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have been replaced by hard ground and impervious roofs, which would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%.