Changes in monthly precipitation and flood hazard in the Yangtze River Basin, China

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 28: (2008) Published online 12 December 2007 in Wiley InterScience ( Changes in monthly precipitation and flood hazard in the Yangtze River Basin, China Tong Jiang, a Zbigniew W. Kundzewicz b,c, * and Buda Su a a Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing, China b Potsdam Institute for Climate Impact Research, Potsdam, Germany c Research Centre of Agricultural and Forest Environment, Polish Academy of Sciences, Poznan, Poland ABSTRACT: A study of change of meteorological and hydrological variables (precipitation, intense precipitation, and river flow) in the Yangtze River Basin was conducted for particular calendar months. Significant changes were found in many monthly precipitation datasets collected between 1961 and The strongest relative increase was detected in January, when the monthly precipitation is low in absolute terms. Changes in the monthly precipitation in spring and summer, from April to August, some of which are statistically significant, are of direct importance to seasonal flood hazard. The significant precipitation rise detected in June, July, and August tends to aggravate the flood hazard. Results of change detection in time series of intense precipitation in the Yangtze River Basin indicate that more precipitation falls in intense events at the expense of moderate and weak events. Significant upward trends in the discharge of the River Yangtze in summer months in the middle and lower regions were also detected. Finally, precipitation in the Yangtze River Basin and discharge in the mainstream Yangtze (gauges Yichang and Datong) in summer 1998 have been examined, in order to interpret the background of the catastrophic flood which is considered to be the costliest flood ever, worldwide. Copyright 2007 Royal Meteorological Society KEY WORDS flood hazard; precipitation; river flow; temporal and spatial trend; Yangtze River Basin; Yangtze River; Mann Kendall test Received 6 July 2005; Revised 1 August 2007; Accepted 6 September Introduction The Yangtze River (Changjiang in Chinese) is the largest river in China (Figure 1). It is 6397 km long, extending more in latitudinal than in longitudinal direction, and its basin covers an area of 1.8 million km 2. In terms of mean discharge, it is the third largest river worldwide. Water resources of the Yangtze River Basin are of fundamental importance to the food security of the Chinese nation. However, the river flow and the water level of the Yangtze vary in a broad range. Frequent destructive abundance of water in the River Yangtze and its tributaries continue to play havoc on the riparian population of hundreds of millions of people. There have been numerous destructive floods in the Yangtze system throughout history, until the recent times. The instrumental observation records have been extended by rich information about the past floods based on historical documents and stone inscriptions. Several different classifications of flood severity have been introduced. As stated by Huang and Zhang (2004), during the time period of 2100 years (185 BC 1911 AD), * Correspondence to: Zbigniew W. Kundzewicz, Potsdam Institute for Climate Impact Research, Potsdam, Germany and Research Centre of Agricultural and Forest Environment, Polish Academy of Sciences, Poznan, Poland. zbyszek@pik-potsdam.de; zkundze@man.poznan.pl there were 214 heavy floods defined according to the information based on local chronicles (at the regional administrative level). The term heavy flood is used if many local chronicles directly indicated this flood and its impact extended over several regions. According to another classification (CWRC, 2002), there have been 31 very heavy floods (which impacted several provinces and lasted several months) on the Yangtze in the last 1000 years 1 each in the 12th, 13th, 16th, and 18th century; 8 in 19th century, and 19 in the 20th century. In the last 150 years, six extraordinarily heavy floods occurred in: 1860, 1870 (this was probably the largest flood on the Yangtze during the last 1000 years), 1931, 1935, 1954, and 1998, leading to high death toll, material damage, and social losses. In the 20th century, dramatic destruction was caused by floods in 1931 and During the 1931 flood, a large city of Wuhan was inundated for 100 days and about 2200 small boats were floating in the city providing transportation, while in the 1954 flood, there were about flood fatalities. The severity of disturbances caused by the flood can be illustrated by the fact that, in 1954, the railway line Beijing Guangzhou, of strategic importance, was not operating for 100 days (Hu and Luo, 1992; Luo, 1996). A particular intensification of material flood damage was observed since In 1998, flooding in China turned out to be the costliest ever flood event Copyright 2007 Royal Meteorological Society

