2.1.1. Observed Precipitation

To evaluate the capabilities of various products in capturing precipitation changes, sub-daily (i.e., daytime, Pd, 0000–1200 UTC; and nighttime, Pn, 1200–2400 UTC) accumulated precipitation data observed from 2003 to 2017 at 2481 weather sites across China (Figure 1), including basic, benchmark, and general meteorological stations, were collected from the China Meteorological Administration (CMA). Although both datasets had undergone a series of quality control measures and homogenization, e.g., outlier identification, internal consistency checks, and spatio-temporal consistency checks [84], there were still missing values within the records. Therefore, to maximize the observational information, we processed the datasets following the procedures described below. First, the number of daytime and nighttime values was computed for each year at each site. If the days with missing values for daytime or nighttime observations exceed 50 at a site, the site was removed. Secondly, for the remaining sites, the bilinear interpolation method was employed to fill the missing values with the observations at the two closest sites. There were 2393 sites remaining after this process (Figure 1), and the accumulative precipitation for a whole day (abbreviated as Pwd) was then obtained as the sum of Pd and Pn.

**Figure 1.** Geographic distribution of the selected 2281 grids (0.25◦ × 0.25◦), at least corresponding to a weather site. The digital elevation model (DEM) with a spatial resolution of 90 m is available at http://srtm.csi.cgiar.org/. [85] Crosses and triangles correspond to 1 and more than 2 sites within a given grid, respectively, followed by the percentage of grid shown in the bracket.

China is located in a typical monsoon region (i.e., the East Asian monsoon region), with evident spatio-temporal variability of precipitation and the related mechanisms [86]. Mainland China (MC) is divided into ten Water Resources Regions (WRRs, Figure 1), which is beneficial for examining regional differences in the performance of each product to detect precipitation trends. We conducted annual and seasonal evaluations of Pwd, Pd, and Pn trends during 2003–2017 on national and regional (i.e., MC and WRR, respectively) scales. Here, spring, summer, autumn, and winter were specified as March–May, June–August, September–November, and December–February, respectively.
