Dramatic Inundation Changes of China's Two Largest Freshwater

Aug 7, 2013 - Responses of landscape pattern of China's two largest freshwater lakes to early dry season after the impoundment of Three-Gorges Dam...
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Dramatic Inundation Changes of China’s Two Largest Freshwater Lakes Linked to the Three Gorges Dam Lian Feng,*,†,‡ Chuanmin Hu,‡ Xiaoling Chen,*,†,§ and Xi Zhao∥ †

State Key Laboratory of Information Engineering in Surveying, Mapping, and Remote Sensing, Wuhan University, Wuhan 430079, People’s Republic of China ‡ College of Marine Science, University of South Florida, 140 Seventh Avenue South, St. Petersburg, Florida 33701, United States § Key Laboratory of Poyang Lake Wetland and Watershed Research, Ministry of Education, Jiangxi Normal University, Nanchang 330022, People’s Republic of China ∥ Chinese Antarctic Center of Surveying and Mapping, Wuhan University, Wuhan 430079, People’s Republic of China ABSTRACT: Ever since its planning in the 1990s, the Three Gorges Dam (TGD) caused endless debate in China on its potential impacts on the environment and humans. However, to date, synoptic assessment of environmental changes and their potential linkage with the TGD is still lacking. Here, we combine remote sensing, meteorological, and hydrological observations to investigate the potential influence of the TGD on the downstream freshwater lakes. A 10 year Moderate Resolution Imaging Spectroradiometer (MODIS) time series from 2000 to 2009 revealed significantly decreasing trends (3.3 and 3.6%/year) in the inundation areas of the two largest freshwater lakes of China (Poyang Lake and Dongting Lake) downstream of the TGD since its impoundment in 2003, after which both relative humidity and surface runoff coefficient of the lakes’ drainages also dropped dramatically. These environmental changes appear to be linked to the TGD.

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areas reaching >3000 and >2200 km2, respectively, are critical in providing local water supplies and modulating dry/wet conditions. Feng et al.7 showed that the drought was directly linked to local precipitation. Then, a natural question would be whether the extreme weather was also related to the TGD. Longterm and synoptic assessment of the lakes’ environmental changes (e.g., wetland coverage, lake inundation, and dry/wet conditions in the lakes’ drainage area) based on scientific evidence will help us understand the potential linkage between these changes and the TGD. For example, several studies have documented the influence of the TGD on the downstream environments, such as decreased sediment discharge,8,9 elevated salinity in the Yangtze River estuary,10 and reduced primary productivity in the East China Sea.11 Researchers also tried to model the climate sensitivity to land use variations associated with the TGR.12,13 Recently, on the basis of a hydrological model, Zhang et al.14 demonstrated the potential effect of the dam operation to the water level of Poyang Lake. However, to date, long-term assessment of the lakes’ inundation and their regional weather conditions are still lacking, let alone the potential linkage between the environmental changes and the TGD.

INTRODUCTION Located on the Yangtze River of China and impounded in June 2003, the Three Gorges Dam (TGD, 30° 49′ N and 111° 0′ E) is the world’s largest hydroelectric dam (see location in Figure 1). Regardless of the initial motivation on flood control and power generation as well as the controversy on the resettlement of 1.3 million people and enormous cost ($26 billion),1 its construction has faced numerous criticisms on the potential environmental impacts ever since the planning phase. The associated issues, including ecological and biodiversity impacts, have received considerable concerns worldwide.1−5 It is believed that the terrestrial and aquatic environments in the Three Gorges Reservoir (TGR) region, downstream of the Yangtze River and in the East China Sea, could be disturbed, altering the ecosystems pertaining to wildlife and posing threats to the well-being of humans.1,5,6 Recently, the government authorities have admitted its potential impacts to the environment of the Yangtze River basin, yet most claims or statements have been based on pure speculations or arguments without any scientific evidence. Indeed, ever since the planning stage of the TGD in the 1990s, the debate on whether it should be constructed and how humans and the environment may be affected has remained. In recent years, the Yangtze River basin, especially the Poyang and Dongting Lake regions (see locations in the Abstract graphic and Figure 1) immediately downstream of the TGD, has experienced the most severe droughts in decades.7 The two largest freshwater lakes of China, with their maximum inundation © 2013 American Chemical Society

Received: Revised: Accepted: Published: 9628

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Figure 1. (a) Locations of Poyang Lake and Dongting Lake (small blue polygons), with their drainage basins shaded in yellow and orange, respectively. Annotated on the map are the Yangtze River (blue line) and the location of the TGD. (b) Locations of the hydrological stations (solid triangles) in the Poyang drainage basin, where daily runoff data were collected. (c) The 2000−2010 mean climatological monthly precipitation (TRMM data in mm/ month) and standard deviation of the lakes’ drainage basins.

