The Relationship between Flood Disaster and Rainfall in Xiangjiang River Basin
Bo Chen1,2, Weihua Fang1,2,Peijun Shi1,2, Fei he1,2, Zhiqiang Wang1,2, Hong Xu1,2 1. Key Laboratory of Environmental Change and Natural Disaster, Ministry of Education of China, Beijing Normal University, Beijing 100875, China; 2. Academy of Disaster Reduction and Emergency Management, Ministry of Civil Affairs & Ministry of Education, the People’s Republic of China, Beijing 100875, China [email protected]
Flood disaster is one of the most serious natural disasters in China, it has caused numerous deaths and injury, as well as tremendous economic losses, and restricted the local sustainable development sharply. Took the total rainfall and frequency of rainfall events beyond downfall (daily rainfall greater than 25mm) from April to August as indices, the relationship between the rainfall and flood disaster from year 1951 to 1998 in Xiangjiang River Basin was analyzed. Four conclusions were found, 1) an abrupt change of flood disaster degree was detected around year 1981 and since then it climbed up to a more serious state. According to its fluctuation character, the flood disaster degree from 1951 to 1998 can be divided to three phases, namely 1951-1961’s aggravating phase, 1965-1975’s mitigatory phase and 1975-1998’s aggravating phase. 2) The total rainfall and frequency of rainfall events beyond downfall from April to August in recent fifty years have no trend but with inter-decadal fluctuation, based on this their variation can be cut to four periods, namely 1951-1967’s decreasing period, 1967-1977’s increasing period, 1977-1992’s declining period and 1992-1998’s climbing up period. 3) Start from April 1th of every year, the total amount of rainfall beyond downfall every five days were calculated, and rainfall events beyond downfall were found to be centralized to May and June. 4) As the increasing of socio-economic wealth, flood disaster aggravates and adaptive strategies can mitigate flood disaster to some extent.
Keywords: flood disaster, rainfall, extreme rainfall events, socio-economic system, Xiangjiang River Basin
1. Introduction
Xiangjiang River Basin is an important economic belt along Jing-Guang railway and Jing-Zhu high way in the middle and lower reach of Yangtz River Basin, which mainly locates in Hunan Province of China (90.2% area within Hunan Province).It covers a total area of 94660 km2 with a length of 856 km. Several major cities including Changsha, Xiangtan, Zhuzhou, Hengyang, Yongzhou, Chenzhou and Loudi locate in Xiangjiang River Basin. Xiangjiang River Basin belongs to subtropical maritime monsoon climate with an annual average precipitation of 1500mm.The upper reach of Xiangjiang River Basin is high mountains with steep slope and the middle and lower reach are flat plain with well-grown rivers. Due to these, Xiangjiang River Basin is one of the most serious flood-prone areas in China. Xiangjiang River Basin suffers much from frequent floods. It was affected by flood every 1.7 years in recent fifty [1] years and hit by five extreme floods in 1954, 1976, 1994, Figure1 Sketch map of Xiangjiang River Basin in Hunan Province, China (with field investigate route) 1996 and 1998 respectively. And in July of 2007 the heavy rain brought by rainstorm Bilis resulted in about one billion US dollars loss in Hunan Province which mainly in Xiangjiang River Basin. From the perspective of disaster system, flood disaster results from the interaction between hazard (intense rainfall) and vulnerability of socio-economic system. This paper aims at analyzing the rainfall character of Xiangjiang River Basin in recent fifty years and discussing its relationship with the flood disaster in study area.
2. Data and Method
All the loss data were obtained from the previous study by Mao[1]. The loss of flood disaster was described by a synthetic flood index named flood disaster degree which was integrated from economic losses, deaths and injuries. Daily rainfall data of nine weather stations in Xiangjiang River Basin from year 1951 to 1998 were used for analysis and the nine weather stations are: Changsha, Pingjiang, Shuangfeng, Nanyue, Zhuzhou, Hengyang, Changning, Daoxian and Chenzhou. Information unilateralism of single station was avoid by calculating the average rainfall and frequency of rainfall events beyond downfall of the nine stations to get the spatial data of study area.
3. Flood Disaster Characters in Recent Fifty Years
Using Mann-Kendall[2,3] test, an abrupt change of flood disaster degree was detected around year 1981 with a confidence of 95% (Figure 2), and the flood disaster degree tends to jump to a more serious state since 1981.Also from its variation character, the process can be divided to three phases, namely 1951-1965’s aggravating phase,1965-1975’s mitigatory phase and 1975-1998’s aggravating phase(Figure 3).
