Jiao Meiyan1, Song Lianchun2, Wang Jun3, Ke Yiming4, Zhang Cunjie2, Zhou Tianjun5, Xu Ying2, Jiang Tong2, Zhu Changhan2, Chen Xianyan2, Gao Xuejie2, Tang Shihao6, Zhang Peiqun2
The China Three Gorges Project has generated extensive concerns both domestically and abroad. People have been arguing the pros and cons of building such a large-scale dam and debating its possible impacts on the local environment. There have been frequent reports of extreme events, including both droughts and floods, throughout the region since the Three Gorges Reservoir started to raise its water level.
At the same time, the Project has brought important social and economic benefits in flood control, power generation, shipping and water resources redistribution. Most remarkable is the huge amount of clean electric power that it generates, which helps to slow down the depletion of non-renewable natural resources. Therefore, in the current social context, a comprehensive assessment of the dam’s climate effects is paramount for ensuring its smooth operation and providing the public with a sound scientific analysis of whether the extreme events observed in the last few years can be attributed to the Project.
Is the climatic pattern of the reservoir area, the neighboring localities, and even the entire Yangtze River Basin, changing? Can a scientific, objective and quantitative climatic assessment be made to determine whether the severe weather events in recent years are linked to the project’s construction and operation? If so, then what is the magnitude of the project’s impact?
To answer these questions, the Beijing Climate Center of the China Meteorological Administration (CMA) initiated a systematic study of the climate effects of the reservoir with the participation of renowned scientists. The study yielded a range of objective, scientific conclusions on the climate effects of the project that can be use to develop climate services for informing future decision-making.
The Three Gorges Water Control Project includes a river-blocking dam, a water reservoir, power generators, navigation structures, and more. It is one of the largest water conservancy projects in the world, running across the canyons and mountains of central Hubei Province and the hilly areas in the eastern part of Sichuan. The Three Gorges Dam controls a drainage area of 1 million square kilometers – 55.6 per cent of the total drainage area of the Yangtze River Basin.
When the Dam’s water level is raised to the desired capacity of 175 m, the 632 km2 of land flooded in the Three Gorges Reservoir form a man-made lake 600 km long and 1-2 km wide, with a total area up to 1,084 km2. It is able to retain 39.3 billion m3 of water, with a flood control capacity up to 22.15 billion m3. The reservoir feeds the Three Gorges Hydropower Station, which has 34 generator units with an installed capacity of 22.5 million kw and an annual generating capacity of 88.2 billion kwh.
Assessing the climate effects
The CMA study included assessments of the climate effects of large water reservoirs or lakes in various regions of the world, including on Lake Volta, Ghana; Lake Nasser or the Aswan High Dam, Egypt; the Itaipu Dam on the border between Brazil and Paraguay; and the Longyang Gorge and Liujiaxia Gorge Dams in China. The results show that in the summer, the lake and water surface temperatures in the different localities are lower than over their surrounding land surfaces. As a result, the waters absorb heat, and the exchange of energy from the neighboring areas to the waters is enhanced. In the winter, the process is reversed, and the lakes and reservoirs become heat sources. This enhances the exchange of energy from the reservoirs to the neighboring areas.
An enhanced energy exchange between the reservoir and the neighboring areas could make the atmospheric structure unstable, which could result in a changed precipitation pattern and a changed geographical distribution of precipitation. However, these studies of the climate effects of large reservoirs have concluded that the impoundment of a large reservoir does not significantly impact the climate of a large region.
The study of the Reservoir’s climate effects would involve three scientific issues: microclimate, interaction between the reservoir and weather events at different scales and the Reservoir’s cumulative climate effects and associated risk assessment
Basic climatic features of the Reservoir Area
The Three Gorges Reservoir area has a predominantly subtropical monsoon climate. The area is warmer than the eastern China, though at the same latitude, due to the terrain effects of Mount Qinling. It has mild winters, hot summers, rainy seasons with high temperature, and moderate rainfall. The area has an annual mean temperature ranging from 17°C to 19°C with a seasonal temperature variation similar to that of the Yangtze River Basin, but a smaller annual and diurnal range. Warm winters are a unique climatic feature of the area.