Table 1. Temporal Distribution of Cloud-Free MODIS Images Used in This Study for Poyang Lake (Bold) and Dongting Lake 2000 Jan Feb March April May June July Aug Sept Oct Nov Dec

1 2 2 4 1 3 2 5 2

2001

1 2 3 3 1 3 2 6 4

2 1 2 1 3 1 5 1 5 4 7 2

2002 2 3 1 1 4 3 3 2 9 3 6 2

5 1 1 2 2 1 9 3 5 11 5 1

2003 2 3 2 1 2 2 10 11 7 10 6 2

6 4 3 3 4 1 6 4 7 14 9 7

2004 5 3 7 3 3 3 17 7 7 6 9 3

2 5 5 5 3 2 4 3 7 12 10 12

2005 2 4 5 4 5 3 7 6 5 7 4 9

2 1 3 7 2 6 2 1 8 10 5 12

2006 3 2 3 7 1 3 4 2 7 8 3 8

6 1 2 3 3 1 2 3 9 4 6 10

2007 5 0 5 2 5 2 3 2 4 1 4 8

8 8 3 3 6 1 2 3 7 8 13 3

2008 7 3 2 3 4 1 3 4 8 3 10 5

6 6 6 4 5 1 2 2 5 4 15 9

2009 3 3 4 2 4 2 2 1 3 6 9 8

9 4 4 8 9 1 4 5 5 11 10 4

2010 5 2 6 5 6 8 3 3 3 7 3 2

3 3 5

3 4 2

exchanges water with the Yangtze River at Hukou in the north. The main water supplies of the lake are from precipitation and local rivers (Ganjiang River, Fuhe River, Xiushui River, Xinjiang River, and Raohe River). Dongting Lake (28° 30′−30° 20′ N and 110° 40′−113° 10′ E) is the second largest freshwater lake of China, which has a drainage area of 262 800 km2 in Hunan province. Similar to Poyang Lake, Dongting Lake also exchanges water with the Yangtze River in the north with water supplies from local rivers (Xiang River, Zi River, Yuan River, and Li River).18 Local weather over the two drainage areas is similar because it is controlled by subtropical monsoon with significant seasonality in precipitation (Figure 1c). Connecting to the Yangtze River and lying in the downstream of the TGD, the two lakes have similar geographical and hydrological characteristics.

Satellite remote sensing provides synoptic and frequent measurements of the water, land, and atmospheric environments and, therefore, is particularly suitable to assess large-scale environmental changes in response to natural and/or anthropogenic impacts.15,16 Here, we combine remote sensing data, meteorological data, and a simple hydrologic model to study the long-term environmental changes of Poyang Lake and Dongting Lake, with the following objectives: (1) to document the longterm changes of the lakes’ inundation areas and hydrological (surface runoff coefficients) and meteorological [e.g., relative humidity (RH)] conditions between 2000 and 2009 and (2) to investigate the potential linkage between changes of the above parameters and the TGD.

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STUDY AREA AND ENVIRONMENTAL SETTINGS With a drainage area of 162 225 km2 representing 97% of Jiangxi province,17 Poyang Lake (28° 22′−29° 45′ N and 115° 47′− 116° 45′ E) is the largest freshwater lake of China. The lake

DATA AND METHODS Three types of data were used in this study: Moderate Resolution Imaging Spectroradiometer (MODIS) data, meteorological data, 9629

