Trends of Extreme Flood Events in the Pearl River Basin During 1951–2010

ADVANCES IN CLIMATE CHANGE RESEARCH 4(2): 110–116, 2013 www.climatechange.cn DOI: 10.3724/SP.J.1248.2013.110

CHANGES IN CLIMATE SYSTEM

Trends of Extreme Flood Events in the Pearl River Basin during 1951–2010

WU Zhi-Yong1,2, LU Gui-Hua1,2, LIU Zhi-Yu3, WANG Jin-Xing3, XIAO Heng1

1Institute of Water Problems, Hohai University, Nanjing 210098, China 2National Engineering Research Center of Water Resources Eﬃcient Utilization and Engineering Safety, Hohai University, Nanjing 210098, China 3Water Resources Information Center of the Ministry of Water Resources, Beijing 100053, China

Abstract

The study investigated the trend of extreme flood events in the Pearl River basin during 1951–2010. Stream flow data at 23 gauging stations were used for the study. The Pearson type III distribution was selected for the flood frequency analysis. Results indicate that extreme flood events increase significantly in the Pearl River Basin since 1980. At the 23 gauging stations, there are 16 (70%) stations show positive (increasing) trends in 1981–2010. Most of the 16 stations are located along the West River and North River. While 7 (30%) stations show negative (decreasing) trends, and are found in the East River and the southeast region of the West River Basin. Keywords: flood; trend; climate change; Pearl River Basin Citation: Wu, Z.-Y., G.-H. Lu, Z.-Y. Liu, et al., 2013: Trends of extreme flood events in the Pearl River Basin during 1951–2010. Adv. Clim. Change Res., 4(2), doi: 10.3724/SP.J.1248.2013.110.

1 Introduction The effect of extreme hydro-meteorological events caused by climate change is a public issue. Recently, Global warming in recent decades is unequivo- the IPCC has released a special report [IPCC, 2012], cal, as is now evident from observations of increases which focused on the relationship between climate in global mean surface air temperatures, widespread change and extreme weather and climate events, the melting of snow and ice, and rising global sea level impacts of such events, and the strategies to manage [IPCC, 2007]. Observed warming over several decades the associated risks. This report provides a careful has been linked to changes in the large-scale hydro- assessment of the newest scientific, technical, and so- logical cycle [Bates et al., 2008]. In the last 20 years, cioeconomic findings in this field. As there are great floods are becoming more severe or more frequent in uncertainties in predicting precipitation changes in China (e.g., Yangtze River flood in 1991, Pearl River general circulation models, it is still difficult to rea- flood in 1994 and 1996, Haihe River flood in 1996, sonably evaluate the influence of climate change on Minjiang River flood in 1998, Hanjiang River flood in extreme precipitation and floods [Guo, 1995; IPCC, 2003 and 2005, Huaihe River flood in 2003, 2005 and 2012]. Thus, most of current researches focus on the 2007), which has led to losses of about 1.5% of the detection of trends, periods and change in river flow gross domestic product (GDP). series based on observed data [Kundzewicz et al., 2005;

Received: 28 January 2013 Corresponding author: WU Zhi-Yong, wuzhiyong−[email protected]

1 WU Zhi-Yong et al. / Trends of Extreme Flood Events in the Pearl River Basin during 1951–2010 111

Cunderlik and Ouarda, 2009; Xu et al., 2009]. How- in the past. Since 1990, extreme floods have succes- ever, only few studies on trends in flood magnitude sively occurred (in the whole basin in June 1994, the and frequency can be found [Collins, 2009]. There- Liujiang River in July 1996, and the West River in fore, this study is mainly to investigate the trends in June 1998 and June 2005) [She and Xie, 2007], which the magnitude of extreme flood events using long-term have a seriously negative impact on the regional social stream flow records in the Pearl River Basin. and economic development.

