Water for people.pdf - WHO Thailand Digital Repository

T H E N A T U R A L W A T E R C Y C L E / 8 5Table 4.7: The largest rivers in the world by mean annual discharge with their loadsSuspended DissolvedMean annual Maximum Minimum solids solidsBasin area discharge discharge discharge Runoff Volume (million (millionRiver (km 2 ) (m 3 /sec) (m 3 /sec) (m 3 /sec) (mm/year) (km 3 ) tons/year) tons/year)Amazon 4,640,300 155,432 176,067 133,267 3,653 4,901 275 1,200(South America)Congo 3,475,000 40,250 54,963 32,873 1,056 1,296 41 43(Central Africa)Orinoco 836,000 31,061 37,593 21,540 1,172 980 32 150(Venezuela)Yangtze 1,705,383 25,032 28,882 21,377 463 789 247 478(China)Brahmaputra 636,130 19,674 21,753 18,147 975 620 61 540(India)Yenisei 2,440,000 17,847 20,966 15,543 231 563 68 13(Russian Federation)Lena 2,430,000 16,622 19,978 13,234 216 524 49 18(Russian Federation)Parana 1,950,000 16,595 54,500 4,092 265 516(Argentina)Mississippi 3,923,799 14,703 20,420 10,202 118 464 125 210(United States)Ob 2,949,998 12,504 17,812 8,791 134 394(Russian Federation)The world’s largest river, the Amazon, contributes by itself some 16 percent of the global total annual stream water flow, and the Amazon and the other four largest river systems (Congo, Orinoco,Yangtze, Brahmaputra) combined account for 27 percent.Sources: GRDC, 1996; Berner and Berner, 1987.Oder (Germany) in July 1997, those generated by the supercyclone inOrissa (India) in October 1999 and those from cyclone Eline, whichaffected Mozambique and neighbouring countries in February 2000(Cornford, 2001).Water qualityThe quality of natural water in rivers, lakes and reservoirs and belowthe ground surface depends on a number of interrelated factors.These factors include geology, climate, topography, biologicalprocesses and land use, together with the time the water has been inresidence. However, over the last 200 years human activities havedeveloped to such an extent that there are now few examples ofnatural water bodies. This is largely due to urban and industrialdevelopment and intensification of agricultural practices, combinedwith the transport of the waste products from these activities bysurface water and groundwater and by the atmosphere. The scale andintensity of this pollution varies considerably. Table 4.8 shows someof the chemical determinants of the world’s average rivers, bothnatural and polluted.There are global problems such as heavy metals, regional problemslike acid rain and much more localized ones – groundwatercontamination, for example. In many places groundwater has becomeTable 4.8: The chemical composition of average river water (concentration in milligrams/litre)Silicon TotalCalcium Magnesium Sodium Potassium Chlorine Sulphate Bicarbonate dioxide Dissolved(Ca ++ ) (Mg ++ ) (Na + ) (K + ) (Cl) (SO 4 ) (HCO 3 ) (SiO 2 ) Solids (TDS)Actual 14.7 3.7 7.2 1.4 8.3 11.5 53.0 10.4 110.1Natural 13.4 3.4 5.2 1.3 5.8 8.3 52.0 10.4 99.6The difference shown in this table between the natural and the actual chemical composition of river water highlights the importance of the water pollution all over the world.Source: Meybeck, 1979.