Historical Estimates of Limnicity

The 'limnicity' of land surfaces has been speculated upon since the early 1900s. In 1925, August Thiene-mann1 concluded from map data summarized in 1914 by Wilhelm Halbfass, that "... around 2.5million km2, that is about 1.8% of dry lands, are covered with freshwaters; in Germany, the area of all lakes covers about 5200 km2, or about 1% of the land surface. The largest lake on Earth is the Caspian Sea which has a surface area of 438 000 km2.'' Several modern assessments of the global area covered by lakes and ponds have been made but were probably underestimates. These estimates were also made using similar map-based methods and have ranged from 2 x 106 to 2.8 x 106km2. Even 75 years after Thienemann's work, based on sometimes poor and incomplete maps, lakes and ponds were still thought to constitute only 1.3-1.8% of the Earth's non-oceanic area.

1 Text located by Lars Tranvik. Translation supplied by Lars

Tranvik and JAD.

The historical stability of estimates of limnicity of land surfaces is due to the prevailing assumption that large lakes make up the majority of Earth's lake area so that map inventories of the world's largest lakes would offer an estimate deficient in only a minor area represented by small water bodies. This idea was substantiated by analyses of the size distribution of lakes. One of the earliest of these was performed by R.D. Schuiling of the Netherlands in 1977. Schuiling plotted the number of lakes found in various size categories of world lakes and European lakes against the surface area of these lakes and found a consistent pattern. These data and others are shown expressed as measures of limnicity (dL) of different size ranges of lakes in Figure 1. On the basis of such analyses, Schuiling and others concluded that lakes are numerically dominated by small water bodies but globally dominated in area by a small number of large lakes. The only dissenting voice in this dogma was Robert Wetzel who designed a graph of world lake abundance (reportedly, first on the back of a napkin) that showed a disproportionately great density of small lakes in the world, relative to large ones. Although meant to be a conceptual analysis rather than a quantitative one, Wetzel's perceptions concerning the relative abundance of different sizes of lakes in the world were remarkably accurate (Figure 1). He postulated that small lakes dominate the area of land surface covered by water.

The great stability from 1925 to 2002 in the estimate of the cumulative continental area occupied by lakes and other water bodies is due to agreement in the area of the world's largest lakes shown on maps. Table 1 shows some characteristics of world lakes of greatest area, down to a size of 2000 km2. The Caspian Sea, the lake with the largest area, alone makes up about 15% of the 2.5millionkm2 represented by the largest mapped lakes of the world. Lake Superior, the world's second largest lake, makes up 3.3% of the area of these lakes, while it takes the second through seventh largest lakes (Superior, Victoria, the Aral Sea, Lake Huron, Lake Michigan, and Lake Tanganyika) to have a cumulative area as large as the Caspian Sea. Lakes of progressively smaller sizes are represented by exponentially increasing numbers of lakes. The area distribution thus appears to approximate the Pareto distribution (explained later), which is precisely the type of distribution one would expect for objects created by random or directed processes in fields as diverse as linguistics and astrophysics. In fact, the word-frequency distribution in this Encyclopedia very likely follows a similar distribution.

The large area lakes of the world are interesting phenomena in themselves. They are found across a broad range of latitudes and elevations and represent lakes of divergent shape, receiving input from small and large watersheds. Of the world's lakes with greatest area (Table 1), the most northerly is Lake Taymyr at 74.5° N on the Taymyr Peninsula in Krasnoyarsk Krai, Russia. It is ice-covered much of the year, with a brief ice-free season from June to September. The most southerly of the large lakes is Lake Buenos Aires (also known as Lake General Carrera) at 46.5° S in Argentina and Chile, which is large enough to create its own climate in the Patagonia region. The large lake at the highest elevation is Lake Nam Co in Tibet, a holy lake large enough that it takes pilgrims 10 days to walk its circumference. Other well-known high-elevation lakes are Lake Titicaca on the border between Peru and Bolivia and Lake Tahoe in the USA. Some lakes are found below sea level, and many of these are very saline. The Caspian Sea is found at —28 m and several other somewhat smaller large lakes are also found below sea level (e.g., Lake Eyre in Australia, the Dead Sea in Israel and Jordan, the Salton Sea in the USA, and Lake Enriquillo in the

10000000 1 000000 100000 10 000 1000 100 10 1

10000000 1 000000 100000 10 000 1000 100 10 1

11 ii^ I iii^ I iii^ I iii^ iiiii^ iiiii^ I iii^ mii^ nun

Lake area (km2)

Figure 1 Relationship between lake surface area and areal frequencies of different sized lakes measured as dL (number of lakes per 106 km2). The filled triangles indicate the frequencies of lake sizes digitized from Schuiling's work. The dashed line represents the hypothesis advanced by Wetzel, digitized from Figure 5 of his 1990 publication. All data represented by filled circles are from Meybeck's publications.

