Dynamics of the lakes in the middle and lower ... - Semantic Scholar

Environ Monit Assess (2013) 185:4005–4018 DOI 10.1007/s10661-012-2845-0

Dynamics of the lakes in the middle and lower reaches of the Yangtze River basin, China, since late nineteenth century Lijuan Cui & Changjun Gao & Xinsheng Zhao & Qiongfang Ma & Manyin Zhang & Wei Li & Hongtao Song & Yifei Wang & Shengnan Li & Yan Zhang

Received: 30 December 2011 / Accepted: 13 August 2012 / Published online: 11 September 2012 # Springer Science+Business Media B.V. 2012

Abstract The middle and lower reaches of the Yangtze River basin have the most representative and largest concentration of freshwater lakes in China. However, the size and number of these lakes have changed considerably over the past century due to the natural and anthropogenic impact. The lakes, larger than 10 km2 in size, were chosen from relief maps and remotely sensed images in 1875, 1950, 1970, 1990, 2000, and 2008 to study the dynamics of lakes in the middle and lower reaches of the Yangtze River basin and to examine the causes and consequences of these changes. Results indicated that there was a dramatic reduction in lake areas, which decreased by 7,841.2 km2 (42.64 %) during the study period (1875–2008), and the number of lakes in this region changed moderately. Meanwhile, a large number of lakes in the middle and lower reaches of the Yangtze River basin were directly converted into paddy fields, ponds, building lands, or other land-use types over the study period. Therefore, all kinds of lake reclamation should be identified as the major driving factors for the loss of lake in this region. Furthermore, flooding, soil erosion, and sedimentation were also the main factors which triggered lake changes in different periods. Some L. Cui (*) : C. Gao : X. Zhao : Q. Ma : M. Zhang : W. Li : H. Song : Y. Wang : S. Li : Y. Zhang Institute of Wetland Research, Chinese Academy of Forestry, No. 1, Dongxiaofu, Haidian District, Beijing 100091, People’s Republic of China e-mail: [email protected]

wetland conservation and restoration projects have been implemented since the 1970s, but they have not reversed the lake degradation. These findings were of great importance to managers involved in making policy for the conservation of lake ecosystems and the utilization of lake resources. Keywords Lake change . Lake reclamation . Human activity . Yangtze River

Introduction Lakes, as one of the most important natural wetland types, contain over 90 % of the world's liquid surface freshwater (ILEC 2007). They not only support sustainable human livelihoods and economic development but also have many ecosystem functions, such as climate mediation, flood and drought mitigation, nutrient retention and cycling, and provision of habitats for wildlife, water supplies, esthetics, and educational resources (Beeton 2002; Bronmark and Hansson 2002; Tiner 2005). However, many lakes around the world are shrinking or disappearing, and the overall conditions of the remaining lakes are deteriorating because of natural events (e.g., climate change, flooding, and soil erosion) and anthropogenic factors (e.g., overreclamation, water pollution, and misguided management policies) (Mitsch and Gosselink 2000; Xie et al. 2010). These changes in lake conditions greatly affect

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the availability of freshwater resources and reduce the ecosystem functionality and economic value of lakes. To prevent further loss of lakes and protect existing lake ecosystems, the accurate and timely detection of lake changes and the examination on causes and ecological effects of these changes are, hence, imperative to support decision making for the sustainable lake ecosystem management. The Yangtze River, which is the longest river in China and the third one in the world, plays a key role in the social and economic development in China. In this region, there exist complex ecosystems including lakes, marshes, and rivers (Yin et al. 2007; Yu 2003). Most lakes (especially those larger than 10 km2) play important roles in supplying water resources and controlling flooding and in providing transit habitats for many rare or endangered migratory birds (Yang et al. 2007). However, over the past century many lakes have undergone various transformations, largely as a result of natural factors such as climate, hydrologic, and ecosystem changes, and human activities including the reclamation of lakes, agricultural irrigation, and domestic and industrial water usage (Du et al. 2011; Ma et al. 2010a). Ultimately, these changes have adversely affected the lake ecosystem in the middle and lower reaches of the Yangtze River basin. Most studies relating to this region (Du et al. 2011; Wang et al. 2008; Xu et al. 2010) focused on an individual lake, such as Taihu Lake, Poyang Lake, or Dongting Lake, and several other preliminary research works paid more attention to investigating China's lake resources in recent decades (Gong et al. 2010; Ma et al. 2010a; State Forestry Administration of China 2004; Wang and Dou 1998; Wang et al. 1989), while analysis on lake changes at long-term timescale (especially on a century scale) were relatively few in this region. Changes of the lake around the Yangtze River basin have significant spatial differences over the past century, and it was difficult to map and detect these changes based on field survey. With the availability of global spatial datasets today, it was a common way to detect the landcover changes over a long-term timescale through satellite image classification (Kashaigili et al. 2006; Liu et al. 2005; Olang et al. 2011; Zhou et al. 2009). Such an approach has been successfully used in monitoring lake changes associated with natural and/or human-based influences (Bai et al. 2008; Bai et al. 2011; Lyon and McCarthy 1995; Wang et al. 2008; Wright and Gallant 2007). The objective of this study was to provide a scientific base of lake changes in the middle and lower reaches of