2 Data and methodology 2.2 Data

2.1 Study region Daily stream flow data at 23 gauge stations in 1951–2010 were obtained from the Information Center The Pearl River is the second largest river (in of the Ministry of Water Resources. Figure 1 shows terms of stream flow magnitude) in China with three the geographic distribution of the 23 stations in the major tributaries: West River, North River and East Pearl River Basin. Of all the stations, 16 are located River. The Pearl River Basin is located in the tropical along the West River, 3 along the North River, 2 along and sub-tropical climate zone with annual mean tem- the East River, and 2 are found in the eastern re- peratures ranging from 14◦C to 22◦C and annual mean gion of Guangdong province and the southern region of precipitation ranging from 800 mm to 2,500 mm. Pre- Guangxi autonomous region. There are 4 stations con- cipitation during April–September accounts for 70%– trolling a catchment area of more than 100,000 km2, 80% of the annual total amount. Influenced by the 13 stations covering catchment areas between 10,000 southwest monsoon, its geographic location and land- and 100,000 km2 and 6 stations with a catchment area form, flood disasters happened frequently in this basin less than 10,000 km2.

Figure 1 Geographic distribution of the 23 gauging stations in the Pearl River Basin

Meteorological data are obtained from the daily of 89 meteorological stations located in the Pearl River climate database with 752 basic meteorological sta- Basin and nearby to calculate the annual mean tem- tions nationwide from 1951 to 2010, collected from perature and precipitation during 1951–2010. Figure the China Surface Climate Daily Value Set on 2 shows that the annual mean temperature has an in- China Meteorological Data Sharing Service System creasing trend during 1951–2010 and becomes signif- (http://cdc.cma.gov.cn/index.jsp). We used the data icantly higher after the mid-1980s. For the annual 112 ADVANCES IN CLIMATE CHANGE RESEARCH precipitation, no obvious trends are apparent. How- 40 years covered in their study, which could increase ever, Wang et al. [2006] found that precipitation is the probability of ﬂood and drought disasters in the unevenly distributed within a year during the recent Pearl River Basin.

Figure 2 Annual mean temperature (a) and annual precipitation (b) and the linear trend in the Pearl River Basin during 1951–2010

2.3 Methodology least 10 days for catchments with areas between 3,000 km2 and 45,000 km2, at least 15 days for catchments According to the manual Standard of Hydrologi- with areas between 45,000 km2 and 100,000 km2, and cal Information and Hydrological Forecasting (GB/T at least 25 days for catchments greater than 100,000 22482-2008) officially issued by the Ministry of Water km2. Resources, extreme flood events are defined as at least The Pearson type III frequency distribution has 10-year flood events. This definition is taken for this been extensively used in China, specifically in the study. southern region [Li, 1984], and is recommended for The analysis of extreme flood events can be con- calculating the design flood. Moreover, it is easily to ducted using the annual-maximum (AM) approach be compared with related studies. Therefore, we se- and the peaks-over-threshold (POT) approach. Three lected this frequency distribution for flood frequency indices were calculated to describe the characteristics analysis. of floods using both approaches. The first index is the annual maximum daily mean stream flow (AM1), 3 Results i.e., only the maximum daily mean stream flow of each year is retained. We also selected the largest indepen- 3.1 Frequency of flood events dent flood event per year (POT1) and average three largest events per year (POT3) for 1951–2010, because The frequency of extreme flood events is shown no generally acknowledged method has been proposed in Figures 3 and 4. The results indicate that the flood to determine a value of threshold [Guo, 2005]. The events have significantly increased after the 1980s, in peaks in POT series were considered to be indepen- particular for the extreme flood events in the last two dent of each other if they were separated by a par- decades. For example, floods with return periods 20– ticular time interval. Based on the results of other 50 years have occurred 13 times during 1991–2000 and researches [Svensson et al., 2005; Petrow and Merz, more than 50-year-flood events have happened 5 times 2009] and inspection of the time series, the time in- during 2001–2010. These are the highest frequen- terval between peaks was set to at least 5 days for cies of the two return period categories during 1951– catchments with areas no more than 3,000 km2, at 2010. WU Zhi-Yong et al. / Trends of Extreme Flood Events in the Pearl River Basin during 1951–2010 113

Figure 3 Occurrence of ﬂoods at the 23 stations in the Pearl River Basin during 1951–2010

periods are shown from Figure 5 to Figure 7. Among the 23 analyzed stations, 16 (70%) stations show positive (increasing) trends and 7 (30%) stations show negative (decreasing) trends for the 50- year flood based on the AM1 time series. Most stations with positive trends are located along the West River and the North River. The highest increase of the 50-year flood with 53.1% is found for Yongwei station along the West River. The majority of stations with negative trends are found in the southwest part of the West River Basin and the East River Basin. Figure 4 Decadal occurrences of extreme flood events in Heyuan station shows the highest decrease of 35.2%. the Pearl River Basin during 1951–2010 The trends of the 10-year flood and the 20-year flood are similar to the 50-year flood, but the magnitudes 3.2 Trends in extreme flood events are relatively smaller.

