Rainstorm flood
Change in precipitation
Xiongan New Area is located on the northern edge of the East Asian monsoon. The annual climatological precipitation is 481 mm, with 78% of rainfall concentrated between June and September. Precipitation in winter accounts for < 2% of the total annual precipitation. The annual variability of precipitation is large and is influenced by the intensity of the summer monsoon. The highest annual precipitation (869 mm in 1988) is more than three times the lowest annual precipitation (254 mm in 1968). During the time period 1961–2018, the annual precipitation and number of rainfall days slightly decreased at rates of 8.8 mm/10a and 1.5 days/10a, respectively (Fig. 2a, b). The inter-decadal variation in precipitation is very clear. The average annual precipitation reached an historical high of 538 mm in the 1960s and an historical low of only 460 mm in the 2000s. The annual number of normal rainstorm days is 1.4 (Fig. 2c) and the longest period of continuous precipitation is 5.0 days. The average maximum daily precipitation is 71.7 mm, and the maximum amount of continuous precipitation is 96.2 mm. During the time period 1961–2018, the hourly maximum rainfall intensity showed an increasing trend of an average of 1.5 mm/10a (Fig. 2d). The rainfall intensity at different return periods during 1991–2018 increased by 16–29% from the 1961–1990 level (Table 1).
Table 1 Return periods of daily maximum precipitation (mm) during two time periods in Xiong’an New Area Flood hazard
Rainstorm flood refers to a flashflood or waterlogging with heavy rainfall. It is one of the most common natural hazards in Xiongan New Area and occur at a high frequency, causing great destruction and heavy economic losses. A total of 139 flood events have occurred in Xiongan New Area over the past 300 years, with, on average, a flood event occurring once every 2 to 3 years. Floods were frequent and severe during the time periods 1796–1827, 1886–1898, and 1948–1965 (Hao et al. 2018). A catastrophic flood took place in the Hai River Basin in early August 1963. As a result, the water level in Baiyang Lake rose and almost all of Anxin and Xiong counties were flooded. Xiongan was affected by a large-scale and long-term flood hazard for nearly 2 months. From 19 to 21 July 2016, the cumulative volumes of precipitation in Rongcheng, Anxin, and Xiong counties were 176, 213, and 192 mm, respectively, and the maximum daily precipitation in Rongcheng and Anxin counties were at record highs of 168 and 205 mm, respectively, affecting 517,000 people in Xiongan.
We use the extreme value distribution function to fit the return period of area rainfall in Xiongan New Area and obtain the area rainfall of Xiongan New Area with a return period of N years. The dynamic submergence process of rainstorm floods is simulated by the hydrodynamic model FloodArea. In combination with the impact-based flood risk assessment criteria, the submergence range and depth of rainstorm flood in Xiongan New Area are obtained (Table 2). For a rainstorm that occurs once every 10 years, 80% of Xiongan New Area reaches a flood depth < 20 cm, which has only a small impact. With the extension of the return period, the area affected by heavy and extremely heavy rainstorms increases. For a rainstorm that occurs once every 50 years, the area with a flood depth of 20–60 cm reaches 904.6 km2, and the area with a flood depth of > 60 cm reaches 37.1 km2. Based on these criteria, we obtain the flood risks in Xiongan New Area (Fig. 3). In general, the northern and southeastern parts of the area are subject to high flood risks. Most of the initial development zone is at moderate and high risk of floods, and part of Xiongan New Area is at extremely high risk of floods.