Environ Monit Assess (2013) 185:4005–4018

the Yangtze River basin since late nineteenth century by analyzing and quantifying the spatiotemporal dynamics of lakes using relief maps and available remotely sensed images.

Materials and method Study area The middle and lower reaches of the Yangtze River basin (106°7′–121°47′ E, 24°30′–33°54′ N) in Eastern China are primarily located in Hunan, Jiangxi, Hubei, southern Anhui, and southern Jiangsu provinces, and Shanghai municipality, covering an area of 7.8×105 km2 (Fig. 1). The mean annual temperature in this region normally ranges between 14 and 18 °C, and the mean annual precipitation is about 1,300 mm. The region is low-lying with an elevation of less than 50 m. The middle and lower reaches of the Yangtze River basin have the most representative and largest concentration of freshwater lakes in China; there are up to 100 lakes (area ≥10 km2) covering a total area of 10,593.4 km2, accounting for 38.17 % of the total area of all freshwater lakes in China. Specifically, there are nine Ramsar sites (which are considered to be wetlands of international importance) in this region (http://www.ramsar.org/pdf/ sitelist.pdf), and these wetlands are important for the conservation of regional biological diversity and for sustaining local human life. Meanwhile, the middle and lower reaches of the Yangtze River basin are one of the most important agricultural and industrial regions in China, and the region has experienced rapid social and economic growth in recent decades. With rapid socioeconomic development, one third of lakes in this region have been reclaimed, and more than 1,000 lakes (area ≥0.1 km2) disappeared because of excessive drainage and cultivation (Chinese Academy of Science Sustainable Development Research Group 2007), which has caused serious ecological and environmental problems, such as eutrophication, reduction of lake water quantities, and destruction of ecosystems. Thus, the conflict between lake conservation and economic development has become increasingly irreconcilable. Data sources and lake mapping Relief maps and satellite images have been collected and integrated for analysis of lake changes in the


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Fig. 1 Location of the study area

middle and lower reaches of the Yangtze River basin. In this study, lake maps for six periods (1875, 1950, 1970, 1990, 2000, and 2008) were generated. The available data sources were as follows. For the 1875 map, one relief map of the lake distribution in the study area with a scale of 1:7,500,000 published by Stieler and Adolf (http:// www.davidrumsey.com/maps1654.html) was taken. For the 1950 map, 22 sections of relief maps of the study region at a scale of 1:250,000 compiled by the United States Army Corps of Engineers in 1953, were collected. Relief maps for the Yangtze River basin in 1875 and 1950 were compiled mainly in the growing vegetation season, and they included clear lake boundaries that were sufficient for the study's purpose. Landsat Multispectral Scanner (MSS) images taken around 1970, Landsat Thematic Mapper (TM) images taken around 1990 and 2000, and Landsat Enhanced Thematic Mapper plus (ETM+) images taken around 2008 were used to map the lake distribution in the four time phases of 1970, 1990, 2000, and 2008; the images were obtained from the International Scientific Data Service Platform

(http://datamirror.csdb.cn) and the Federation of Earth Science Information Partners of the National Aeronautics and Space Administration (http://www.landsat.org/ ortho/index.php). Lake boundary is usually defined as the average water surface boundary over a certain number of years. Because lake water surface boundaries are subject to constant change, the determination of lake boundaries must follow a set of consistent rules to allow for temporal and spatial comparisons. In this study, the rules that we adopted for remote sensing interpreting of lake boundaries were proposed by Ma et al. (2010b). Some primary interpreting rules were listed as follows: (1) multiphase satellite images from more than 2 years were used to confirm their annual average normal water level in dry seasons; (2) when wetlands were present within the main water body in dry seasons or adjacent to it, the area was included in the lake dimensions; (3) when determining lake boundaries, the water level was ascertained and supported by reference hydrological and meteorological data and expert; and (4) the lake area included all areas of water surface and internal/adjacent wetlands.