Figure 2 Mann-Kendall test of flood disaster degree in Xiangjiang River Basin from 1951 to 1998 (Two horizontal dashed lines are confidence lines, solid curve is UF value and dashed curve is UB value)
4. Rainfall Characters in Recent Fifty Years
According to previous study, flood disaster mainly occurred between April and August in Xiangjiang River Basin. Based on analysis of the rainfall data, about 80% of the annual total rainfall was found to be between April and August, so only the rainfall characters from April to August were analyzed. No trends of the total rainfall and frequency of rainfall events beyond downfall from April to August in recent fifty years have been detected, but inter-decadal variation exist, based on this, their variation can be cut to four periods, namely 1951-1967’s decreasing period, 1967-1977’s increasing period, 1977-1992’s declining period and 1992-1998’s climbing up period(Figure 3). Variation of Flood Disaster Degree and Rainfall 12 1800 1600 10 Figure 3a 1400 8 1200 1000 6 800 4 600 Rainfall(mm) 400 2
Flood Disaster Degree 200 0 0 1951 1953 1955 1957 1959 1961 1963 1965 1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997
Year
Variation of Flood Disaster Degree and Frequency of Extreme Rainfall Events 12 18 16 10 Figure 3b 14 8 12 10 6 8 4 6
4 Rainfall Events 2 Flood Disaster Degree 2 Frequency of Extreme 0 0 1951 1953 1955 1957 1959 1961 1963 1965 1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997
Year
Figure 3 Variation of flood disaster degree and rainfall characters in Xiangjiang River Basin between 1951 and 1998(Blue curves describe rainfall character (Rainfall in Figure 3a and frequency of rainfall events beyond downfall in Figure 3b), red curves describe flood disaster severity and short upright lines are phase divisions)
Intense rainfall centralizes in space and time will result in flood disaster, start from April 1th of every year, the total amount of rainfall beyond downfall every five days were calculated, and rainfall events beyond downfall were found to be centralized to May and June(Figure 4).
The 8th five days The 11th five days The 14th five days The 18th five days
Figure 4 Variation of rainfall beyond downfall during the 8th, 11th, 14th and 18th five days in Xiangjiang River Basin from 1951-1998 (The curve is five-year moving average)
5. Conclusions and Discussion
Flood disaster in Xiangjiang River Basin has changed abruptly to a more serious situation around year 1981, and it has inter-decadal fluctuation. The total rainfall and frequency of rainfall events beyond downfall from April to August in recent fifty years have no trend but with inter-decadal variation. Based on this, the relationship between flood and rainfall characters in Xiangjiang River Basin can be cut to three phases, namely, 1) The total rainfall and frequency of rainfall events beyond downfall from April to August decreased with flood disaster aggravated from 1951-1965. During this period, the government paid much attention to the development of agriculture, Hunan, as the food basis of China, developed rapidly, and the vulnerability of the socio-economic system increased because of deforestation and socio-economic activities moving much more into flood-prone areas so although rainfall dropped flood disaster degree climbed up. 2) The total rainfall and frequency of rainfall events beyond downfall from April to August increased with flood severity mitigated from 1965-1975. During this period, on one hand the governmental policy call farmers to build thousands of reservoirs which helped a lot in mitigating flood disaster, on the other hand, the economy in study area held still due to the Cultural Revolution, so in despite of the total rainfall and frequency of rainfall events beyond downfall increased the flood disaster degree declined. 3) The total rainfall and frequency of rainfall events beyond downfall from April to August decreased first then increased with flood disaster aggravated from 1975-1998.During this period, Chinese economy developed greatly due to the policy of reform and opening up and population increased rapidly, as limited land resources people moved into areas with high flood risk which resulted in increasing of socio-economic vulnerability. Due to those above, flood disaster aggravated acutely. Flood disaster’s changing abruptly to a more serious situation around year 1981 also indicates that the increase of Flood disaster severity has a strong relationship with the development of social wealth.
References
[1] Dehua Mao,Jingbao Li et al. Study on the flood-waterlogging Disaster in Hunan Province [M]. Hunan Normal University Press, 2000. [2] Congbin Hu, Qiang Wang. Definition of Abrupt Climate Change and its detection method [J].Atmospheric Science，1992,16（1）,111-119. [3]Fengying Wei. Modern weather statistic and diagnose forecast technology [M].Weather press, 1999.