The Pearl River Basin has experienced a warmer 4 Discussion climate since the mid-1980s (Fig. 2). In order to bet- ter understand changes in flood occurrences, the whole In general, the trends in extreme flood events in period (1951–2010) was divided into two sub-periods the downstream region are consistent with that in the (1951–1980 and 1981–2010). Geographic distributions upstream. However, the trend at Nanning station is of the relative changes of extreme flood events based opposite to the trend at Xinhe station, which is lo- on the 50-, 20-, and 10-year return period in the two cated in the Yong River. There are two main reasons 114 ADVANCES IN CLIMATE CHANGE RESEARCH

Figure 5 Geographic distributions of the relative changes Figure 6 Same as Figure 5, but for the 20-year flood (%) of the 50-year flood for (a) AM1 series, (b) POT1 series and (c) POT3 series during 1981–2010 compared with increase in catchment area. For instance, the differ- the period 1951–1980 ence is below 10% for the stations with a catchment area of more than 100,000 km2, such as Qianjiang, for the inconsistency. Firstly, the drainage area of Dahuangjiangkou, Wuzhou and Gaoyao station. Xinhe station (5,791 km2) is much less than that of It is easy to measure the influences of extreme Nanning station (72,656 km2). Secondly, heavy pre- precipitation on such extreme flood events, as flood cipitation often occurs outside the drainage area of is usually a rainstorm-based event in the Pearl River Xinhe station. Therefore, the flood events at Xinhe Basin. Therefore, we calculated the annual maximum station contribute less to the flood potential at Nan- consecutive 3-day precipitation. The results show that ning station. the change of the extreme precipitation is consistent Figure 8 shows the differences in the AM1, POT1 with that of the extreme flood events. For example, and POT3 series for 50-year flood events at the 23 there is an increase of 6.3% in the annual maximum stations. The smallest difference in these flood series consecutive 3-day precipitation and an increase of the is 2.7%, which is observed at Changle station, while 50-year flood event by 20.2% at Pingle station located the largest difference of 37.5% is found at Shuangjie in the West River Basin in 1981–2010 compared with station. In general, the difference decreases with an those in 1951–1980. In addition, hydraulic engineering WU Zhi-Yong et al. / Trends of Extreme Flood Events in the Pearl River Basin during 1951–2010 115

structures might have significant effects on the trends of extreme flood events. In the East River Basin, there are three major reservoirs, the Xinfeng River reservoir, the Fengshuba reservoir and the Baipenzhu reservoir, which control nearly half of the catchment area. Here, the annual maximum consecutive 3-day precipitation decreased by 5.1%, while the 50-year flood magnitude decreased significantly by 45.8%.

5 Conclusions

In general, the frequency of extreme flood events increased significantly in the Pearl River Basin since 1990. During the two sub-periods (1951–1980 and 1981–2010), results show that among the 23 analyzed stations, 16 (70%) show positive (increasing) trends. Most of the 16 stations are located along the West River and North River. Around 7 (30%) stations show negative (decreasing) trends. These are mostly found along the East River and the southeast region of the West River Basin. The trends in extreme flood events in the downstream sections are consistent with that in the upstream sections. For three flood indices, the difference in index values decreases with an increase in catchment area. However, the reasons for the changes in extreme flood events are only briefly discussed in this study, which need to be thoroughly analyzed in Figure 7 Same as Figure 5 but for the 10-year flood the future.

Figure 8 Relative changes (%) of the 50-year ﬂood for AM1, POT1 and POT3 series in the period 1981–2010 compared with 1951–1980 116 ADVANCES IN CLIMATE CHANGE RESEARCH

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