Correspondence/Rebuttal pubs.acs.org/est
Dramatic Inundation Changes of China’s Two Largest Freshwater Lakes: Natural Process or Influenced by the Three Gorges Dam? A Revisit
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Amazonas. The second is that water level cannot be used to derive lake inundation area even if they are highly correlated.6 This effect can be clearly visualized by comparing Figure 1B of SK and Figure 7 of Feng,3 where the interannual changes of maximum inundation in Feng3 are not mimicked by interannual changes of maximum water level in SK. On the other hand, at annual scales although lake inundation is highly correlated with precipitation (Figure 7 of Feng,3 R2 = 0.62), precipitation cannot explain all changes in lake inundation. Indeed, the mismatch between the declining inundation and relatively stable precipitation for the period of 2003−2009 (Figures 2a and 4a of Feng2) strongly suggests that there must be reasons other than precipitation that could explain the inundation declining trend. Note that once validated (Figure 2 of Feng et al. 2012b4 or Feng4), satellite-derived precipitation over large scales, as used in Feng,2,3 should be more statistically meaningful than limited number of rain gauges because rainfall can be highly heterogeneous in space. (3) SK commented that the explanation of the mismatch between field-measured runoffs and predicted runoffs in 2004 and 2005 “cannot stand up to scrutiny” because field-measured runoff “is not a direct indicator or the main factor of water balances of Poyang lake”. Then what could cause the mismatch as well as the significant decline in annual runoff coefficient after 2003? Note that similar to precipitation, runoff is also highly correlated with lake inundation as runoff is the major input of the lake, as shown by the strong correlation between runoff and precipitation (Figure 3 of Feng3). However, similar to precipitation, runoff cannot explain all lake inundation changes. Indeed, the purpose of Figure 3 in Feng4 is not just to show the interannual runoff changes but also to show the mismatch between predicted runoff and measured runoff as well as interannual changes in runoff coeff icients, which are all possibly tied to the TGD’s impoundment. SK also questioned the validity of the MODIS statistical results in Feng.2 Such validity has already been addressed in Feng3 where MODIS coverage frequency was detailed for the entire study period (Table 1 of Feng3). To our best knowledge, this is the only data set that can provide medium-resolution, long-term, and cloud-free data at such a frequency to account for seasonal variability in annual means. In contrast, it is unclear how the results of Figure 1A in SK were obtained, through either digitizing the figures in Feng2,3 or new calculations. Also note that the annual means shown in Feng1 are from hydrological years instead of calendar years.
nvironmental changes downstream Yangtze River have drawn consistent attention and sometimes debates in the past decades from both the research community and the public, particularly after the construction of the Three Gorges Dam (TGD). A recent example is raised by Song and Ke (SK),1 who argue that existing data are not sufficient to link the 2003−2009 area shrinkage of the two largest freshwater lakes of China (Poyang Lake and Dongting Lake) to the impoundment of the TGD in 2003, as proposed by Feng et al.2 (hereafter referred to as Feng2). Although we agree that in general it is extremely difficult to prove a causal effect in nature, we certainly disagree with the data interpretation and arguments by SK, thus standing by our original conclusions. Indeed, except for longer time series of water level (derived from satellite altimetry) and precipitation (derived from discrete rain gauges), most arguments by SK are already fully addressed in Feng2 or in a previous paper specifically focused on Poyang Lake’s inundation (Feng et al., 2012a3 or Feng3). Here we revisit these arguments and our original conclusions through addressing the three major comments of SK. (1) The “10-year MODIS data is not long-enough to derive reliable the so-called long-term trends of lake changes.” However, this is not the objective of Feng,2 which is to document changes “between 2000 and 2009” and “to investigate the potential linkage between changes of the above parameters and the TGD”. We further acknowledged that “Ideally, more data are required before 2000 to assess the pre-TGD conditions” but did not suggest or even imply the requirement of more data af ter 2009, as the potential TGD impact may be smeared by other factors after many years. Thus, adding another year of 2010 (or more years after 2010) is irrelevant here because 2010 is a wet year and both groups (Feng2,3 and SK) agreed that lake area is strongly influenced by precipitation. For the period of 2000 − 2009, even after excluding the abnormal wet year of 2002, the declining trend for both lakes is still statistically significant (R2 = 0.64 for Poyang Lake and R2 = 0.54 for Dongting Lake respectively, p < 0.05 for both). (2) SK suggested that the water-level data (derived from satellite altimetry) did not show significant declining trend for Dongting Lake (1993−2010) or Poyang Lake (2003−2010). However, the data cannot prove or disapprove whether there was not a declining trend in the lake area for two reasons. The first is the data accuracy. Satellite altimetry is well-known to contain large uncertainties for shallow waters. Unlike what is claimed in SK (i.e., uncertainties in “centimeter level”), the cited works4,5 did not show any quantitative uncertainty estimates. To the opposite, the limited validation results in Figure 3 of Berry et al.4 showed occasionally very large errors (several meters) over the © 2014 American Chemical Society
Published: January 15, 2014 2088
dx.doi.org/10.1021/es500042k | Environ. Sci. Technol. 2014, 48, 2088−2089
Environmental Science & Technology
Correspondence/Rebuttal
Therefore, given the above lines of evidence, we strongly stand by the original conclusions of Feng2 that the statistically significant declining trends in the lake inundation and the mismatch between precipitation-predicted runoff and measured runoff as well as the statistically significant decline in the runoff coefficient are all likely linked to the impoundment of the TGD. We do agree, however, that more data (from either validated satellite or field measurements) may be added to increase the observation frequency and temporal coverage to understand how these lakes respond to climate variability and human activities in the future.7
Lian Feng*,†,‡ Chuanmin Hu‡ Xiaoling Chen*,† †
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State Key Laboratory of Information Engineering in Surveying, Mapping and Remote Sensing, Wuhan University, Wuhan 430079, China ‡ College of Marine Science, University of South Florida, 140 Seventh Avenue South, St. Petersburg, Florida 33701, United States
AUTHOR INFORMATION
Corresponding Author
*E-mails:
[email protected] (L.F.);
[email protected] (X.C.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NO: 41331174, 41023001), the openfund projects of LIESMARS (Wuhan University) and the University of South Florida. We thank our colleagues (Song and Ke) for their thoughtful comments on this difficult environmental problem, and thank the editor for the opportunity to revisit the data, methods, and conclusions presented in our original manuscript.
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REFERENCES
(1) Song, C.; Ke, L., Recent dramatic variations of China’s two largest freshwater lakes: natural process or influenced by the Three Gorges Dam? Environ. Sci. Technol. 2014, DOI: 10.1021/es405500s. (2) 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. (3) 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 (0), 80−92. (4) Feng, L.; Hu, C. M.; Chen, X. L. Satellites capture the drought severity around China’s largest freshwater lake. IEEE J. Sel. Top. Appl. Earth Observ. Remote Sens. 2012, 5 (4), 1266−1271. (5) Berry, P.; Garlick, J.; Freeman, J.; Mathers, E., Global inland water monitoring from multi-mission altimetry. Geophys. Res. Lett. 2005, 32, (16). (6) Papa, F.; Legrésy, B.; Rémy, F. Use of the Topex−Poseidon dualfrequency radar altimeter overland surfaces. Remote Sens. Environ. 2003, 87, 136−147. (7) Easterling, D. R.; Wehner, M. F., Is the climate warming or cooling? Geophys. Res. Lett. 2009, 36, (8).
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