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as the averages of the 12 monthly means. The mean annual changing rate was estimated as

and surface runoff data. All data were grouped by hydrologic years,19 starting April 1 of the current year and ending March 31 of the next year. This is because the month of April is the dry− wet transition month (Figure 1c). The first data type is remote sensing data collected by the MODIS instruments on the Terra and Aqua satellites (launched in 1999 and 2002, respectively). These data were used to quantify the changes in the lakes’ inundation areas. MODIS data from 2000 to 2009 were obtained from the U.S. National Aeronautics and Space Administration (NASA) Goddard Space Flight Center (GSFC) and processed with the calibration and algorithms embedded in the software package of SeaDAS (version 6.2). Of the thousands of available images, 564 and 510 cloud-free images covering Poyang and Dongting Lakes were used, respectively, to calculate the inundation areas. The temporal distribution of these images is listed in Table 1. There is at least one image in any given month (except for February 2006 for Dongting Lake), and on average, there is about one image per week, suggesting that the potential aliasing because of infrequent measurements should be minimal. The inundation area was delineated using the MODIS floating algae index (FAI) and a gradient method.20,21 FAI is defined as the Rayleigh-corrected reflectance at 859 nm normalized against a baseline formed linearly between 645 and 1240 nm to partially remove the aerosol reflectance. FAI is essentially an atmospherically corrected reflectance at 859 nm and is therefore stable against variable atmospheric and observational conditions.21 The 500 m resolution data at 1240 nm were resampled to 250 m resolution to match the 859 and 645 nm data. For each pixel, the gradient was determined by a surrounding 3 × 3 window [gradient = ((1/8)∑i 8= 1(dyi/dxi)2)1/2dyidxi]. The high water absorption at 859 nm (>4 m−1) led to a sharp boundary between land and water pixels, from which the inundation area was derived using the gradient method. The second data type is meteorological data collected by both satellites and ground stations. Monthly precipitation data between 2000 and 2009 were obtained from NASA’s Tropical Rainfall Measuring Mission monthly product (TRMM 3B43, V6). These data showed excellent agreement with those obtained from the ground-based rain gauges in the study region20 and were integrated over the drainage basins to represent local precipitations. MODIS Global Terrestrial Evapotranspiration (ET) Data Set (MOD1622) was obtained from the Numerical Terra dynamic Simulation Group (NTSG) (http://ntsg.umt. edu/project/mod16) and was integrated over the two lakes’ drainage basins. Other meteorological data (air temperature, RH, sunshine hours, and wind speed) were collected from 14 and 26 ground stations, respectively, from the Poyang and Dongting drainage basins. The data were spatially interpolated using the inverse distance weighting (IDW) approach23 and then averaged to represent the conditions for the two regions. The third data type is runoff data of the Poyang Lake drainage basin between 2000 and 2008, provided by the Jiangxi Provincial Institute of Water Sciences (http://www.jxsks.com/class. aspx?&boardid=278). These data were collected daily from hydrologic stations in the five main tributaries of Poyang Lake. The runoff measurements were integrated over each hydrological year, and the summation of all tributaries was used to represent the total runoff to Poyang Lake. Runoff data for the Dongting Lake drainage basin was unavailable from public sources. The inundation areas determined from the individual MODIS images during each calendar month were averaged to derive the monthly mean inundations, and the annual means were derived

R = (S(y2 )/S(y1))1/(y2 − y1) − 1

(1)

where S(y1) and S(y2) are the mean inundations in the starting (y1) and ending (y2) years, respectively. To ensure the same contribution of each year to the calculation, S(2000) and S(2009) were first derived using all annual data and a linear leastsquares fitting, which were then used with eq 1 to estimate R. The surface runoff of the Poyang Lake drainage basin could be estimated with the water balance between three driving forces

runoff = P − ET − G

(2)

where P is the precipitation of the drainage basin, ET is the evapotranspiration, and G is the groundwater exchange. A positive G means that some of the precipitated water is retained in the ground without generating surface runoff. However, longterm G data are generally unknown because of measurement difficulties. If G is neglected in the estimation, a simplified model to calculate runoff would be runoff predict = P − ET

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(3)

