**2. Materials and Methods**

### *2.1. Study Area*

China, which covers an area of about 9.6 million km2, is located in the eastern Asia. The terrain of China is complex and varied, with flat plains and hills in the east and high mountains and plateaus in the west. The elevation of China has an overall descending tendency from west to the east. Specially, the world's "Third Pole", the Tibetan Plateau (TP), is situated in southwest China. Generally, the climate on the China features hot and rainy summers and cold and dry winters due to the effects of monsoon system and topography. In addition, the annual precipitation in China varies greatly, with an obvious gradient from southeast to northwest, except for some parts of northwestern Xinjiang. Over southeast China, a huge amount of water vapor from the Pacific Ocean is brought by the Asian monsoon in the summer, producing abundant precipitation (exceeding 800 mm/year). Over the inland areas of western China, which are far from the ocean, monsoon precipitation becomes negligible and the continental climate dominates (with annual precipitation less than 400 mm). Considering the diverse climate in China, following Yong et al. [39], we separated mainland China into four representative climate regions based on the annual precipitation (Figure 1a): (1) The humid region, mainly covering the southeast of China and dominated by the subtropical monsoon with the average annual precipitation (AAP) above 800 mm; (2) the semi-humid region (AAP between 400–800 mm), including northeast China, North China, and southeast parts of TP; (3) the semi-arid region (AAP of 200–400 mm), which consists of two parts, one region extending from the southwest TP into the North Inner Mongolia with a narrow strip, and another region located in the Ili river valley; (4) the arid region (AAP < 200 mm), a main desert areas in China, including the most of Xinjiang and western part of Inner Mongolia.

**Figure 1.** (**a**) Distribution of station density used in this study. Four climate regions (-1 –-4 : Humid, semi-humid, semi-arid, arid) are outlined in Mainland China. (**b**–**d**) Daily mean precipitation over Mainland China during the period of September 2017–August 2019, from gauge observations, GSMaP\_NRT, and GSMaP\_Gauge\_NRT precipitations.