Table 2 Return periods of 24-h precipitation (mm), flood depth (m), and flood area (km2) in Xiong’an New Area Future flood risk
The regional climate model RegCM4 is used to predict the future changes in climate in Xiongan New Area and the surrounding areas based on statistical downscaling data at a high resolution of 6.25 km under the RCP4.5 scenario (Shi et al. 2019) in the reference period 1986–2005. The results show that R95P, RX5day, R1mm, R10mm, and R20mm will generally increase in the future; in particular, intense precipitation during the time period 2026–2045 will increase by 34.1%. Most of the Beijing–Tianjin–Hebei region is grade I risk in the reference period, accounting for 88.2% of the total area, whereas grade III and higher risks are mainly in the downtown regions of the plain areas. Xiongan is mainly at grade I risk (Fig. 4a). It is estimated that the area of Xiongan at grade II risk will increase from 11.8% in the reference period to 84.3% at about 2035, whereas the area at grade V risk will increase to 15.7% (Fig. 4b). Both the population and economic volume of Xiongan are planned to increase markedly, and the risk of the initial development zone will also rise to grade V by about 2035.
Drought and water shortage
Drought hazard
Xiongan New Area is usually affected by a moderate and higher drought risk on 57.3 days and a longest continual drought of an average of 31.6 days annually. During the time period 1961–2018, the average annual number of drought days showed a clear inter-decadal feature, with the most drought days (247 days) in 1968, followed by 1975, 1999, and 2006, all with > 150 drought days (Fig. 2d). There were six extreme drought events in North China after 1951. A spring–summer drought occurred in Hebei Province in 1975, leading to 190 days of drought in Xiongan New Area. The groundwater levels decreased sharply and the production of crops such as wheat and maize was severely affected. Drought continued for a long time in central and southern Hebei Province in 1999, coupled with abnormally high temperatures that led to serious losses of soil moisture. There were 160 drought days in Xiongan New Area in 1999 and a serious drought in Xiong County. Serious winter–spring and autumn droughts swept through Hebei Province in 2006. The average number of drought days in Xiongan New Area reached 170. Drought had a profound impact on the growth of winter wheat and autumn sowing, and some areas were affected by a shortage of drinking water.
Change in water resource
Xiongan New Area contains Baiyang Lake in the Daqing River Basin. The annual average runoff is about 2.23 billion m3, with 85% of the total from mountainous areas. Influenced by a monsoon climate, 60–80% of the annual surface runoff is concentrated in June–September, resulting in extreme inter-annual variations: the maximum runoff is 15 times the minimum runoff. During the time period 2005–2016, the average water resources in three counties of Xiongan New Area totaled 160 million m3 with a per capita resource of 147 m3. The total water supply was 250 million m3, with a water deficit of nearly 90 million m3 per year (about 36%). More than 90% of the total water supply is from groundwater.
The decrease in annual precipitation has led to a sharp decrease in natural runoff in the mountainous areas of the Daqing River Basin. The annual runoff at three hydrological stations (Zijingguan in the Juma River Basin, Fuping in the Sha River Basin, and Daomaguan in the Tang River Basin) in the mountainous areas of Baiyang Lake in the Daqing River Basin during the time period 1961–2016 decreased at a rate of 130 million m3/10a, significant at the 95% level (Fig.5a), mainly in the summer months. The surface water resources in Xiongan New Area were insufficient to meet demand, and a large amount of groundwater was over-exploited for a long time, leading to a rapid decrease in groundwater levels. During the time period 1976–2016, deep groundwater levels in Anxin County showed a significant long-term downward trend (Fig. 5b). The groundwater depth in Anxin County decreased by 38%, from 7.8 to 10.2 m, during the time period 2006–2015, whereas the groundwater depth in Rongcheng County decreased from 9.2 to 22.5 m and that in Xiongxian County from 17.8 to 19.2 m.
Projection of future water resource
Temperatures in Daqing River Basin are projected to rise in the future, precipitation will increase slightly, and the number of extreme heavy precipitation events will increase. Extreme droughts will increase in northern and central areas, whereas the longest period of continual drought days will decrease in the southern area and the basins below Baiyang Lake. The SWAT model predicts that surface runoff in the Daqing River Basin will increase by 11% in the near future, with a larger increase in the northern area than in the south. Runoff is expected to increase mainly in autumn and winter by 80 and 45%, respectively, whereas runoff in spring will decrease by about 5% and the runoff in summer will be unchanged (Fig. 6).