RESULTS Long-Term Changes in the Lakes’ Inundation Areas. Both Poyang Lake and Dongting Lake showed strong seasonality in their inundation areas because of seasonal variations in precipitation and water exchange with the Yangtze River20 (Figure 1c). The annual maximum/minimum inundation ratio ranged from 2.1 to 3.9 for Poyang Lake and from 3.0 to 6.2 for Dongting Lake between 2000 and 2009. An example of the maximum and minimum inundation conditions in a typical year of 2007 is shown in the Abstract graphic. The most striking result is the significant decrease in the inundation areas after 2003, during which the TGD was impounded (Figure 2a). Specifically, Poyang Lake showed a mean annual inundation area of 1939.9 ± 265.9 km2 during the pre-TGD period (2000−2002), which decreased by 385.7 km2 to 1554.2 ± 91.5 km2 in the post-TGD period (2004−2009). The difference between the two periods is statistically significant (t test; p < 0.05). Similarly, Dongting Lake’s inundation area decreased by ∼17% (195.1 km2) after the impoundment of the TGD in 2003 (from 1131.4 ± 138.7 to 936.3 ± 119.4 km2, with a p value of 0.06 when testing the difference for significance). The maximum annual mean was 1226.3 km2 for Dongting Lake and 2191.7 km2 for Poyang Lake, both occurring in 2002. In contrast, the minimum annual mean observed in 2006 accounted for only 63.5% (Dongting Lake) and 66.0% (Poyang Lake) of the maximum annual values. Table 2 shows the changing rates of the annual mean inundations in different periods. Both Poyang Lake and Dongting Lake showed decreasing inundation areas during the post-TGD period and increasing inundation areas during the pre-TGD period. On average, from 2000 to 2009, Poyang Lake experienced a shrinking rate of 3.3% year−1 (p < 0.05), while Dongting Lake showed a shrinking rate of 3.6% year−1 (p < 0.05). The statistically significant decreasing trend in the long-term inundation area of Poyang Lake was further revealed by the variations in the dry season starting date during each year (in Julian day). The date was derived as the time when the inundation area started to fall below the mean inundation area of the dry season (from October to March) during the hydrological 9630

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represent eq 2. As shown in Figure 3a, from 2000 to 2003, the predicted and measured runoff were very similar (difference of 30%) than ground measurements, indicating a change of the water balance in the Poyang Lake drainage basin during these 2 years. Such a change was further confirmed by the significantly decreased annual runoff coefficients (defined as the ratio between measured surface runoff and precipitation) during this period (Figure 3b). The mean runoff coefficient was 0.51 ± 0.03 from 2000 to 2002, which decreased to 0.38 ± 0.04 during 2004 and 2005. In other words, the same amount of precipitation during these 2 years resulted in much less surface runoff in the Poyang Lake drainage basin. All evidence showed that the two largest freshwater lakes of China shrunk after the impoundment of TGD, suggesting a potentially causal effect of the TGD (see also the Discussion). However, could other factors (e.g., meteorological or hydrologic variability) explain the observed changes? To reject this possibility, we analyzed local precipitation data and the Yangtze River discharge, the two major water sources of the lakes.17 Figure 4a reveals that, except for a peak in 2002, the precipitation of the two drainage basins was relatively stable in the 10 year period from 2000 to 2009. In particular, there is no increasing or decreasing trend in precipitation after 2003. Likewise, no trend was observed in the Yangtze River discharge data (Figure 4b) collected at the closest hydrologic station down the TGD (Yichang station), except the 2006 minimum because of the severe drought in the Yangtze River basin.24 Thus, the shrinking trend of the two lakes could not be explained by either precipitation or river discharge. To determine the possible linkage of the TGD with local weather, several meteorological parameters of the last 2 decades were plotted (panels c−f of Figure 4; wind speed data from 1995 to 2009). Annual mean air temperature increased gradually since 1993, while annual total sunshine hours showed no apparent trend. No trend was observed in the annual mean wind speed after 2000. However, RH dropped significantly after the impoundment of the TGD in 2003 (red box in Figure 4f). From 1988 to 2002, RH was stable and generally followed a 3−4 year circle, yet an abrupt decrease occurred after 2003. Lower RH means that the air was drier, implying that the land could also be drier and, therefore, could retain more water, leading to the observed lower runoff coefficient. Indeed, RH is highly correlated (R2 = 0.78; p < 0.05) with the runoff coefficient (Figure 3c), suggesting that they were both influenced by the TGD after 2003.