Dominican Republic). Although many large lakes have somewhat rounded shapes (Table 1; shoreline development ratios <2), Lake Saimaa, Finland's largest lake, has shores that are extremely convoluted (development ratio >60). Some large lakes of the world have watersheds that are quite small relative to the lake area (i.e., 2-10; Table 1), whereas others such as Lake Chad and Lake Eyre drain areas >100-times their size.

Large lakes have a remarkable range of depths (Table 2), owing to regional hypsometry and the diverse geologic age and composition of their watersheds. The world's deepest lake is Lake Baikal in southern Siberia, which has a maximum depth of >1700 m. With Lake Tanganyika and the Caspian Sea, it is one of three lakes with a maximum depth >1000 m. In contrast, Lake Patos, a floodplain lake in Brazil the size of which is about 30% of the surface area of Lake Baikal, has a maximum depth of only about 5 m. Shallow lakes often are subject to extreme fluctuations in area owing to flooding or drying during periods of climatic variation. Despite the likelihood that the largest lakes might be expected to cover the greatest range of topography, there is only a weak general relationship between lake size and maximum depth in lakes larger than about 400 km2 (Figure 2). For lakes >5000 km2, however, there is a relationship between the minimum size of maximum depth observed in a size class of lakes and their areas. This relationship tells us that very large contiguous areas of the Earth's surface are unlikely to lack some level of significant relief.

The main problem with analyses of lake size distributions performed prior to the current decade is that small water bodies have been omitted from, or are poorly represented on, many maps. Therefore, the resolution of maps has dictated the perceived relative abundance of small lakes. This can be illustrated using modern satellite imagery by comparing the three panels of Figure 3, all with the same geographic center, but shown at three levels of spatial resolution. In the top panel, we perceive only the largest of the lakes (the North American Great Lakes and a few others). At intermediate resolution, perhaps similar to the resolution seen on maps in the early part of the twentieth century, we see large lakes and many of intermediate size. In the bottom panel, which still has inferior resolution to modern GIS coverages, a myriad of water bodies appear. These likely equal or exceed the area of larger lakes when summed over great land areas. In fact, if one assumes that some random process created pits and bumps in landscapes that were then filled with water, the aspect of lake regions and the size distribution of lakes is very similar to that observed in high-resolution satellite images

Table 1 The world's lakes >2000 km2 in area, arranged in decreasing order of lake area

Name

Latitude

Continent

Lake area (km2)

WA:LA

Elev. (m)

Zmean (m)