2 1472 T. JIANG ET AL Figure 1. The Yangtze River Basin. Figure 2. Topography of the Yangtze River Basin. Location of the rain gauges and river gauges used in this study is indicated by white circles and black circles, respectively. worldwide, with a damage tag of about 30 billion US$ and the 1996 floods in China, causing material loss of about 26 billion US$ come second on the global list of all times. Even if there are powerful embankments along the River Yangtze and its many tributaries, they do not provide adequate protection of the riparians during large floods (cf Kundzewicz and Xia, 2004). There have been six large floods in the 1990s, namely, in: 1991, 1993, 1995, 1996, 1998, and The exceptional density of floods in the 1990s seems to indicate a possible increase in the frequency of large floods, consistent with the findings presented in IPCC (2007). Floods on the Yangtze may considerably differ in areal coverage, ranging from local, regional, up to basin-wide events. Floods are particularly dramatic if superposition of flood waves occurs: e.g. when high waves from the Dongting and Poyang lake basins meet with a flood wave on the mainstream Yangtze, and when the flood peaks from the upper and the middle/lower reaches coincide, causing a gigantic flood wave crest downstream. The flood hazard depends on a combination of anthropogenic and natural factors. The Yangtze River Basin is densely populated (over 400 million inhabitants) and largely urbanized. Owing to the growing population pressure, activities like deforestation, agricultural land expansion, urbanization, construction of roads, reclamation of wetlands and lakes, and strengthening of embankments of river have been progressing. This has reduced the available storage capacity in the basin, increased the value of the runoff coefficient, and aggravated the flood hazard. Owing to the complicated physical setting of the Yangtze River Basin, it can be divided into three distinct geographical regions characterized by the three morphological terraces, i.e. the Tibet plateau (where the source of the river is located), middle mountains, and the eastern lower plain, cf Figure 2. For the purpose of the present study, the Yangtze River

3 CHANGES IN PRECIPITATION AND FLOOD HAZARD IN THE YANGTZE RIVER BASIN 1473 Basin is conceptually divided into three regions, as follows: the upper region extending between the longitudes 90.6 and E, with a mean altitude of 3720 m a.s.l. (28 precipitation stations available), the middle region ( E, 1560 m a.s.l., 41 stations) and the lower region ( E, 400 m a.s.l. 78 stations). The river has multiple tributaries, including several large lakes, such as the Dongting and the Poyang. A large part of the Yangtze River Basin has a subtropical monsoon climate. The mean annual precipitation in the basin varies from 700 to 1700 mm. The spatial and temporal variations of precipitation are closely related to the activity of the summer monsoon that transports huge amounts of warm and humid air masses from the ocean to the Yangtze River Basin. In an average year, the summer monsoon starts to influence the basin in April and retreats in October. The precipitation is mostly concentrated in the summer season, from June to August, accounting for nearly half of the annual total. This is the time when abundant water may not be contained in the river channel, inundating riparian areas. Figure 3 shows the monthly area-averaged precipitation in three regions and the corresponding discharge in the Yangtze. It can be seen that the mean monthly precipitation over 100 mm occurs in all months from June to September in the upper region, from May to September in the middle region, and from April to September in the lower region. The objective of this paper is to study changes in long-time series of the observed precipitation in the Yangtze River Basin and the discharge of the Yangtze. 2. Data and methods The meteorological data used in this study originate from the National Climatic Centre of the Chinese Meteorological Administration (CMA). A set of 147 gauging stations with uninterrupted climatic records from 1 January 1961 to 31 December 2000 was selected. Location of stations is presented in Figure 2. Monthly discharge and annual maximum flow records collected from 1961 to 2000 at the following three main hydrological stations were examined: the Pingshan gauge in the upper region, Yichang in the middle and Datong in the lower region (CWRC, ). These stations terminate sub-catchment areas of about 0.485, 1.006, and million km 2, respectively, while the annual runoff at these stations is about 149, 440, and 890 billion m 3, respectively. The homogeneity of precipitation and river flow was checked using three different methods namely, von Neumann ratio, cumulative deviations, and Bayesian procedures (cf Buishand, 1982; Maniak, 1997). The datasets of 147 meteorological stations and three hydrological stations were found to be homogeneous at the 5% significance level. Figure 3. Monthly area-averaged precipitation and corresponding discharge of the River Yangtze at Pingshan, Yichang, and Datong stations ( ). The trend analysis methodology used in the present study includes the non-parametric Mann Kendall (MK) test (Kendall, 1975) and a linear regression method. For a general discussion of the applicable methodology, see Kundzewicz and Robson (2004) and Radziejewski and Kundzewicz (2004). The MK test is a rank-based procedure, applied