Figure 2. (a) Annual mean inundation areas of Poyang Lake and Dongting Lake between 2000 and 2009 (hydrological year). Both Poyang Lake and Dongting Lake showed significantly decreasing trends during the 10 year period. (b) Dry season starting dates of Poyang Lake and Dongting Lake between 2000 and 2009. Poyang Lake showed an early starting trend, while no significant trend was found for Dongting Lake. The vertical gray dashed line indicates the year of 2003 when the TGD was impounded, and the “yr” in the legend refers to “year”.

Table 2. Annual Changing Rates of the Annual Mean Inundations during Different Periods for Poyang Lake and Dongting Lakea Poyang Lake (%) Dongting Lake (%) a

2000−2009

2000−2002

2004−2009

−3.3 −3.6

6.0 1.4

−1.7 −4.4

The rates were estimated with eq 1.

year. Poyang Lake showed a significant decreasing trend (−7.53 days year−1; p < 0.05) from 2000 to 2009 (Figure 2c), leading to earlier dry-season starting dates (on average, 53.6 days) between the post-TGD period (2004−2009; 296.4 ± 24.3 days) and the pre-TGD period (2000−2002; 350 ± 22.6 days), and the mean dates were also significantly different (t test; p < 0.05). The general decreasing trend was modulated in 2008 when the TGR increased its water discharge in late autumn (http://hb. xinhuanet.com/zhuanti/2008-11/08/content_14941455.htm), clearly indicating the impact of the TGD. In contrast to Poyang Lake, no significant trend in the dry season starting dates was found for Dongting Lake, possibly because of a closer proximity to the TGR, yet the exact reason needs to be further investigated once more hydrological and meteorological data are available. Linkage with the TGD. To understand whether the observed trends were linked to the TGD or were simply a direct result of changes in local precipitation, the water balance of Poyang Lake drainage basin was investigated together with meteorological data. In situ data for Dongting Lake was unavailable; therefore, the analysis was only applied to Poyang Lake. The annual surface runoff of Poyang Lake drainage was estimated with eq 3 and compared to in situ measurements that

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DISCUSSION Could the large mismatch (>30%) between predicted and measured runoff during 2004−2005 be an artifact because of uncertainties in the satellite-based estimates of P and ET in eq 3? Validation of the TRMM P estimates using local rain gauges showed a root-mean-square (RMS) difference of 8−19% on monthly scales.7 Such a difference may, at least in part, be due to their measurement scales: while the rain gauges provided point measurements, each TRMM data point covered an area of 0.25° × 0.25° (about 27.5 × 27.5 km2 at the equator). The MODIS ET data (MOD16 data product) were derived using the well-known Penman−Monteith equation25 together with a suite of environmental parameters, such as surface energy partitioning, environmental constraints, evaporation from canopy interception, wet 9631

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Figure 3. (a) Predicted and measured surface runoff of the Poyang Lake drainage. The difference (shaded in green) represents the water exchanges with groundwater. (b) Annual runoff coefficient of the Poyang Lake drainage basin showing significant decreases after 2003 (the year of the TGD impoundment), especially during 2004−2005 (outlined in a red circle). (c) Relationship between the runoff coefficient of the Poyang Lake drainage basin and its local air RH.

Figure 4. (a) Mean annual precipitation (TRMM data) of Poyang Lake and Dongting Lake from 2000 to 2009. (b) Yangtze River discharge measured at the closest hydrological station (Yichang) downstream of the TGD. (c−f) Weather parameters of the drainage basins from 1988 to 2009 (collected at meteorological stations in the drainages basins), including mean air temperature, cumulated sunshine hours, mean wind speed (1995−2009), and RH, respectively. Note the dramatic drop in RH since 2003 when the TGD was impounded (highlighted by the red box). The gray dashed lines indicate the year of 2003 when the TGD was impounded.

monthly and daily scales, respectively, do not represent systematic biases for long-term trend analysis. At annual scale, these uncertainties should be significantly reduced. This

and moist soil surfaces, and transpiration from canopy stomata. Validation of the daily MOD16 ET data showed uncertainties of 10−30%.25 However, such reported uncertainties in P and ET at 9632