Dev. ratio

Caspian

42.0

Asia

374000

10

-28

209

2.8

Superior

47.6

N. America

82100

2

183

149

4.7

Victoria

-1.0

Africa

68460

4

1

40

3.7

Aral

45.0

Asia

64100a

25

53

16

2.6

Huron

45.0

N. America

59500

2

177

59

5.9

Michigan

44.0

N. America

57 750

2

177

85

3.1

Tanganyika

-6.0

Africa

32900

8

774

574

3.0

Baikal

54.0

Asia

31 500

21

456

730

3.5

Great Bear

66.0

N. America

31 326

5

156

76

4.3

Great Slave

61.8

N. America

28568

34

156

73

3.7

Erie

42.2

N. America

25 657

2

171

19

2.4

Winnipeg

52.5

N. America

24387

40

217

14

2.5

Nyasa

-12.0

Africa

22490

3

475

273

2.8

Ontario

43.7

N. America

19 000

4

75

86

2.4

Balkhash

46.0

Asia

18 200a

10

343

6

5.0

Ladoga

61.0

Europe

17 700

4

52

2.0

Chad

13.3

Africa

16 600a

151

240

3

2.2

Maracaibo

9.7

S. America

13010

7

0

22

1.5

Patos

-31.1

S. America

10140

0

2

2.7

Onega

61.5

Europe

9700

33

30

3.3

Rudolf

3.5

Africa

8660

427

29

1.6

Nicaragua

11.5

N. America

8150

32

13

2.4

Titicaca

-15.8

S. America

8030

8

3809

103

3.5

Athabasca

59.2

N. America

7935

20

213

26

2.8

Eyre

-28.5

Oceania

7690a

146

-12

3

4.5

Reindeer

57.3

N. America

6640

10

337

17

5.3

Issykkul

42.4

Asia

6240

1608

277

2.7

Tungting

29.3

Asia

6000a

11

3

1.5

Urmia

37.7

Asia

5800a

9

1275

8

1.8

Torrens

-31.0

Oceania

5780a

12

30

<1

2.7

Vanern

58.9

Europe

5648

8

44

27

7.3

Albert

1.7

Africa

5590

617

27

1.8

Netilling

66.5

N. America

5530

30

3.8

Winnipegosis

52.6

N. America

5375

3

3

3.7

Bangweulu

-11.1

Africa

4920a

20

1140

1

2.0

Nipigon

49.8

N. America

4848

320

63

2.9

Gairdner

-31.6

Oceania

4770a

2

34

<1

2.3

Manitoba

50.9

N. America

4625

248

3

3.4

Taymyr

74.5

Asia

4560a

3

3

3.7

Koko

37.0

Asia

4460

3197

14

1.5

Kyoga

1.5

Africa

4430

1036

6

7.5

Saimaa

61.3

Europe

4380

14

76

14

63.3

Great Salt

41.2

N. America

4360

12

1280

4

2.1

Mweru

-9.0

Africa

4350

922

7

1.5

Woods

49.3

N. America

4350

323

8

4.9

Peipus

57.3

Europe

4300

11

30

6

2.0

Khanka

45.0

Asia

4190a

69

5

1.5

Dubawnt

63.1

N. America

3833

236

3.5

Mirim

-32.8

S. America

3750

17

1

5

2.7

Van

38.6

Asia

3740

4

1646

55

2.3

Tana

12.2

Africa

3600

1811

1.5

Poyang

29.0

Asia

3350

10

8

6.4

Uvs

50.3

Asia

3350

759

1

1.6

Amadjuak

64.9

N. America

3115

113

3.5

Lop

40.5

Asia

3100

768

2

5.4

Melville

53.8

N. America

3069

0

97

2.7

Rukwa

-8.0

Africa

2716a

28

793

<1

1.9

Hungtze

33.3

Asia

2700

15

1.9

Wollaston

58.2

N. America

2690

9

398

17

5.6

Alakol

46.2

Asia

2650

347

22

Continued

Table 1 Continued

Name

Latitude

Continent

Lake area (km2)

WA:LA

Elev. (m)

Zmean (m)

Dev. ratio

Hovsgol

51.0

Asia

2620

1624

183

2.1

Iliamna

59.5

N. America

2590

15

123

2.2

Chany

54.8

Asia

2500

105

2

4.1

Nam

30.8

Asia

2500

4627

1.6

Sap

13.0

Asia

2450a

33

1

4

2.2

From

-30.7

Oceania

2410

35

49

<1

1.5

Kivu

-2.0

Africa

2370

1460

240

3.3

Mistassini

50.9

N. America

2335

8

372

75

4.5

Mai-Ndombe

-2.0

Africa

2325a

340

5

2.6

Nueltin

60.2

N. America

2279

278

2.5

South Indian

57.1

N. America

2247

254

7

5.7

Buenos Aires

-46.5

S. America

2240

217

1.6

Tai

31.3

Asia

2210

3

2

2.2

Edward

-0.4

Africa

2150

912

35

1.7

Ilmen

58.3

Europe

2100a

28

18

6

1.5

Helmand

31.0

Asia

2080a

168

510

4

2.9

Michikamu

54.1

N. America

2030

460

33

3.9

A superscripted 'a' indicates that a lake's area is variable In time and that the area may be nominal. WA:LA Is the ratio of watershed to lake area, 'Elev.' Is the elevation of the lake above mean sea level, Zmean is the average depth, and 'Dev. ratio' is the shoreline development ratio (high numbers mean less circular). Latitudes are expressed in decimal degrees with latitudes in the southern hemisphere expressed as negative numbers. Data are after Herdendorf's work.

A superscripted 'a' indicates that a lake's area is variable In time and that the area may be nominal. WA:LA Is the ratio of watershed to lake area, 'Elev.' Is the elevation of the lake above mean sea level, Zmean is the average depth, and 'Dev. ratio' is the shoreline development ratio (high numbers mean less circular). Latitudes are expressed in decimal degrees with latitudes in the southern hemisphere expressed as negative numbers. Data are after Herdendorf's work.

(Figure 4). Therefore, analyses of limnicity based on maps drew faulty conclusions about the amount of land surface covered by lakes and ponds, as well as the relative areal importance of small and large lakes.

Was this article helpful?

0 0
Project Earth Conservation

Project Earth Conservation

Get All The Support And Guidance You Need To Be A Success At Helping Save The Earth. This Book Is One Of The Most Valuable Resources In The World When It Comes To How To Recycle to Create a Better Future for Our Children.

Get My Free Ebook


Post a comment