National Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences (CAS), Beijing 100029, China
Graduate University of the Chinese Academy of Sciences, Beijing 100049, China
National Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences (CAS), Beijing 100029, China.
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The Three Gorges Dam (TGD) is a hydroelectric river dam that spans the Yangtze River in Yichang, Hubei Province, China. It is the largest hydroelectric power station in the world. After its construction, a huge reservoir was created with an average width of 1.1 km (0.7 miles) and length of 600 km (370 miles). Such a huge reservoir has received increasing attention because of the effects of climatic fluctuations and the land use changes in the reservoir area. Previous studies (Zhang et al., 2004; Mao et al., 2005) suggest that the TGD reservoir area will alter local scale (∼km) meteorological conditions such as patterns of wind and temperature. However, the local climatic impacts have not been systematically quantified because of the change in surface area. Recently, a paper by Wu et al. (2006) (hereinafter referred to as WU2006) demonstrated an effect of the filling of the TGD reservoir. WU2006 examined the effect of the TGD on the regional precipitation around the vicinity of the TGD by analysing the Tropical Rainfall Measuring Mission (TRMM) rain rate data from the past 9 years (January 1998 to January 2006), and the authors concluded that after the TGD filling, precipitation had increased in the region between the Daba and Qinling Mountains (Figure 1 shows the geographical features in the TGD region) and had decreased in the vicinity of the TGD. Their study considered the climatic effects of the TGD at the regional scale (∼100 km) rather than at the local scale (∼10 km). Their results neglected the decadal variation of precipitation over the region because of the limited temporal coverage of the dataset, leading to an arguable conclusion. The analysis of local climatic variation with a limited dataset must be done carefully when trying to differentiate anthropogenic influences from natural factors because the dataset may not truly represent the actual characteristics of the climate for the area examined. Here, using daily rain-gauge data with a longer time series, from 1960 to 2005, we conclude that the variation in precipitation around the TGD shown by WU2006 is part of the natural interannual oscillation of precipitation.
2. Dataset and methods
To study the temporal and spatial variability of precipitation in the vicinity of the TGD (28–34°N, 106–112°E), daily rainfall data from 27 rain-gauge records in the TGD region from 1960 to 2005 were analysed. The daily rain-gauge data were obtained from the national climatic reference network and national weather surface network of China. The daily precipitation was recorded by manual observations at 6-h intervals. The rain-gauge data were collected and quality-controlled by the National Meteorological Information Center (NMIC) of the China Meteorological Administration (CMA), and they were available on the China Meteorological Data Sharing Service System (http://cdc.cma.gov.cn/). The error of the rain-gauge data is less than 0.1 mm. The error of the dataset would not affect the analyses in this study. To effectively understand precipitation variations in the mid-west region of China that includes the TGD vicinity, the method of the empirical orthogonal function (EOF) is used to obtain the temporal and spatial variations of precipitation over that region.
3. Natural variability of precipitation around the TGD
The temporal and spatial variations of mean annual precipitation in the mid-west region of China that includes the TGD vicinity are analysed by the EOF method. Figure 2 shows the first and second leading modes of the EOF, including their spatial patterns. The results indicate that the first EOF mode (Figure 2(a)) accounts for 42.6% of the total variance, which represents the same amount of precipitation variation between the northern and the southern areas of the Yangtze River. The second EOF mode (Figure 2(b)) explains 18% of the total variance. The spatial pattern of the second EOF mode indicates that the trend in the precipitation variation is opposite for the northern and the southern areas of the Yangtze River, which reveals that precipitation increased in the northern area and decreased in the southern area. This mode of change, which coincides with the pattern plotted in Figure 1 of WU2006, may be due to the natural variation in precipitation patterns, rather than the effect of TGD mentioned in WU2006. From Figure 2(c) and (d), it can be seen that the time-series curves of both the first and the second modes fluctuate normally after 2003. These results also indicate that the TGD has not had a remarkable influence on regional precipitation. Furthermore, the 9-year moving smoothed time series of the second mode (crossed line in Figure 2(d)) clearly shows two transformations in the spatial mode, one is from the end of the 1970s to the 1980s and another from the end of the 1990s to the 2000s. The above EOF analysis validates the conclusion that the natural precipitation change as shown in Figure 1 of WU2006 should have occurred at least once before the construction of the TGD, such as in the 1970s to 1980s.