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lakes’ inundation areas are likely to be attributed to the impoundment of the TGD. While the long-term MODIS observations revealed a statistically significant decreasing trend in the inundation areas of the two largest freshwater lakes of China downstream of the TGD after its impoundment in 2003, analyses of the meteorological/hydrological data (precipitation and runoff coefficient) and Poyang Lake’s water budget led to the conclusion that there appeared to be a causal effect of the TGD. In addition to the shrinking lakes, other consequences included the sharp drops in RH and surface runoff coefficient as well as early starts of the dry season. These findings provide unprecedented information on the linkage of the TGD and the downstream lake environments. These environmental changes are expected to cause significant impacts on the local ecosystems, yet quantification of such impacts requires long-term efforts, particularly in a changing climate.

assessment is further supported by the agreement between predicted and measured runoff during 2000−2003 and 2006− 2008 (Figure 3a). Thus, the large mismatch between predicted and measured runoff during 2004 and 2005 is unlikely due to uncertainties in the satellite-based P and ET estimates but rather due to the missing groundwater component in the prediction. Indeed, the TGR stored a water volume of ∼10 billion m3 after its impoundment in June 2003 (http://news.xinhuanet.com/ politics/2009-10/07/content_12190960.htm), leading to a sharply decreased water level downstream of the Yangtze River. Consequently, a huge amount of water in Poyang Lake was discharged to the Yangtze River in 2003.26 The TGR continued to store water after 2003. Thus, the abnormal conditions in the two subsequent years (i.e., >30% difference between predicted and measured runoff in 2004 and 2005; shaded in Figure 3a) may have resulted from the fact that some of the precipitated water was used to recharge the groundwater system instead of generating surface runoff. Although the water budget was balanced again after 2006, the lower mean runoff coefficient during 2006−2008 than that during 2000−2002 still indicates the continued effect of the TGD. The RH decreases could have contributed to the severe drought in early 2011, although decreases in local precipitation were found to be the primary reason.7 The lake environments are complex systems under influence of many factors, including climate variability, yet the findings here provide critical information and knowledge to help inform policy and decisions to mitigate the potential environmental impacts of both human activities and climate variability. For example, the local government of Jiangxi province is planning to build a dam in Poyang Lake to retain more water during the dry season. The impacts of the dam, once constructed, can be assessed using the same techniques as outlined here. Climate variability and human activities have led to large-scale changes in many of the world’s lakes, for example, in the Arctic27 and in China.28 Will the decreasing trends of Poyang Lake and Dongting Lake continue in the future? The air temperature in China has continuously increased in the past few decades.29 A higher temperature could cause more evaporation,30 leading to drier land and lower runoff coefficient. Should the temperature continue to increase, the observed trends in lake inundation areas, RH, and runoff coefficients may also continue. The study here is based on the 10 year MODIS data between 2000 and 2009. Ideally, more data are required before 2000 to assess the pre-TGD conditions to provide baseline data before the dam’s impoundment in 2003. However, this limitation is induced by the data availability. MODIS data did not start until 2000, and data collected by other sensors are not suitable for the purpose of trend analysis of inundation changes (e.g., highfrequency sensors, such as SeaWiFS, have coarse spatial resolution, while high-resolution sensors, such as Landsat, have low-frequency revisits). The use of multiple sensors will also create potential problems in cross-sensor consistency. Indeed, the conclusion of the linkage between the lakes’ inundation changes and the TGD is not solely based on the temporal coincidence between the TGD impoundment (2003) and the lake changes before and after the impoundment but also other observations (precipitation, Yangtze River discharge, and changes in RH) that provide indirect supporting evidence. Thus, although the 10 year period from MODIS observations is relatively short, especially when considering that only 3 years of data are available before 2003, the decreasing trends of the two

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [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 open-fund projects of LIESMARS (Wuhan University) and the University of South Florida. We thank NASA for providing MODIS and TRMM data, and the NTSG of the University of Montana for providing ET. We are indebted to local environmental agencies and groups who collected and provided surface runoff and rain gauge data. Three anonymous reviewers provided extensive comments that helped improve the manuscript.

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REFERENCES

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dx.doi.org/10.1021/es4009618 | Environ. Sci. Technol. 2013, 47, 9628−9634