, and \(\sim\)3.5 months, respectively. The only forward direction of the cloud motion vector propagation algorithm was adopted in the Early run. In addition to Early, backward morphing was added in the Late run. Meanwhile, in the Final run, climatological calibration coefficients were added on the basis of Late run [33]. The rain-gauge data were not assimilated in Early and Late run, while the Final product was adjusted using 1\({}^{\circ}\) GPCC gauge analysis which is interpolated to 0.1\({}^{\circ}\) and applied equally to every half hour in the month. The Final run is mostly recommended for research purposes. IMERG applies the wind-loss correction scheme [59] to the GPCC gauge analyses during the calibration process. The latest IMERG version 06 products were released to the public in March 2019. The Final version of the IMERG product includes two precipitation fields, precipitationUncal and precipitationCal. The Early and Late precipitation products are identical to the Final precipitationUncal since the gauge correction is only applied to the precipitationCal field in the Final product. IMERG V6B had an upgrade of full intercalibration to GPM combined instrument datasets (2 BCMB), also in this version the input precipitation rates were increased from 50 to 200 mm/h to adjust fractional coverage [44]. For this study, data from precipitationUncal (hereafter IMERG-UC) and precipitationCal (hereafter IMERG-C) of 0.1\({}^{\circ}\) spatial resolution from the Final run IMERG version 06B between March 2014 and December 2016 were obtained from the PMM website ([[https://gpm.nasa.gov/data-access/downloads/gpm](https://gpm.nasa.gov/data-access/downloads/gpm)]([https://gpm.nasa.gov/data-access/downloads/gpm](https://gpm.nasa.gov/data-access/downloads/gpm))). A detailed description of the IMERG algorithm can be found in [PERSON] et al. [60]. GSMaP is a SBP product developed by Japan Science and Technology (JST) under the Core Research for Evolutional Science and Technology (CREST) program [61; 62]. To provide high-precision precipitation products, GSMaP combines various available PMW and IR sensors [63].