The annual rainfall is 1000-1300mm, in a wet-dry-wet distribution pattern from the west to the east. Under the influence of monsoon, the climate presents a distinctive seasonal variation, with rainfall mainly in April-October, and most rainy days in May and October, in a dual-peak pattern that is distinctively different from the single peak pattern that prevails in other parts of the Yangtze River Basin. The area has a sophisticated spatial distribution of climate, especially vertically. The river valley is warm in the winter, while the hilly areas experience cool summers but cool, foggy, wet microclimate in winter.
Climate evolution in the Reservoir Area and the Yangtze River Basin
In the past 50 years, the Reservoir Area has registered an ascending trend for annual mean temperature, with the largest increase in the last 10 years, though the increase is significantly lower compared with the increase of the Yangtze River Basin. In both the Reservoir Area and the Yangtze River Basin, annual precipitation does vary significantly, but inter-decadal figures do show significant variation. The Reservoir Area saw limited annual rainfall variations from the 1960s to the 1990s, though with significantly reduced rainfall in the last 10 years. Meanwhile, the Yangtze River Basin became abnormally wet in the 1990s, but abnormally dry in other years. In the past 50 years or so, both the Reservoir Area and the Yangtze River Basin witnessed a reduced numbers of rainy days, of annual mean wind speed, and of relative humidity, though the former is on the slower side compared with the latter.
In the past 50 years, the Reservoir Area reported a drought and flood variation trend that is basically consistent with the lower and middle streams of the Yangtze River. The Reservoir Area saw significantly reduced annual incidences of consecutive rainy processes, consecutive rainy days, and total rainfall from consecutive rainy processes, but significantly increased numbers of drought days, implying a worsening trend for droughts and an unclear trend for floods. The entire Yangtze River Basin registered an insignificant increase of high temperature days, while the Reservoir Area had a significant increase of high temperature days. Both the Reservoir Area and Yangtze River Basin claimed significantly reduced annual numbers of thunderstorm and foggy days.
The Yangtze River Basin has seen an ascending trend of annual mean temperature over the past 100 years (1883-2011). However, the upper stream of the Yangtze River and the Three Gorges Reservoir Area reported a cooling trend. Both the Three Gorges Reservoir Area and the Yangtze River Basin have registered a significant warm-cold-warm variation pattern. However, the Reservoir Area’s warming trend slowed noticeably in the 1990s, compared with the other parts of the country. The Reservoir Area has a drought and flood variation trend that is similar to that of the Yangtze River Basin, though significantly correlated to the upper stream.
Main factors affecting the Reservoir Area’s climate
Both the Reservoir Area and the Yangtze River Basin are sensitive to climate change impacts. The climate variations in the two areas are not only affected by local geographical conditions, but are also by large climate systems. The most important factor is the variation of atmospheric circulation in Asia. Other external forcing factors, such as sea surface temperature and snow cover, also play a significant role. Earlier studies showed that there is an important correlation between the inter-decadal variation of major climatic factors and the precipitation variation of the Yangtze River Basin.
After 1987, the western Pacific subtropical high shifted significantly west and south. The southern Asia high also became noticeably stronger, with more snow cover on the highlands and abnormal wetness in the summer, especially in the lower and middle streams of the Yangtze River from the mid-1980s to the 1990s. Then the southern Asia high became abnormally weak from the 1990s, resulting in a significantly reduced snow cover on the highlands that still lasts. In the eastern part of China, rain bands started to make a northward decadal shift, noticeably reducing precipitation over the Yangtze River Basin as of 2000.
In addition, the East Asian summer monsoon saw a significantly weakened inter-decadal variation in the late 1970s. As a result, the Reservoir Area and the Yangtze River Basin experienced an inter-decadal increase of rainfall in the summer, though the Yangtze River Basin became abnormally dry after 2000, under the influence of steadily enhanced East Asian summer monsoon.
Sea surface temperature, another important external forcing factor, would also affect drought and flood variations mainly by changing the position and intensity of the western Pacific subtropical high.
