2.1 Study Site

Nanchang (28◦ 68′ N, 115◦ 89′ E), the capital of Jiangxi province, China, and a highly populated and urbanized area (5.46 million inhabitants in 335 km2) in the MLRYR, was selected as the study region for spatial analysis and hydrologic modeling (Fig. 1). The open water bodies of each district of Nanchang city are natural flood storage areas that have formed over thousands of years. However, a large number of natural rivers and lakes have been narrowed, blocked, and landfilled during the rapid urbanization process. Furthermore, the permeable soils have been replaced by hard ground and impervious roofs, which would hinder ground water recharge and increase the loading of drainage systems. Because of the unique climate characteristics of the MLRYR region and the human factors in urban expansion, Nanchang is also suffering from urban flooding and groundwater depletion.
The city has humid subtropical (Cfa) (Li et al., 2018) climate conditions. The mean yearly temperature is about 17.5 °C, with July being the hottest month. The mean annual precipitation is about 1,600 mm yr-1, most of which falls during the plum rain season (June - July) and the typhoon season (July - September). The plum rain season, with characteristics of persistent precipitation, is a specific and important climate phenomenon over the MLRYR region, South Korean Peninsula, and south-central Japan from June to July, associated with the East Asian Monsoon (EAM; Ge et al., 2007; Lu et al., 2013). The EAM system includes the cold dry northwestern East Asian Winter Monsoon (EAWM) and the warm and moist southeastern East Asian Summer Monsoon (EASM) (Zhou, 2009). The beginning of plum rain is the main part of the EASM system that occurs over eastern China, the South China Sea, Korea, Japan, and the adjacent seas (Tao and Chen, 1987; Tao et al., 1998; Ding, 1992, 2004; Wang et al.2018; Wang and Lin, 2002; Ding and Chan, 2005; Ding et al., 2007; Lu et al., 2013). In Nanchang, during the plum rain season, the northward warm moist EASM and the shallow cold air mass of Central East China from the north meet and create a quasi-stationary front, creating high precipitation June and July, (243.8 and 306.7 mm month-1 in June and July, respectively (Xu, 2008;http://www.weather.com.cn/cityintro/101240101.shtml )). From July to September, abundant precipitation is associated with typhoons. Because of rapid urbanization, the storm water network in this city is very sensitive to intense precipitation, which can cause local flooding.
Groundwater dynamics in Nanchang city are affected by extraction, rivers, and rainfall (Liu, X.Y. et al., 2006). Because of the overexploitation of groundwater, three big regional cones of depression were formed during the 1960s (Lan et al., 2017). There continues to be an expansion of the cones of depression, and groundwater levels have dropped, which has also influenced by a multi-decadal drought (Lan et al., 2017; Liu et al., 2006).
Evaporation, humidity, radiation and wind speed data in 2015 as well as the precipitation and temperature data in 1985-2015 for Nanchang city were provided by the China Meteorological Administration. All data were daily recorded. The missing precipitation data in the total data per year is shown in the Fig. S1. All days without precipitation data are considered zero rainfall. Continuous precipitation data from 1985 to 2015 were used in three aspects: first, to analyze seasonal variations of rainfall; second, to synthesize 2-year, 10-year and 100-year precipitation events; and third, to analyze cumulative changes of surface runoff and evaporation with the temperature data. Daily evaporation, humidity, wind speed and precipitation data in 2015 were used to calculate ET by different methods. The 31-years precipitation dataset has a strong seasonal cycle, with distinct wet (from February to August) and dry (from September to January) seasons (Fig. 2). The wet season precipitation accounts for 79.1% of annual total precipitation. Monthly data showed that the maximum rainfall always occurs in June, with a mean precipitation of 320 mm month-1.
The study was conducted on Xiangjiang International Furniture Square (XIFS) (28◦ 39′ 16.23′′N, 115◦ 56′ 36.26′′E) (Fig. 3a). XIFS is a commercial service facility area located in the northern part of Nanchang city. The total area of XIFS is 0.16 km2 with a building density of 45% sharing the homogeneous climate condition as well as the soil and vegetation conditions. The buildings are all flat roofs with an average slope of 2%.