**2. Materials and Methods**

#### *2.1. Study Area*

Xi'an, the capital of Shaanxi Province, is a megacity in western China, located at the central part of the Guanzhong Plain, with the Loess Plateau to the north, and the Qingling Mountains to the south. Xi'an is situated in a sub-humid and warm temperate region under the influence of the East Asian monsoon with hot and humid summers, and cold winters. During the warm period (May–October), winds blow in from southern China; during the cold period (November–April), winds from the desert regions of north-western and western China are dominant. Over the period of 2001–2017, the average annual temperature was 13.6 ◦C, and the annual precipitation was 558.2 mm (http://data.cma.cn/, accseed on 1 March 2020).