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Meanwhile, aquaculture ponds and reservoirs were excluded from this study because they have humandominated features and their natural ecological and hydrological processes have been modified greatly. Additional details of the rules for interpreting lake boundaries can be found in Ma et al. (2010b). After comparison of multiphase images in each period, the images that we finally chose were listed in Table 1, and most of which were taken between December and April. All data obtained were well processed and freely available. To generate lake maps for the six periods, which were subsequently used to analyze the lake changes, all available relief maps and satellite images were georectified to an Albers equivalent conical projection, with root-mean-square uncertainties of less than 1 pixel. Three bands of satellite imagery (bands 4, 3, and 2 for Landsat-MSS and bands 5, 4, and 3 for Landsat-TM/ETM+) were combined as the optimal setting for lake mapping, and other bands were used for reference. All relief maps and satellite images were then used to map lakes in ArcGIS 9.3 software. Visual interpretation was applied to ensure mapping accuracy, and screen scales were controlled within 1:50,000

to ensure the geometric accuracy in screen-based lake delineation. The minimum mapping unit for areal features was kept at 9 ha (equivalent to 10×10 image pixels). In addition, information from Google Earth, published literature, and field investigations were used to help delineate lake boundaries and verify results. In this study, we only extracted natural lakes, including shallow-water mud flats that were defined by the Ramsar Convention (Ramsar Convention Secretariat 2006). After extracting natural lakes in the study area for the six periods, we further obtained spatial and attribute databases for lakes larger than 10 km2 in this region in ArcGIS 9.3. The distribution of lakes (larger than 10 km2) in the middle and lower reaches of the Yangtze River basin during 1875–2008 is illustrated in Fig. 2.

Results Dynamics of lake area and number The area of lakes (larger than 10 km2) in five provinces (Hubei, Hunan Jiangxi, Anhui, and Jiangsu)

Table 1 Acquisition dates of the Landsat images used to map the lake distribution Path/row

Acquisition date

Path/row

Around 1970 (MSS)

Acquisition date Around 1990 (TM)

Around 2000 (TM)

Around 2008 (ETM+)

