Runoff

The unequal runoff distribution among the continents (see Table 2.2) conceals an even greater runoff complexity resulting from the coupled spatial

Table 7.1. Large rivers of the world listed according to annual discharge

Length

Drainage

Average annual

River

Continent

Discharge into

(km)

area (103 km2)

discharge (km3)

Amazon

South America

Atlantic Ocean

6308

6915

6923

Ganges-

Asia

Bay of Bengal

2897

1621

1386

Brahmaputra

Congo

Africa

Atlantic Ocean

4370

3457

1320

Orinoco

South America

Atlantic Ocean

2740

948

1007

Yangtze

Asia

East China Sea

6300

1959

1006

La Plata

South America

Atlantic Ocean

4700

3100

811

Yenisei

Asia

Kara Sea

5540

2580

618

Lena

Asia

Laptev Sea

4345

2490

539

Mississippi

North America

Gulf of Mexico

5971

3221

510

Mekong

Asia

South China Sea

4500

810

505

patterns of precipitation and evapotranspiration for individual continents. Global continental runoff is an estimated 35% of terrestrial precipitation, and the world's 50 largest rivers account for 57% of the global runoff. However, large differences exist among the discharges into individual ocean basins. The Atlantic Ocean receives over twice the runoff delivered to the Pacific Ocean and more than four times the river discharge into the Indian Ocean (Dai and Trenberth, 2002). The data in Table 7.1 illustrate the disparity of river discharge into ocean basins.

Figure 7.21 reveals latitudinal differences in runoff evident by river discharge into the oceans. The discharge spike for the zone from the equator to 5° S is due to the Amazon and Congo rivers discharging into the Atlantic Ocean at these latitudes. These rivers are the world's two largest by volume (Dai and Trenberth, 2002). The domination of the Northern Hemisphere zones at latitudes higher than 5° results from the relatively greater land areas at middle and high latitudes in the Northern Hemisphere. Another notable feature is the runoff minimum at 27.5° N which corresponds to the expanse of arid lands at these latitudes. However, seasonal runoff variations related to snowmelt at higher latitudes and high altitudes (Lee et al., 2004), the onset of evapotranspiration during the growing season in middle latitudes (Czikowsky and Fitzjarrald, 2004), and runoff changes at the close of the twentieth century from Arctic and sub-Arctic watersheds (McClelland et al., 2006) are not evident in Figure 7.21. These influences are evident in the comprehensive analysis of streamflow seasonality and variability for over 1300 sites globally reported by Dettinger and Diaz (2000).

Latitude

Fig. 7.21. Annual mean runoff into the global oceans by latitude zone using a 5° latitude running mean. (After Dai and Trenberth, 2002, Fig. 8. Used with the permission of the American Meteorological Society.)

Latitude

Fig. 7.21. Annual mean runoff into the global oceans by latitude zone using a 5° latitude running mean. (After Dai and Trenberth, 2002, Fig. 8. Used with the permission of the American Meteorological Society.)

Fig. 7.22. Global annual mean runoff. (From Barry, 1969, Fig. 1.1.8. Methuen and Co. Ltd. Reproduced by permission of Taylor and Francis Books UK.)

Figure 7.22 provides a high-resolution view of the variable nature of annual runoff for the continents. This map is based on observational data, which are considered more reliable than model-derived runoff (Kalnay et al., 1996; Coe, 2000). The vast expanse in Africa, Asia, Australia, and North America with runoff less than 5 cm is striking. The low runoff area of South America is modest and overshadowed by the abundant runoff characteristic of the Amazon Basin and the northern coastal zone. In North America, the contrast

Table 7.2. Large lakes of the world listed according to the lake surface area

Lake

Continent

Surface area (km2)

Lake volume (km3)

Caspian Seaa

Asia

374 000

78 200

Superior

North America

82 367

12 232

Victoria

Africa

68 800

2 750

Aral Seaa,b

Asia

62 000 (28 000)

1 066 (181)

Huron

North America

59 570

3 540

Michigan

North America

58 016

4 870

Tanganyika

Africa

32 000

17 800

Baikal

Asia

31 500

23 000

Great Bear

North America

31 153

2 240

Nyasa (Malawi)

Africa

30 044

8 400

a Considered a saline lake because it is landlocked.

b Irrigation diversions from tributary streams beginning in the 1960s reduced inflows. Area and volume are now variable. Current representative values are shown in parentheses.

a Considered a saline lake because it is landlocked.

b Irrigation diversions from tributary streams beginning in the 1960s reduced inflows. Area and volume are now variable. Current representative values are shown in parentheses.

between the heavy runoff along the Pacific Coast contrasts markedly with the low to modest runoff extending from the central United States into northern Canada. Europe displays moderate runoff increasing at higher latitudes. Peel et al. (2001) provide a further assessment of continental differences in annual runoff variability.

7.12.1 Lakes and reservoirs

Lehner and Doll (2004) estimate there are nearly 250 000 lakes and reservoirs worldwide larger than 0.1 km2 covering a total area of 2.7 million km2. The volume of water in freshwater lakes is 90 000 km3, but large lakes account for a disproportionately high percentage of this volume (Table 7.2). The volume of water stored in over 10 000 reservoirs worldwide, not including regulated natural lakes, is estimated at 4286 km3. The largest reservoirs represent a minority of the total number of reservoirs, but large reservoirs are the common focus of public attention.

Lakes and reservoirs are unequally distributed globally. The majority of lakes occur in the Northern Hemisphere, the maximum number occurs at 35° N, and over 60% of the world's lakes are in Canada. The maximum number of large reservoirs occurs between 30° N and 55° N. However, a significant number of large reservoirs exist between 30° N and 30° S, reflecting major dam construction in South America, Africa, and Southeast Asia (Lehner and Doll, 2004). There are over 40 000 dams worldwide higher than 15 m, and more

Table 7.3. Large reservoirs of the world listed according to reservoir water storage volumea

Surface

Surface

Table 7.3. Large reservoirs of the world listed according to reservoir water storage volumea

Reservoir

River

Country

Continent

Volume (km3)

area

Bratskoye

Angora

Russia

Asia

169.3

5478

Nasser

Nile

Egypt, Sudan

Africa

162

6000

Kariba

Zambesi

Zambia, Zimbabwe

Africa

160

5400

Volta

Volta

Ghana

Africa

148

8502

Guri

Caroni

Venezuela

South America

129

4250

Krasnoyarskiye

Yenisei

Russia

Asia

73.3

2000

Williston

Peace

Canada

North America

70.3

1779

Zeiskoye

Zeya

Russia

Asia

68.4

2119

Kujbyshevskoye

Volga

Russia

Europe

58

5900

Cabora Bassa

Zambezi

Mozambique

Africa

55.8

2739

a Natural lakes regulated by a dam are not included.

a Natural lakes regulated by a dam are not included.

than half of these large dams are in China (ICOLD, 2003). Table 7.3 shows selected characteristics of several large reservoirs emphasizing the water volume impounded by large dams. It is estimated that 77% of water discharged from rivers in the Northern Hemisphere is from watersheds regulated by dams or other human influences.

7.12.2 Wetlands

Permanent and intermittent wetlands cover 8-10 million km2 in a variety of hydroclimatic settings. Wetlands have a maximum development around 60° N with a secondary peak near the equator. They are more widespread in the Northern Hemisphere than in the Southern Hemisphere (Lehner and Doll, 2004).

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