Reservoir Area climate after impoundment
The Three Gorges Reservoir started to raise its water level from 69 m to 135 m in June 2003. The water level hit the mark of 156 m in October 2006. During 2008 – 2009, one-month trial runs at 172 meters were staged. When the water level was raised to the highest level, 175 m, on 25 October 2010, the Reservoir entered another trial run phase, during which the water level sat at 175 m in the non-flood season (October-April) then dropped to its lowest level, 145 m, before the onset of flood season (usually in June).
Variation of basic meteorological elements
After the impoundment (2004-2011), most parts of the Three Gorges Reservoir Area reported a temperature rise when compared before the operation (1996-2003), with an annual mean temperature around 17.9°C – 18.9°C, or a 0.2°C increase of temperature. The most significant temperature rise occurred in the center of the Reservoir Area, or Qianjiang (0.47°C) in the south of the Reservoir Area, followed by Laifeng (0.39°C). However, the temperature rise apparently failed to pass the 95 per cent statistical confidence. More specifically, the winter and spring months had a mean temperature rise from 0.3°C to 1.1°C, and the summer months from 0.1°C to 0.4°C. The Reservoir Area reported a large inter-annual fluctuation of both extreme maximum and minimum temperatures, though the two temperatures showed a general ascending trend of 0.3°C and 0.6°C, respectively.
The coastal sites along the Reservoir had a smaller temperature rise, compared with other sites in the Reservoir Area. The sites near the Three Gorges Dam area appear to experience a cooling trend. A comparison between the near and distant sites in the Reservoir Area shows that after the impoundment, the sites near the Reservoir Area reported some temperature changes due to the expanded water surface, along with warming winters and slightly cooling summers.
Analysis of MODIS satellite data indicates that the surface temperature near the Reservoir Area has sustained a descending trend since 2001, especially in the Dam area and the upper and middle streams. Taking into account the temperature rise trend across the Reservoir Area in the past 50 years, this is consistent with the temperature variation trends of the southwestern part of China and the Yangtze River Basin. Thus, one can conclude that the impoundment has not significantly impacted temperature variations in the Reservoir Area.
After the impoundment, most parts of the Reservoir Area claimed a declined annual precipitation, though to different extents, mostly between 3 per cent and 4 per cent. The eastern section of the Reservoir Area reported significantly declined precipitation, though there is no obvious change of precipitation distribution in the entire Reservoir Area. Analysis of TRMM satellite data shows that the southern section of Mount Qinling and areas near the Dam experienced a noticeable increase in rainfall, while the downstream area and the southeastern part of the Reservoir Area experience a decline in rainfall (Fig. 10). A wide area analysis suggests that the Reservoir Area is positioned at a transitional section that connects a relatively wet to a relatively dry south. In this context, the Reservoir Area’s precipitation variation bears the regional mark of climate change over a wider area.
After the impoundment, the Reservoir Area registered an annual mean relative humidity between 74 per cent to 78 per cent with a limited inter-annual variability, a 2.4 per cent decline. The decrease was 1 per cent to 3 per cent in the summer and less than 1 per cent in the winter. Absolute humidity increased slightly. The Reservoir Area registered a declined trend for annual mean wind speed at 0.4-2.1 m/s. The wind speeds measured at sites near or distant from the water body showed limited variation, implying that the Reservoir had little effect.
Changed occurrences of major climatic events
After the impoundment, the Reservoir Area registered a 32 per cent increase of high-temperature days and 21 per cent decrease of low temperature days, both passed the 95 per cent confidence check. However, the annual number of drought days reported was slightly reduced, though drought days increased slightly in the late summer. The rainstorm days also came diminished, but at the expense of an increased intensity. Consecutive rainy process and lightning weather tended to decline in occurrence. Hazy days started to attract attention for their increased occurrences.
Numerical modeling of climate effects
Regional climate modeling shows that the Reservoir would produce some impact on the neighboring areas’ climate, but not beyond a 20 km radius. The Reservoir would noticeably bring down the air temperature above the water surface, by 1°C in the winter, and 1.5°C in the summer, with only 0.1°C for the land surface close to the water. The water body’s evaporative cooling effect would cause the air to sink, and as a result there would be less precipitation. However, the decline in precipitation would be small in winter, a 1 to 2 per cent drop within a 10km radius, and higher in summer, a 10 per cent drop. That 10 per cent would weaken to 3 per cent at a 10km radius and to 1 per cent at a 20 km radius.