127/038

Feb. 13, 1974

118/038

May. 18, 1989

Feb. 7, 2000

Feb. 29, 2008

128/038

Nov. 16, 1973

119/038

May. 22, 1992

Mar. 1, 2000

Apr. 24, 2008

128/039

Nov. 16, 1973

119/039

May. 30, 1989

Mar. 17, 2000

May 8, 2007

129/038

Oct. 30, 1973

120/038

Oct. 15, 1990

Mar. 24, 2000

May 1, 2008

129/039

Oct. 30, 1973

120/039

Oct. 15, 1990

Mar. 8, 2000

May 17, 2008

130/038

Dec. 24, 1973

120/040

Oct. 20, 1992

Jan. 20, 2000

Nov. 9, 2008

130/039

Dec. 24, 1973

121/038

Mar. 26, 1995

Dec. 10, 1999

Oct. 15, 2008

130/040

Dec. 24, 1973

121/039

Feb. 13, 1989

Jan. 27, 2000

Oct. 15, 2008

130/041

Dec. 23, 1975

121/040

Feb. 13, 1989

Apr. 16, 2000

Dec. 2, 2008

131/039

Dec. 25, 1973

122/038

Oct. 24, 1994

Mar. 6, 2000

Dec. 9, 2009

131/040

Dc. 25, 1973

122/039

Oct. 18, 1992

Mar. 6, 2000

Oct. 6, 2008

132/038

Nov. 2, 1973

122/040

Oct. 18, 1992

Mar. 6, 2000

Jan. 10, 2009

132/039

Nov. 2, 1973

122/041

Nov. 9, 1994

May. 12, 2001

Dec. 9, 2008

132/040

Dec. 8, 1973

123/038

Jan. 26, 1989

Apr. 17, 2001

Oct. 13, 2008

133/038

Nov. 3, 1973

123/039

Jan. 26, 1989

Jan. 11, 2001

Nov. 14, 2008

133/039

Mar. 31, 1975

123/040

Dec. 4, 1989

May. 3, 2001

Nov. 14, 2008

133/040

Dec. 26, 1975

124/038

Mar. 11, 1988

Oct. 30, 2000

Jan. 24, 2009

134/038

Nov. 22, 1973

124/039

Oct. 25, 1995

Oct. 30, 2000

Feb. 7, 2008

134/039

Oct. 16, 1975

124/040

Oct. 25, 1995

May. 10, 2001

Apr. 14, 2009


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Fig. 2 Lakes distribution in the middle and lower reaches of the Yangtze River basin in 1875, 1950, 1970, 1990, 2000, and 2008

around the middle and lower reaches of the Yangtze River basin and the total area and number of lakes in this region are presented in Table 2 for each of the studied time periods. Table 2 indicates that there was significant difference on temporal variation of lake area among the five provinces during 1875–2008. Although Hunan province has the largest area of lakes among the five provinces around the middle and lower reaches of the Yangtze River in 1875, the most dramatic reduction of lake area also occurred in this province during the period of 1875– 2008, and it has decreased to 926.4 km2 in 2008. The slight change of lake area happened in Jiangxi and Jiangsu provinces in 1875–2008, and the lake area in

these two provinces decreased by 308.4 and 881.4 km2, respectively, over the past 133 years. The lake area in Anhui province increased sharply from 1981.1 km2 in 1875 to 2,030.6 km2 in 2008, but it continually decreased by 1,022.9 km2 from 1950 to 2008. There was a similar changing process of lake area in Hubei province as compared to that in the whole study area between 1875 and 2008. There was a general trend of decrease in total lake area and in the number of lakes in the study region over the past 133 years. Although there was an obvious trend of increase in total lake area during 1970–1990, the total lake area reduced remarkably from 18,467 km2 in 1875 to 10,593.4 km2 in 2008. The number of lakes

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Table 2 Lake changes in the middle and lower reaches of the Yangtze River region at six time intervals Period

Hubei Province

Hunan Province

Jiangxi Province

Anhui Province

Jiangsu Province

Total

Area (km2)

Area (km2)

Area (km2)

Area (km2)

Area (km2)

Area (km2)

Number

1875

4,213.7

4,923.3

3,552.1

1,981.1

3,796.8

18,467.0

70

1950

2,928.5

3,064.3

3,101.0

3,053.5

3,668.2

15,815.5

149

1970

2,127.8

2,671.7

2,976.9

2,602.0

3,392.9

13,771.3

122

1990

2,978.5

2,246.9

4,338.1

2,507.1

3,209.3

15,288.9

113

2000

1,689.4

1,951.9

3,610.8

2,320.2

2,694.7

12,267.0

107

2008

1,486.3

926.4

3,234.7

2,030.6

2,915.4

10,593.4

100

increased sharply from 70 in 1875 to 149 in 1950 and then gradually reduced to 100 in 2008. Temporal change of lakes Table 3 indicates that there were dramatic changes in lake area in the middle and lower reaches of the Yangtze River basin in 1875–2008. The lake area in the middle and lower reaches of the Yangtze River basin decreased by 2,651.5 km2 with the relative change rate of 33.68 %, and the annual rate of decrease in lake area in this region was 35.35 km2/year during the period of 1875–1950. The lake area in this region also decreased greatly in the period of 1950–1970. The only dramatic expansion in the lake area in the study area occurred between 1970 and 1990. During this period, the lake area increased by 1,517.6 km2, and the annual rate of expansion in lake area was 75.88 km2/year. The smallest relative change rate in lake area in this region was 19.27 %, and it also appeared in the period of 1970–1990. Table 3 also indicates that the most dramatic reduction in lake area took place in the middle and lower reaches of the Yangtze River basin during the period of 1990–2000. In this period, the lake