for detecting trends in the records of monthly precipitation in the basin and of the records of the Yangtze at the Pingshan, Yichang, and Datong stations at 10, 5, and 1% significance levels. A linear trend approach is applied to analyse the tendency rate (trend slope) by way of the least squares fit at a 5% significance level. There have been several recent global (cf Kundzewicz et al., 2005; Svensson et al., 2005) and national studies (e.g. Lopez-Moreno et al., 2006; Niel et al., 2005; Pillco Zola and Bengtsson, 2006; Vincente-Serrano, 2006; Yang et al. 2005) of changes detected in long-time series of hydrological records. Detection of abrupt or gradual changes in river flow records is of considerable scientific interest and practical importance, e.g. for planning future water resources and flood preparedness (Kundzewicz, 2004).

4 1474 T. JIANG ET AL Figure 4. Number of stations showing statistically significant trend in monthly precipitation totals at confidence level of 95%. 3. Observed precipitation trends 3.1. Monthly precipitation trends The analysis indicates remarkable differences in the direction and strength of trends in precipitation for individual months (Figure 4, for the significance level of 5%). The highest number of trends was detected in January, when precipitation at 91 (out of 147) stations showed a statistically significant upward trend at the 10% significance level, 76 stations at the 5% significance level, and at 42 stations at the 1% significance level. However, the prevailing upward trend in precipitation of January is of little importance for flood risk, since in this winter month precipitation only accounts for less than 3% of the annual total and exerts little influence on river flows (CWRC, 2002). In March, June, and July 45 stations showed a significant upward trend at the 10% significance level, and this is much more than the number of stations with a significant downward trend. Other stations in these months showed mostly upward, albeit statistically insignificant changes. In contrast, in April, September, and December many stations indicated a significant downward trend (e.g. in April, 35 stations at the 10% significance level). The MK test for monthly areal precipitation over the whole basin reveals statistically significant upward trends in January, March, June, and July and downward trends in April, May, September, and December (Figure 5(a)). The trends in precipitation have clear regional features (cf Figure 5(b)). In the upper region, significant upward trends were found for four months (January, February, March, and July), while there was no significant downward trend detected for any month. In the middle region, statistically significant downward trends were found in April, May, September, October, and December while the two months with significant upward trends were June and July. In the lower region, significant upward trends were found in four months (January, March, June, and July), while for only one month (April) a downward trend was detected. Summarizing, in all three regions, a significant upward trend in July was detected, while in June a significant trend was found in two out of three regions (upper and lower regions). The upward trend of precipitation in June and July is of considerable importance for a flood hazard. Figure 5. Monthly areal precipitation trend of the Yangtze for ; (a) trend for the whole basin; (b) trend of different regions of the Yangtze. The dashed lines denote the threshold of 90, 95, and 99% confidence levels. Figure 6 illustrates spatial variability of trend in precipitation for the summer months (June, July, and August). The upward trend of precipitation in June dominates the east-central part of the middle region and the northern part of the lower region (Figure 6(a)). The result for July also indicates an upward trend in the middle and the lower regions, but the location shifts from the north of the basin to the south. An upward trend also occurs in the upper drainage basin at the Pingshan station (Figure 6(b)). In August, only a small area in the southeastern lower Yangtze River Basin experiences an upward trend (Figure 6(c)). The advance and retreat of monsoon determines the timing of the rainy season and the amount of precipitation in the Yangtze River Basin (Becker et al., 2003; Gemmer et al., 2004). The shifting of the significant upward precipitation trends from the north towards the southeast of the Yangtze River Basin between June and August (Figure 6(a) (c)) indicates the migration of the rain belt in the summer season (Qian et al., 2003). The variation in the summer precipitation can be explained by the weakening of the summer monsoon since the 1970s, resulting from the decrease in thermal contrast between the tropical ocean and the Asian continental region. Weakening of the summer monsoon prevents the major rain belt from migrating further north and keeps it stagnant over the southern Yangtze River Basin for a longer time (Qian et al., 2003). This may explain observed changes in the regional characteristics of summer precipitation: increase in the Yangtze River Basin and decrease in the north and northeast of China (Yang and Lau, 2004).