Numerical modeling results show that the Reservoir may affect temperature, humidity and wind variations on a local scale - within 10 km – but to what point is undetermined. Whether impacts could be regional (within 100 km) remains controversial.
Possible causes behind the extreme events
The special environment, unique climatic conditions and human activities around the Reservoir Area also affect the weather. In recent years, the Reservoir Area saw reduced incidences of foggy days due to climate warming but, unfortunately, ever-increasing human activity exacerbates the problem, so there are more hazy days. The extreme climatic events observed in the region are also closely associated with the changed status of atmospheric circulations in East Asia, Arctic sea ice, tropical sea surface temperature and the Qinghai-Tibet Plateau’s thermal anomaly.
The major droughts and floods that occurred are mainly the results of the abnormal variations of large-scale climatic factors that run across the country. There have been a range such events across the country, including severe rainstorms and floods occurred in Chongqing in the summer of 2007, low temperatures caused disasters in the southern part of China in early 2008, and exceptional consecutive rainy days – a 1 in 50 years events – occurred upstream of the Reservoir Area in the autumn of 2008.
The extensive droughts in the lower and middle streams of the Yangtze River in the spring of 2011 and the sudden shift between droughts and floods in the summer were caused by the abnormal pattern of the tropical Pacific and Indian Ocean sea surface temperatures, the Qinghai-Tibet Plateau snow cover, and the Eurasian atmospheric circulations.
Anticipating climate change
Further analysis of the possible future impacts of climate change on the Project revealed that:
• an increase in annual precipitation variability in the reservoir area may cause runoffs in the upper reaches to fluctuate, leading to greater variation in reservoir water levels, thus increasing instability and hindering safe operation;
• an increase in the frequency and intensity of extreme climate events may lead to exceptional floods, increasing the burden of flood control;
• an increase in the interannual variability of precipitation may result in more droughts, especially in the dry season, affecting the impoundment of the Three Gorges Reservoir and associated power generation, shipping and the water environment; and
• a temperature rise may aggravate the eutrophication of water bodies, thus increasing the vulnerability of natural ecosystems in the reservoir area.
To ease any negative impacts that climate change may have, an adaptation management system will be established and constantly improved at the micro-policy level to ensure the safe operation of the reservoir. At the macro-policy level, national and local government support will be secured for adaptation-related planning and policy-making. This will make climate change adaptation and disaster-risk management a part of the development planning for the reservoir area, allowing sustainable socioeconomic development.
Specific measures to be adopted could include:
• studying and defining the threshold values that may lead to the occurrence of meteorological disasters in the reservoir area;
• enhancing the weather observing network and constructing transport infrastructures in the locality;
• strengthening water pollution prevention and control activities in the reservoir area;
• optimizing the scheduling of the Three Gorges Project and strengthening coordination with the water conservancy projects in the upper stream;
• strengthening the development of innovative hydrologic forecasting techniques that can be applied to different forecast periods;
• optimizing the project’s drought resistance plan;
• taking positive industry adaptation measures; and
• strengthening the ecological environmental protection activities in the reservoir area.
Towards climate services
CMA’s study of climate in the reservoir area has led to a clearer understanding of the causes of the extreme events observed since impoundment and to the conclusion that the effects of the Project are minimal and within a 10 km radius. Further analysis of future impacts of climate change has revealed that there may be risks and/or benefits for the reservoir area. Climate services for the Three Gorges Water Control Project that build on CMA’s scientific study and the adaptation management system will be developed under China’s national framework for climate services in order to assist in decision-making to mitigate the risks and maximize the benefits.
2 National Climate Center, CMA, Beijing Climate Center
3 Changjiang Water Resources Commission, Ministry of Water Resources (MWR), China
6National Satellite Meteorological Center (NSMC), CMA
7The full text of this report, which includes an analysis of the “Future climate variation projection for the Reservoir and neighboring areas,” methodology and a complete list of reference is available in the online version of the Bulletin.