area decreased by 3,021.9 km2 with the largest annual and relative change rate of 302.19 km2/year and 38.38 %, respectively. Although the lake area in the middle and lower reaches of the Yangtze River basin continued to decrease between 2000 and 2008, the annual rate of reduction in lake area had slowed down to 209.2 km2/ year as compared to that in the period of 1990–2000. The table (Table 3) also indicates that the number of lakes in the middle and lower reaches of the Yangtze River basin changed obviously in the period of 1875– 2008. The only dramatic increase in lake number in the middle and lower reaches of the Yangtze River basin occurred in the period of 1875–1950. During this period, the number of lakes increased by 79 with the largest relative change rate of 263.33 %. The number of lakes in this region began to decrease in the period of 1950–1970. During this period, the number of lakes decreased by 27 with the largest annual reduction rate of 1.35 %. Although the number of lakes in this area continued to decrease between 1970 and 1990, the annual rate of reduction in lake number had slowed down to 0.45 %. From 1990 to 2008, the tendency to lake number decline still existed

Table 3 Lake area and number change in the middle and lower reaches of the Yangtze River basin Period

Lake area change Area change (km2)

Lake number change Annual change rate (km2/year)

Relative change rate (%)

Number change

Annual change rate

Relative change rate (%)

1875–1950

−2,651.5

−35.35

33.68

79

1.05

263.33

1950–1970

−2,044.2

−102.21

25.96

−27

−1.35

90.00

1970–1990

1,517.6

75.88

19.27

−9

−0.45

30.00

1990–2000

−3,021.9

−302.19

38.38

−6

−0.60

20.00

2000–2008

−1,673.6

−209.2

21.26

−7

−0.88

23.33


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Figure 3 shows the spatial change of lakes in the middle and lower reaches of the Yangtze River basin in the period of 1875–1950, 1950–1970, 1970–1990,

1990–2000, and 2000–2008. Two kinds of lake changes including lakes decreased and lakes increased are shown. It is noticed from the lake change map in each of the studied time periods that there is a visible difference between lakes decreased and lakes increased in their spatial location in the five provinces around the middle and lower reaches of the Yangtze River basin.

Fig. 3 Spatial change of lakes in the middle and lower reaches of the Yangtze River basin in the period of 1875–1950, 1950– 1970, 1970–1990, 1990–2000, and 2000–2008. Lakes decreased include lakes that disappeared and lakes that shrunk; lakes increased include new lakes that appeared and lakes that

expanded. When the area of a lake was less than 10 km2, it was excluded from this research. Correspondingly, if its area expanded to or were larger than 10 km2, it would be included in our research again. In this study, most decreased lakes did not disappear altogether but instead shrank to sizes below 10 km2

in this region, and the annual rate of reduction in lake number had slightly increased to 0.88 %. Spatial change of lakes

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The decreased lakes almost spread over the whole study area, but the dramatic increase of lakes were mostly within Anhui province during the period of 1875–1950. The decreased lakes were mainly located in Anhui, Hubei, Hunan, and Jiangxi provinces in the period of 1950–1970. During this period, the increased lakes were primarily within Hunan and Jiangxi provinces. The lake changes map in (1970– 1990) implies that the significant increase of lakes mainly happened in Hubei and Jiangxi province. On the contrary, large decrease of lakes also occurred in these two provinces in the period of 1990–2000. Large scale of lake decrease was mostly located in Hunan provinces, while several small scale of lake decrease happened in Hubei and Anhui provinces from 2000 to 2008. During this period, a little increase of lakes also appeared in Hubei, Jiangxi, and Jiangsu provinces. Lake area changes in five provinces (Hubei, Hunan, Jiangxi, Anhui, and Jiangsu provinces) around the middle and lower reaches of the Yangtze River basin are also observed in Table 4 for each of the studied time intervals. There was a significant decrease in lake area in Hubei and Hunan provinces during 1875– 1950, and the reductions of lake area in these two provinces were 1,285.2 and 1,859 km2, respectively. On the contrary, during this period, the lake area in Anhui province increased by 1,072.4 km2. However, the lake area in Anhui province began to reduce from 1950 to 1970. In the same period, the lake area in Hubei, Hunan, Jiangxi, and Jiangsu provinces continued to decrease, and the annual rates of reduction in lake area in these three provinces were larger than that in the period of 1875–1950 except for Hunan province. There were obvious increasing trends for the lake area in Hubei and Jiangxi province in 1970–1990. In this time, lake area in Hunan, Anhui, and Jiangsu

provinces still decreased. Since 1990 to 2008, the lake area in these five provinces around the middle and lower reaches of the Yangtze River basin has been decreasing, and the dramatic decrease of lake area mainly occurred in Hubei, Hunan, and Jiangxi provinces. It is important to note that the lake area in Jiangsu province increased by 220.7 km2 in 2000– 2008.