5 CHANGES IN PRECIPITATION AND FLOOD HAZARD IN THE YANGTZE RIVER BASIN 1475 (a) Significance <5% (+) <10% (+) >10% <10% ( ) <5% ( ) (b) Significance <5% (+) <10% (+) >10% <10% ( ) <5% ( ) (c) Significance <5% (+) <10% (+) >10% <10% ( ) <5% ( ) Figure 6. Interpolated Mann-Kendall trend of monthly precipitation in (a) June, (b) July, and (c) August for ; symbols (+) and( ) denote positive and negative trends, respectively. A regular feature is the occurrence of mei-yu (in Chinese: plum rain), during June July. The plum rain season, coincident with the period when the fruit matures, usually begins in mid-june and ends approximately 20 days later. Usually it is caused by a quasi-stationary rain belt covering the middle/lower Yangtze Basin, bringing in abundant precipitation. The onset and length of mei-yu largely drives the flood occurrence and is likely to change due to changes in monsoon circulation. There are indicators that in recent decades onset of the mei-yu period comes earlier and the duration of this period is longer (Becker et al., 2004) Rainstorm trends The guidelines of CMA distinguish several categories of point rainfall, according to the daily rainfall sums, namely light rain (with a daily sum of mm), moderate rain (10 25 mm d 1 ), heavy rain (25 50 mm d 1 ), and rainstorm (over 50 mm d 1 ). A useful index measures the summer rainstorm frequency defined as the number of rainstorm days, i.e. number of days with precipitation exceeding 50 mm (Gong et al., 2004). Yang et al. (2004) analysed the daily rainfall data of 109 stations, covering most of the Yangtze Basin except from headwater parts in the Qinghai Province, from 1960

6 1476 T. JIANG ET AL to At most stations, the rainstorm frequency and mean rainstorm volume (both annual and summer) had increased. Yet, in most stations the changes were not statistically significant. For example, while for 75 stations (out of 101), increase of frequency of rainstorm was observed, only 20 stations passed the 5% significance test. For 78 stations, where increase of frequency in rainstorm volume was observed, only 21 stations passed the 5% significance test. The number of stations where decreases were observed was 26 for frequency and 23 for volume, respectively, but only two in each of these groups showed a statistically significant trend. Linear regression for the time range of verifies the obvious trend of increase in both summer rainstorm, i.e. the sum of heavy precipitation (occurring in days with daily values in excess of 50 mm, cf Figure 7(a)) and the total summer precipitation (Figure 7(b)), with the slopes of the regression line being 1.32 and 2.43 mm a 1 for the summer rainstorm and summer precipitation, respectively. The linear regression test on the summer rainstorm frequency (the number of days with precipitation in excess of 50 mm, cf Figure 8(a)) and intensity (amount of intense daily rainfall, cf Figure 8(b)) further suggests that there is an evidence of increasing monotonic trend for rainstorm frequency, but not for rainstorm intensity. An increase in the number of rainstorm days between 1991 and 2000 and (Figure 9) prevailed over a large part of the lower Yangtze River Basin. The increase in the number of rainstorm days per year, in general, is within 5 days per decade, but in the southeastern part of the Yangtze River Basin, the increase is 10 days or more per decade. The frequent occurrence of rainstorm exacerbates the flood hazard. For the whole basin, the averaged summer precipitation total from 147 meteorological stations is composed of 16% of light rain, 27% of moderate rain, 27% of heavy rain, and 30% of rainstorm. The upward slope of linear trend for different rain types is 0.06, 0.34, 0.71, and 1.3 mm a 1 respectively during which means that the increase in summer rainstorm over the Yangtze River Basin (Figure 7(a)) is much more than the total summer precipitation (Figure 7(b)). An increasing