High temperature and heat hazard
Changes in high temperature
The climatological annual average temperature, the average maximum temperature, and the average minimum temperature in Xiongan New Area are 12.6, 18.7, and 7.3 °C, respectively, lower than those in regions such as Beijing, Tianjin, Shijiazhuang, and Baoding. This may be related to the heat island effect of large cities and the regulating effect of Baiyang Lake on temperatures (Cui et al. 2015; Zheng et al. 2012). The extreme average annual maximum temperature in Xiongan New Area was 38.2 °C. During the time period 1961–2018, the annual average temperature, maximum temperature, and minimum temperature in Xiongan New Area showed an upward trend, with linear changes of 0.17, 0.20, and 0.21 °C/10a, respectively, significant at the 95% level (Fig.2e, f). There are, on average, 11.8 high-temperature days annually, an increase of 0.8 days/10a since 1961 (Fig.2 g). The average high temperature first appears on June 2 and lasts until July 22. In 2000, there were 29.3 average high-temperature days in Xiongan New Area, with the highest temperatures in Anxin and Rongcheng counties on July 1 reaching historical records of 41.0 and 41.2 °C, respectively.
Future heat risk
The heat disaster risk areas in the Beijing–Tianjin–Hebei region in the reference time period were mainly urban areas with high populations and a high GDP, such as Beijing, Tianjin, Baoding, and Shijiazhuang; 98% of the Xiongan New Area had a risk below grade II (Fig. 7a). In the near future (2026–2045), however, the ensemble annual average temperature of Xiongan New Area will increase by 1.01 °C and the daily maximum temperature, the daily minimum temperature, and the number of summer days will increase by 1.38 °C, 1.35 °C, and 9 days, respectively. There will be a heat risk zone of grade III along the Hai River Plain east of the Taihang Mountains by about 2035, and the areas with a heat risk of grade III and above will expand. The area with a heat risk of grade III will expand from 1.96 to 27.45%, and the area with a heat risk of grade V will expand to 15.59% (Fig. 7b).
Heavy haze
Xiongan New Area usually has an annual average of 5.1 haze days, significantly fewer than in Beijing, Tianjin, and Hebei (18.4 days). In 2017, the good air quality rate in the Beijing–Tianjin–Hebei Region was between 38.9 and 79.7%, whereas that in Xiongan New Area was 61.9%, and the concentration of PM2.5 was 68.1 μg/m3, better than that in Shijiazhuang and other areas of Baoding.
Located on the leeward side of the Taihang and Yanshan mountains, Xiongan New Area has an annual average wind speed of 1.63 m/s and a 28% frequency of annual breeze (< 1.0 m/s). The self-purification capacity of the atmosphere is calculated using the method of Zhu et al. (2018). Northern Shijiazhuang and Baoding have the lowest self-purification capacity in the Beijing–Tianjin–Hebei region (Fig. 8a). Xiongan New Area lies in the east of Baoding and the average self-purification capacity of the atmosphere was 1.63 tons/km2/day during the time period 2007–2017, lower than that of Beijing, Tianjin, and Shijiazhuang, which is not conductive the diffusion of air pollution and urban ventilation. As a result of climate change, the annual average wind speed and self-purification capacity of the atmosphere in Xiongan New Area significantly decreased at a rate of 0.22 m/s/10a (Fig. 2h) and 300 kg/km2/10a (Fig. 8b), respectively, during the time period 1961–2018. The frequency of heavy pollution in autumn and winter and these particular meteorological conditions has increased significantly in Baoding, where Xiongan New Area is located, since 1961 (Fig. 8c), with an increase of two events every decade on average to 21 events in 1 year. Heavy pollution in autumn and winter and these particular meteorological conditions occurred seven times in 2011–2012 and 2015–2016.