Discussion Analysis of driving factors for lake changes in the different periods Lakes in the middle and lower reaches of the Yangtze River basin have changed considerably with the combined impact of human activities and natural factors. However, as shown by satellite images, field surveys, and previous literature (Du et al. 2011; Ma et al. 2010a; Wang and Dou 1998), the changes in lake area and number in the middle and lower reaches of the Yangtze River basin are more attributed to human activities (e.g., lake reclamation and expansion of lakeside developments and enclosures) than natural events (e.g., climate change, flooding, soil erosion, and sedimentation). Human activities 1875–1950 Most lakes in the study area were located within the floodplains of the Yangtze River. When people were exiguous in some historical period, these fertile floodplains without being exploited for agriculture can provide local people with the ample source of food. However, over a period of time, portions of the

Table 4 Lake area change in five provinces around the middle and lower reaches of the Yangtze River basin Period

Hubei Province Area change (km2)

Hunan Province

Jiangxi Province

Anhui Province

Jiangsu Province

Annual change rate (km2/year)

Area change (km2)


Area change (km2)


Area change (km2)


Area change (km2)


1875–1950

−1,285.2

−17.14

−1,859

−24.79

−451.1

−6.01

1,072.4

14.3

−128.6

−1.71

1950–1970

−800.7

−40.04

−392.6

−19.63

−124.1

−6.21

−451.5

−22.58

−275.3

−13.77

−424.8

−21.24

1,361.2

68.06

−94.9

−4.75

−183.6

−9.18

−295

−29.5

−727.3

−72.73

−186.9

−18.69

−514.6

−51.46

−1,025.5

−128.19

−376.1

−47.01

−289.6

−36.2

220.7

27.59

1970–1990

850.7

42.54

1990–2000

−1,289.1

−128.91

2000–2008

−203.1

−25.39


floodplains (e.g., lakes in this region) must be reclaimed for agriculture to meet the food demands of gradually increasing population. A literature review indicates that there were two peaks in lake reclamation on the Yangtze River during 1875–1950, one in the middle and latter periods of the Qing Dynasty (1870– 1911) and one in the 1930s to 1950s (Bian et al. 1993; Li et al. 2004; Mei et al. 1995). During these two stages subsections of many large lakes in the floodplains were mainly converted into agricultural land to meet the food demands of a rapidly growing population, which led to an initial loss in lake area in the middle and lower reaches of the Yangtze River. During this period, lake reclamation also split some large lakes into several midsized or small lakes, and this was one of the reasons resulting in the increase of lake number in this region. Furthermore, several floods also occurred between the 1930s and the 1950s leading to the increase in lake number and area in the middle reach of the Yangtze River (He and Jiao 2000; Huang 2001; Zhong 2002). In addition, the resolution of relief map in 1875 was lower compared to that of relief maps for other years, and this may prevent some small lakes from being extracted from the relief map in 1875. This may be another reason for the increase of lake number in this region, especially in Anhui province, during this period (Fig. 3). Thus, changes of lake size and number in the middle and lower reaches of the Yangtze River basin in 1875–1950 were triggered by lake reclamation and frequent severe floods. Although the enlargement and contraction of the lake area occurred concurrently, the general trend was a decrease in lake area from 18,467 km2 in 1875 to 15,815.5 km2 in 1950. 1950–1970 There was a period of stable economic development along the Yangtze River during 1950– 1970. In this period, the reclamation of landsurrounding lakes increased at an unprecedented rate, leading to a rapid decrease in lake area and number. During this period, great decrease of lake area mainly occurred in Hubei and Jiangxi provinces. For example, Hubei province used to be known as a province with a thousand lakes. The number of lakes (area ≥0.1 km2) in this province decreased from 1,309 to 612, and the reduction of lake (area ≥6.67 km2) accounted for 84.96 % of the total decrease of lake (area ≥0.1 km2) during the 1950s to 1970s (Zhang et al. 2010). It was noted that large and midsized lakes with an area of

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over 10 km2 had actually become the primary target of human utilization as compared to small lakes (area