part of annual precipitation falls in intense events, at the expense of moderate and weak events. However, increase in rainstorm intensity is not significant. The present findings update and corroborate those by Yang et al. (2004). One of the major impacts of global warming that draws increasing attention is the spatial and temporal redistribution of precipitation, which has become the driving force which induces flood and drought hazards (IPCC, 2007). Since water-holding capacity of the atmosphere has increased with temperature, the potential for intense precipitation has also increased. In the second half of the 20th century, an increase in intense precipitation (though neither uniform, nor ubiquitously significant) was reported in many regions of the mid and high latitudes of the Northern Hemisphere, based on analysis of records at observational stations (cf IPCC, 2007). Regional evidence was reported by Karl and Knight (1998), Brunetti et al. (2000), Zhai et al. (1999), Sen Roy and Balling (2004), and Yang and Lau (2004). 4. Observed runoff trends Summer rainfalls, and particularly intense precipitation spells, covering large areas of the Yangtze River Basin are the primary factors responsible for floods on the Yangtze (CWRC, 2002). The present work which looks into changes in particular months extends, updates, and corroborates the results of previous change detection studies revealing a significant upward trend of summer precipitation in the middle and lower Yangtze (Zhai et al., 1999; Becker et al., 2003; Gong and Ho, 2003; Gemmer et al., 2004; Su et al., 2004, 2005; and Zhang et al., 2005), while Chen et al. (2004) found a significant Figure 7. Linear trend of (a) summer rainstorm total (sum of precipitation in days with daily precipitation above 50 mm) and (b) summer precipitation total in the Yangtze River Basin for

7 CHANGES IN PRECIPITATION AND FLOOD HAZARD IN THE YANGTZE RIVER BASIN 1477 Figure 8. Linear trend of (a) summer rainstorm frequency and (b) summer precipitation intensity in the Yangtze River Basin for Figure 9. Rainstorm frequency change between and downward trend of evapotranspiration in the basin in the last 40 years. The coincidence of the upward trend in summer rainfall and the concurrent decrease of surface evaporation results in enhanced surface runoff and flood potential (Hu, 2004). The monthly Yangtze flow records from 1961 to 2000, examined by the MK test, show a statistically significant upward trend in February and July, and a downward trend in May, October, and November at the Yichang station (Figure 10). Upward trends prevail at the Datong station (January, February, March, July, and August), where downward trend is found only in one month (May) (Figure 10). The data also indicate that a significant upward trend at both gauging stations occurs in July, i.e. the month with the highest discharge in Yichang and Datong, during which numerous floods have taken place. Summer runoff plotted in Figure 10 shows no distinct trend at the Yichang station during the last 40 years (Figure 11(a)), but a clear upward trend is present at the Datong station where the rate of increment of summer runoff is m 3 per year (Figure 11(b)). Since the Yangtze River flow is generated mainly from rainfall, discharge trends from 1961 to 2000 at the Yichang and the Datong stations (Figure 10) are similar to those in Figure 10. Mann-Kendall analysis of the monthly discharge of the River Yangtze at the (a) Yichang station and (b) Datong station. The dashed lines denote 1, 5, and 10% significance levels. the corresponding areal precipitation for regions above these river gauges (Figure 5(b)). In the summer season, the increasing runoff trend is clearly visible at the Datong station (Figure 11(b)), but not at the Yichang station (Figure 11(a)). The annual maximum discharge at the Yichang station also fluctuates and does not exhibit an obvious monotonic trend for (Figure 12(a)). This corroborates

