## 2 Study Area and Data Preparation
### Satellite Precipitation Products  
Two types of satellite precipitation products, namely, the TMPA 3B42V7 three-hourly and IMERG final run half-hourly precipitation products, were adopted in this study.  
The TMPA 3B42V7 precipitation products were produced by the TRMM TMPA Version 7 algorithm [(5)]. TMPA can produce microwave-infrared satellite precipitation estimates combined with gauge adjustments and has the potential to give the best precipitation estimates for the regions between 50\({}^{\circ}\)S and 50\({}^{\circ}\)N on a 0.25\({}^{\circ}\) spatial resolution and a three-hour time interval. In this study, the TRMM 3B42V7 three-hourly precipitation product from 1 April 2014 to 31 December 2015 was obtained from the Precipitation Measurement Mission (PMM) website [(25)], and the three-hourly precipitation data were accumulated to daily values in the study area. The 3B42V7 three-hourly precipitation files are centered on the hour in the file name. For example, a \"03z\"-hour file represents data from 1:30 UTC to 4:30 UTC. To produce the 3B43V7 daily precipitation data in accord with the time span of daily precipitation collection at the local weather stations (03:00-03:00 UTC), we assigned a half weight (0.5) to the \"03z\" files for the current and next days, and defined a full weight (1.0) for the \"06z\", \"09z\", \"12z\", \"15z\", \"18z\" and \"21z\" files for the current day and the \"00z\" files for the next day.  
IMERG is the Level 3 precipitation estimation algorithm of GPM and is intended to inter-calibrate, merge, and interpolate multiple satellite microwave precipitation estimates, microwave-calibrated infrared satellite estimates, precipitation gauge analyses, and other precipitation estimators on a finer spatial resolution (0.1\({}^{\circ}\)) with larger quasi-global coverages (60\({}^{\circ}\)S-60\({}^{\circ}\)N) and a shorter temporal interval (30 min) than those of TMPA products. In this study, the IMERG final run half-hourly precipitation product from 1 April 2014 to 31 December 2015 was downloaded from the PMM website [(26)], and the IMERG daily precipitation data were produced by simple addition of the 48 half-hourly files covering the time span of 03:00-03:00 UTC.  
Both 3B42V7 and IMERG final run products were already bias-corrected on a monthly basis using the GPCC precipitation data set. As stated in Section 2.2, no rain gauges in the basin are included in the GPCC gridded gauge-analysis products. This implies that the GPCC data set covering the study area might not be accurate enough. Therefore, it is necessary to evaluate the quality of these two satellite products using the ground observations, and remove their biases to better suit streamflow modeling. The procedures for quality evaluation and bias-correction of 3B42V7 and IMERG products are described in detail in Sections 3.1 and 3.2.  
To perform hydrological simulations on a spatial resolution of 0.25\({}^{\circ}\) (detailed discussion in Section 3.4), the IMERG daily precipitation product was aggregated from a 0.1\({}^{\circ}\) spatial resolution to 0.25\({}^{\circ}\). Given that four IMERG grid cells are completely located within a 0.25\({}^{\circ}\) grid cell, four fallhalfway inside, and a ninth falls one-fourth within, we assigned areal weights of 0.16, 0.08 and 0.04 for these IMERG grid cells, respectively, for precipitation aggregation.