Correspondence/Rebuttal pubs.acs.org/est
Recent Dramatic Variations of China’s Two Largest Freshwater Lakes: Natural Process or Influenced by the Three Gorges Dam?
F
eng et al.1 (hereafter referred to as Feng1) report that the dramatic decreasing trends (3.3 and 3.6%, p < 0.05) in the inundation area of two largest freshwater lakes in China (Poyang Lake and Dongting Lake) during 2000−2009 are highly attributed to the impoundment of the Three Gorges Dam (TGD). The lake inundation area was retrieved from time-series cloud-free MODIS images by using the method proposed by Feng et al.2 As pointed out in Feng,1 the coverage of employed 10-year MODIS data is not long enough to derive reliable the so-called long-term trends of lake changes. Particularly, the variable “inundation area” of the two lakes shows considerable interseasonal and interannual variability (shown in Figure.1A and B and also Figure 2 of Feng1), thus the estimated linear trends based on limited records are highly sensitive to outliers, the starting and ending values. For example, as shown in Figure.1A (lake area derived from results by Feng et al.2), the decreasing trend of Poyang Lake area is statistically
significant during 2000−2009; however the trend becomes statistically insignificant when just extending the period to 2010 (−1.9%/yr, p > 0.1). In essential the decreasing trend over the study decade is amplified by the extremely large area in 2002. Furthermore, the contrast of lake areas before and after the dam’s impoundment relies greatly on the representation of lake situations during the short period of 2000−2002, considering the dramatically large inundation areas in 2002. Long-term water level changes of the two lakes based on time-series satellite radar altimetry data (Figure.1B) show that Dongting Lake is at a state of peak in 2002. The water level data are provided by Laboratoired’ Etudes en Géophysique et Océanographie Spatiales (LEGOS),3 and the accuracy has been demonstrated in previous studies,4,5 with a root-mean-square error of centimeter level. The water-level changes also agree well with area changes shown in Feng.1 The time series of water-level data show there is no significant declining trend in water level of Dongting Lake
Figure 1. (A) annual (curve) and seasonal (bars) averaged area of Poyang Lake over 2000−2010 (hydrological year); the green, red, and blue lines indicate the linear trends of Poyang Lake area during 2000−2010, 2000−2009, and 2000−2010 (without 2002), respectively; (B) time-series water level of Dongting Lake and Poyang Lake derived from satellite radar altimetry; (C) annual (the solid curve) and seasonal (wet season: the dashed curve, dry season: the dotted curve) gauge-based precipitation (unit: mm) from 1992 to 2010 by averaging observations of 22 and 14 weather stations within Dongting Lake basin and Poyang Lake basin, and the smooth lines indicate precipitation trends; (D) Correlation analysis on precipitation and area/water-level: annual precipitation (P) and annual averaged lake area(S) (red square), P and averaged S in wet season (green circle), P and averaged S in dry season (pink triangle), annual P and annual averaged lake level(H) (green crossing). Published: January 15, 2014 © 2014 American Chemical Society
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dx.doi.org/10.1021/es405500s | Environ. Sci. Technol. 2014, 48, 2086−2087
Environmental Science & Technology
Correspondence/Rebuttal
(3) Crétaux, J. F.; Jelinski, W.; Calmant, S.; Kouraev, A.; Vuglinski, V.; Bergé-Nguyen, M.; Gennero, M. C.; Nino, F.; Abarca Del Rio, R.; Cazenave, A.; Maisongrande, P. SOLS: A lake database to monitor in the near real time water level and storage variations from remote sensing data. Adv. Space Res. 2011, 47 (9), 1497−1507. (4) Berry, P.; Garlick, J.; Freeman, J.; Mathers, E., Global inland water monitoring from multi-mission altimetry. Geophys. Res. Lett. 2005, 32, (16). (5) Papa, F.; Legrésy, B.; Rémy, F. Use of the Topex−Poseidon dualfrequency radar altimeter over land surfaces. Remote Sens. Environ. 2003, 87 (2), 136−147.
(1993−2010) and Poyang Lake (2003−2010). Further analysis reveal that the above-normal water levels match well with extreme wet years (Figure.1C), such as 1997−1999 (Dongting Lake), 2002 and 2010 (Dongting Lake and Poyang Lake), while lake levels are below-normal in the dry years of 1994, 2000− 2001, and 2004−2008. At annual scale, correlation analyses suggests that changes in water level of the two lakes are strongly related to precipitation during 1993−2010 (r > 0.7, p < 0.01) (Figure.1D), and the correlations are even stronger (r > 0.75, p < 0.01) at seasonal (wet/dry) scale. The most four smallest areas during 2000−2010 for Poyang Lake occurred in the dry seasons of 2003, 2006, 2007, and 2010, which agree well with low precipitation in these seasons. These evidence suggest that the lake level changes are tightly associated with precipitation variability and the lake shrinkage during 2004−2008 can be largely related to climatological droughts. One of the critical evidence supporting Feng1’s conclusion is the large mismatch between predicted and field-measured runoffs (deemed as the main input of lake water budget) of Poyang Lake in 2004 and 2005, which is explained by groundwater discharges induced by TGD’s impoundment. However, it cannot stand up to scrutiny, even assuming the employed data are accurate enough. According to Figure 3 in Feng,1 the most serious shrinkage of Poyang Lake should not have occurred in 2006 when the measured runoff was very high, suggesting that the field-measured runoff defined by Feng1 is not a direct indicator or the main factor of water balances of Poyang lake. It is no doubt that the impoundment of TGD, especially in 2003 and 2006, may have impacted on the balance of downstream aquatic and ecological environment. However, in this region with abundant precipitation which shows considerable interannul variability, several critical issues, such as how to differentiate effects of climatological droughts and increased lake-Yangtze discharges on the shrinking stage over 2004−2008, whether there are significant increases in lake− river discharges due to the dam’s operation, and whether the dam altered the downstream flow regimes only in certain months (discharge and impoundment stages), still remain unclear. Therefore, impacts of the TGD project on inundation variations of the two lakes should be examined quantitatively at longer and finer (e.g., monthly and daily) scales in future.
Chunqiao Song† Linghong Ke‡,* †
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Department of Geography and Resource Management, The Chinese University of Hong Kong, 999077, Hong Kong ‡ Department of Land Surveying and Geo-Informatics, The Hong Kong Polytechnic University, 999077, Hong Kong
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Feng, L.; Hu, C.; Chen, X.; Zhao, X. Dramatic inundation changes of China’s two largest freshwater lakes linked to the Three Gorges Dam. Environ. Sci. Technol. 2013, 47 (17), 9628−9634. (2) Feng, L.; Hu, C.; Chen, X.; Cai, X.; Tian, L.; Gan, W. Assessment of inundation changes of Poyang Lake using MODIS observations between 2000 and 2010. Remote Sens. Environ. 2012, 121, 80−92. 2087
dx.doi.org/10.1021/es405500s | Environ. Sci. Technol. 2014, 48, 2086−2087