8 1478 T. JIANG ET AL Figure 11. The summer discharge of the Yangtze and its linear trend for at the stations (a) Yichang and (b) Datong. the findings of Xiong and Guo (2004) who analysed 120 years of data and found no statistically significant trend in annual maximum flow series at Yichang. However, the maximum discharge at the Datong station has been rising since the 1960s, reaching the highest value in the late 1990s ( m 3 s 1 in 1999). The average annual maximum discharge at the Datong station in 1990s was about m 3 s 1, which was higher than in the earlier three decades, (Figure 12(b)). Annual maximum discharge usually occurs in the period from July to October in the middle region and from May to September in the lower region. Typically, the annual maximum occurs in July, in 60 and 70% of cases for the middle and lower region, respectively. The annual maximum discharge of the lower part of the basin has been on the rise since 1960 (Figure 12). 5. Correlation between precipitation and runoff Intense, and/or long-lasting precipitation is the main flood-generating factor on the Yangtze. Hence, as expected, monthly areal precipitation and runoff (cf Figure 3(a)) at the Pingshan station are positively correlated for all months from April to October. However, the correlation is negative (at the 5% significance level) for all winter months between November and March (excluding December), when low-temperature effects are present. At the Yichang station, the Pearson correlation coefficient passes the 10% significance level throughout all months and even the 1% level for ten months. At the Datong station, monthly runoff is positively correlated with monthly precipitation (at the 5% significance level) for all months, except for September to November. The maximum value of the monthly runoff is delayed by one month compared to areal precipitation both at the Pingshan and the Datong stations. Highest precipitation in the upper region and discharge at Pingshan occur in July and August, respectively, while highest precipitation in the lower regions and discharge at Datong, in June and July, respectively. However, at the Yichang station, monthly runoff is concurrent with areal precipitation; the highest value of both variables is in July (Figure 3(b)). Correlation analysis illustrates that the monthly areal precipitation is in close relationship with the monthly runoff in most of the months. However, in the upper part of the Yangtze River Basin during winter months, the link between the monthly runoff and the monthly precipitation is expectedly different than in non-winter months, because snowfall cannot contribute directly to the river flow. In the lower region of the Yangtze River, there are time lags between precipitation and runoff, which can be attributed to human water management, influencing the runoff generation (cf Yin and Li, 2001; Wang, 2004). 6. Intense precipitation and flooding in the summer of 1998 In 1870, the highest Yangtze discharge ever was recorded in Yichang ( m 3 s 1 ) and also a 30-day discharge reached a record level of 165 billion m 3. However, the most destructive flood in terms of material damage was the summer 1998 flooding on the Yangtze and its tributaries. It is instructive to take a closer look at this flood event. The case of the 1998 flood illustrates the typical flood-generating mechanism, described in section 5. A case study of the 1998 flood (Figure 13) shows how rainstorms in late June and early July led to the formation of flood peaks in the initial flood stage and how the subsequent heavy rainfall aggravated the situation, causing a prolonged flood persisting for several months. In the summer of 1998, there were several occurrences of the stable rain belt over the Yangtze River Basin. (a) (b) Figure 12. The annual maximum discharge of the Yangtze and its linear trend for at the stations (a) Yichang and (b) Datong.

