2.1 Study area
We carried out this study in metropolitan Shenyang, Liaoning Province, northeast China (41o11’51” ~ 43o02’13”N,122o25’09” ~ 123o48’24”E, Fig. 1A). Shenyang city is a capital city of Liaoning province and had an estimated 2017 population of 5,864,912 people (Chinese Census Bureau, 2017). The urban built-up area of Shenyang is within the third beltway and about 758.6 km2. Shenyang is an inland city with an average altitude of 51 meter, belonging to the temperate semi-humid continental climate zone. Its average annual temperature and annual precipitation are 8.6℃ and 721.9 millimeter respectively. There are 7 districts and 76 subdistricts within the third beltway of Shenyang city (Fig. 1B).