*3.1. Gauge Precipitation Changes across MC*

Figure 2(a1) depicts observed annual LTs for MC and ten WRRs during 2003–2017. For MC, annual LTwd and LTd were 8.42 mm/yr (*p* < 0.05) and 4.96 mm/yr (*p* < 0.05), respectively, followed by an insignificant LTn of 3.46 mm/yr. Comparing LTwd, LTd, or LTn (i.e., signs and magnitudes) among WRRs, there were evident regional differences, while significant (*p* < 0.05) and larger increases (>13 mm/yr, >8 mm/yr and >7 mm/yr for LTwd, LTd and LTn, respectively) were found in YZRB, SERB, and PRB, followed by the largest reductions (–13.95 mm/yr for LTwd, −4.68 mm/yr for LTd and −9.27 mm/yr for LTn) in HuRB. In spring (Figure 2(b1)), LTs for WRRs and MC were between −4 mm/yr and 4 mm/yr, with the exceptions of SERB and PRB, which showed LTwd > 8 mm/yr, and LTd and LTn > 4 mm/yr. During summer, MC LTwd and LTd (LTn) were positive (negative) with a rate <2 mm/yr (Figure 2(c1)). Among ten WRRs, most exhibited smaller LTwd (LTd and LTn) in summer, generally corresponding to between −3 mm/yr and 4 mm/yr (between −1.50 mm/yr and 2.50 mm/yr); however, significant (*p* < 0.05) decreasing and increasing LTs were detected over HuRB and YZRB (excluding LTn) and SERB, and the LTwd, LTd, and LTn were >6 mm/yr and > 4 mm/yr, respectively. As shown in Figure 2(d1), MC LTwd and LTd (LTn) were 5.60 mm/yr (*p* < 0.05) and 2.80 mm/yr (*p* < 0.05), respectively. Except for two WRRs (i.e., HaRB and YRB), autumn LTwd, LTd, and LTn were consistently positive from 2003 to 2017. However, magnitudes of autumn LTs differed among these WRRs, for which significant (*p* < 0.05) and larger increases (>8 mm/yr for LTwd and 3.20 mm/yr for LTd and LTn) occurred in YZRB, SERB, and PRB. Regarding winter precipitation (Figure 2(e1)), SERB and PRB exhibited the highest LTwd (>2.80 mm/yr) and LTd and LTn > 0.90 mm/yr, followed by the remaining WRRs and MC with an LTwd < 1.50 mm/yr (LTd and LTn < 0.70 mm/yr). Additionally, comparing signs and magnitudes of LTd and LTn (Figure 2(a1–e1)), 10 and 15 of 55 cases (i.e., 11 (MC + 10 WRRs) × 5 (annual + seasonal scales)) showed opposite signs and larger differences, with ratios between LTd and LTn > 2.00 and < 0.50, respectively. These findings imply that LTd and LTn values were not consistent, possibly due to the different precipitating mechanisms during daytime and nighttime, and thus further confirms the necessity to evaluate various precipitation products at a sub-daily scale.

As shown in Table 2 and Figure 2(a2), 33% of grids had decreasing annual LTwd across MC, generally in east LRB, HuRB, the YRB–YZRB border, and most of SWRB and NWRB. Moreover, 3% of grids in north-central HuRB showed significant (*p* < 0.05) negative annual LTwd with a rate of −12 mm/yr. In contrast, 11% of grids had significantly (*p* < 0.05) increasing LTwd, mainly situated in east SHRB, central YRB, northeast YZRB, north SERB, and middle PRB, for which LTwd over the three latter regions exceeded 20 mm/yr. For both annual LTd and LTn (Table 2), negative values were found in > 30% of grids, followed by < 4% of grids with significant (*p* < 0.05) values. Moreover, in spite of smaller magnitudes of difference compared to annual LTwd, similar spatial distributions for LTd and LTn were detected (Figure 2(a3,a4)). Figure 2(b2–b4,c2–c4,d2–d4,e2–e4) illustrate the spatial distribution of seasonal LTwd, LTd, and LTn during 2003–2017. In broad terms, LTwd, LTd, or LTn spatially differ during seasons, while in a given season, a generally similar spatial pattern is observed among LTwd, LTd, and LTn, including for locations with significant (*p* < 0.05) LTs. For example, spring LTwd, LTd, and LTn were negative at 30% of grids, primarily in NWR, SWRB, west YZRB, north HuRB, and HaRB (Figure 2(b2–b4) and Table 2); moreover, 2% of grids with significant (*p* < 0.05) changes were sporadically distributed, and larger reductions (–6 mm/yr for LTwd, but –2 mm/yr for LTd and LTn) were in south SWRB. At the remaining grids, 60% of grids with larger increases for LTwd (12 mm/yr), LTd and LTn (4 mm/yr) in spring for were mainly located in east YZRB, PRB, and SERB, and 5% of grids with significant (*p* < 0.05) changes were generally in SHRB–LRB, YRB–YZRB borderlands, and east PRB. As shown in Figure 2(c2–c4) and Table 2, 44% of grids with a negative summer LTwd, LTd, and LTn were generally situated in central SHRB, LRB, HuRB, YRB–YZRB borderlands, west YZRB, PRB, and north SWRB, and the largest and significant (*p* < 0.05) reductions (−10 mm/yr) in 5% of grids were concentrated in HuRB. Of the remaining grids (>50%), the largest (10 mm/yr) and significant

(*p* < 0.05) summer LTs were detected in 4% of grids mainly in northeast YZRB and north SERB. In autumn (Figure 2(d2–d4) and Table 2), LTwd, LTd, and LTn were at least -6 mm/yr at 30% of grids in south LRB, YRB–HaRB–HuRB and YRB–YZRB borderlands, central NWRB, and central SWRB. Of the grids with increasing LTs, 14% of grids with large (10 mm/yr) and significant (*p* < 0.05) values were situated in central SHRB, central PRB, east YZRB, parts of middle YZRB (i.e., Sichuan basin), and SERB. During winter (Figure 2(e2–e4) and Table 2), there was an approximately equal balance of grids with negative and positive LTwd, LTd, or LTn, which was generally 4mm/yr or −4mm/yr at most grids; moreover, increasing LTs were widely distributed across east coastal WRRs, south SWR, and central YRB. Furthermore, 2% of grids with significant (*p* < 0.05) increases in winter precipitation were patchily distributed across MC.

**Figure 2.** Linear trends (LTs) for mainland China (MC), ten Water Resources Regions (WRRs), and 2281 grids during 2003–2017. Annual and seasonal LTs averaged over MC and ten WRRs are shown in (**a1**) and (**b1**–**e1**), respectively, in which stars represent significant changes with *p* < 0.05. (**a2**) and (**b1**–**e2**) show spatial distributions of annual and seasonal Pwd trends across MC, respectively, with the green cross representing significant changes with *p* < 0.05. (**a1**–**e3**) and (**a1**–**e4**) are the same as (**a1**–**e2**), but for Pd and Pn trends, respectively.

**Table 2.** Percentage of grids with increasing and decreasing LTs across MC for the whole day (LTwd), daytime only (LTd), and nighttime only (LTn).