9 CHANGES IN PRECIPITATION AND FLOOD HAZARD IN THE YANGTZE RIVER BASIN 1479 Figure 13. Area-average daily precipitation and corresponding water level of the River Yangtze in the flood season of summer of 1998 at (a) Yichang and (b) Datong. The dashed line is the warning water level. As reported by Cheng and Chen (2004), the rain belt stayed over the Dongting and the Poyang lakes for 23 days uninterruptedly, and over 29 days over the upper Yangtze and the Hanjiang rivers. On several days, a 24- h precipitation exceeding 100 mm occurred over large areas, e.g , , and km 2 on 13 June, 22 July, and 21 July respectively, while the areas with precipitation in excess of 50 mm were much larger: km 2 on 13 June, and km 2 on 22 July. There have been 84 days of storm rain (defined by occurrence of precipitation in more than ten stations in excess of 50 mm in each of them) from June to August and 52 days of heavy rainstorm (with more than ten stations with daily rainfall over 100 mm). Daily precipitation in excess of 300 mm/d was noted in six days, i.e. three days in June and three in July. Figure 13 shows area-averaged daily precipitation and corresponding river stage of the Yangtze at Yichang (Figure 13(a)) and Datong (Figure 13(b)) from 1 June to 10 September Persistent rainstorms from 12 to 21 June in the lower Yangtze region (Figure 13(b)) and sporadic rainstorms in the middle region in the second half of June (Figure 13(a)) considerably raised the river base flow, so that further intense rainfall caused sharp discharge rise and massive inundations. There were eight flood peaks at the Yichang station during July and August. In the late August, as a consequence of decreasing precipitation, the flood peaks started to decline (Figure 13(a)). At the Datong station, water level remained higher than the warning water level (14.5 m) from the end of June to early September (Figure 13(b)), i.e. for more than 80 days (Cheng and Chen, 2004). During the 1998 floods, many record stages and discharges on main tributaries were exceeded, yet, alltime flow records in the mainstream Yangtze were not reached. Hence, the record material damage is mainly due to the increase in loss potential. 7. Concluding remarks The summer precipitation in the Yangtze River Basin shows an upward trend in the last 40 years, especially in June and July. The most distinct trend was detected in the lower Yangtze region. Increase of the summer

10 1480 T. JIANG ET AL precipitation between and is about 61 mm per decade on average for the whole basin andupto86mminthelowerregion. The MK test for precipitation trend from 1961 to 2000 reveals that the stations with significant upward trend at 10, 5, and 1% significance levels are concentrated in January, March, June, and July, while in April, September, and December, significant negative trends were detected for point as well as for areal values (Figures 4 and 5). Regional properties of changes in spatial precipitation trend are evident in the Yangtze River Basin (Figure 5(b)). The upward precipitation trend in July (Figure 6(b)) covers the whole basin and in June it prevails in the middle and the lower region (Figure 6(a)). The increase of monthly precipitation in June and July is of much importance for flood hazard (CWRC, 2002). Rainstorms (intense precipitation exceeding 50 mm d 1 ) contributed mostly to the increase of summer precipitation in the Yangtze, while the rainstorm frequency significantly increased in the 1990s. However, no obvious trend of rainstorm intensity has being found. Trying to identify the causes of the frequent and intense flooding in the 1990s could help in a better understanding of the change of flooding hazard in the Yangtze River Basin and in a better management of flood risk in the future. Influenced by the increasing summer precipitation and rainstorm trends, the summer runoff and flood discharge of the lower Yangtze region show an upward trend in the last 40 years. Since the low-lying and broad valley of the lower part of the Yangtze River Basin is densely populated, the region is very vulnerable to flooding and recent inundations have caused very high material damage. If the presently observed trend continues into the future, higher flood risk in the lower part of the basin can be expected. Acknowledgements Work leading to preparation of this paper has been financially supported by the Key Project of the Chinese Academy of Sciences (No. KZCX3-SW-331), National Natural Science Foundation of China (No ), and funds from the Nanjing Institute of Geography and Limnology, CAS (No. SS220007). It is also a contribution to the EU FP6 ENSEMBLES Project. The support of the German Science Foundation (DFG) towards a Sino-German Workshop in Nanjing in April 2003, which helped the authors to start collaboration, is gratefully acknowledged. The authors also thank the National Climate Center (NCC) in Beijing, China, and the Changjiang Water Resources Commission (CWRC), Ministry of Water Resources, in Wuhan, China, which have provided valuable climate and hydrological datasets. Discussions with Dr Stefan Becker and Dr Marco Gemmer are also gratefully acknowledged. 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