To investigate this further, the difference in the mean annual precipitation between 1981–1984 and 1977–1980 was calculated. This difference is plotted for the vicinity of the TGD in Figure 3. It clearly shows that precipitation increased on the northern side of the Yangtze River and decreased on the southern side of Yangtze River, i.e. a ‘northern flood and southern drought’ as mentioned in WU2006. This pattern in Figure 3 is very similar to that of Figure 1 in WU2006, whether the change in the spatial pattern of precipitation or the change in precipitation intensity is examined. When we investigated the precipitation difference across a latitudinal distribution (averaged from 109°E to 111°E) for the two periods, the same phenomena of ‘northern flood’ and ‘southern drought’ was found as in WU2006 (the green line in Figure 4 of WU2006). Therefore, the correlation between the increased precipitation in the region between the Daba and Qinling Mountains and the decreased precipitation in the vicinity of the TGD with the filling of the TGD as inferred in WU2006 is unverified.
Figure 2 of WU2006 displayed the time series of the difference in the rate of TRMM rainfall (from 1998 to 2005) between the area north of the Yangtze River (31–34°N, 107–111°E) and the whole TGD area (28–34°N, 107–111°E). In this paper, we plotted this precipitation feature with a longer time series (from 1960 to 2005) based on the rain-gauge data (Figure 4). Figure 4 shows the variation in the mean monthly (black line) and mean annual (dashed line) precipitation for this time period for the following: (a) the whole TGD region (28–34°N, 107–111°E), (b) the region north of Yangtze River (31–34°N, 107–111°E) and (c) the difference between the two regions (the region north of Yangtze River minus the whole TGD region). This figure does not show clearly the abnormal monthly and annual mean precipitation values that occur in these two regions after 2003 when the TGD water level rose to 135 m (Figure 4(a), Figure 4(b)). When we investigated the precipitation difference between the northern region and the whole region (Figure 4(c)), the difference in the mean annual precipitation between the two regions is basically between − 1 and 0 mm/d, which means that a change after 2003 is not evident in the vicinity of the TGD. The same result was found for the difference in the mean monthly precipitation between the two areas (between − 2 and 1 mm/d). The fluctuation in the precipitation curve after 2003 is of the ordinary course of nature. So, the conclusion based on Figure 2 of WU2006 that those authors reach is not reasonable.
4. Discussion and conclusions
In this paper, the variation in precipitation around the vicinity of the TGD was investigated by using rain-gauge records from 1960 to 2005 for the mid-west region of China. The results indicate the dipole pattern of precipitation variation around the TGD as mentioned in WU2006 is just the performance of the second EOF mode. This type of dipole pattern also appeared in the region during the period from 1977 to 1984. The conclusion that the TGD affects regional precipitation obtained by WU2006 is unverified because of a limited temporal coverage of that dataset.
Recently, the climatic effect of the TGD has been studied by using numerical models (Miller et al., 2005; Wu et al., 2006). However, the simulation ability of numerical model is limited in the vicinity of the TGD due to the complicated topography in the region. Actually, Wang et al. (2005) have pointed out the unsuccessful simulations of the rainfall variability in the Asian-Pacific summer monsoon region includes the mid-west region of China. In fact, the numerical results in WU2006 suggest that the effect of the TGD alters the mean monthly precipitation, but the simulation by Miller et al. (2005) produced the opposite conclusion that there is no significant change in precipitation after the filling of the TGD. Determining how to find a suitable regional climate model that performs well in the TGD region and its use for the study of the impact of the TGD on its regional climate should be the subject of further research.
The authors would like to thank Dr. Jian Li, Dr. Hongbo Shi for useful discussion and two anonymous reviewers for helpful comments. This work is supported by the National Natural Science Foundation of China under Grant No. 40705025 and 40806007.