Water for people.pdf - WHO Thailand Digital Repository

8 4 / A L O O K A T T H E W O R L D ’ S F R E S H W A T E R R E S O U R C E SThe Natural Water CycleMap 4.4: Long-term average runoff on a global grid(in mm/year)< 0 0 10 50 100 200 300 500 1,000 [max 6,160]The enormous variation in climate around the Earth leads to great variability in the streamflow, which is in line with the rainfall. This map shows similar patterns to map 4.1.Source: Map prepared for the World Water Assessment Programme (WWAP) by the Centre for Environmental Research, University of Kassel, based on Water Gap Version 2.1.D, 2002.swamps associated with them, for example from the Indus (Pakistan),the Niger (West Africa), the Nile (eastern Africa) and the Colorado(Argentina). About 1,100 km 3 of runoff per year is lost in this way(Shiklomanov, forthcoming). For the large number of rivers with agroundwater component, this is included in the determination of flow,even though some groundwater flows to the oceans directly.The world’s largest river, the Amazon in Latin America, contributessome 16 percent of the global total annual runoff, while the fivelargest river systems (Amazon, Ganges with the Brahmaputra in India,Congo in Central Africa, Yangtze in China and Orinoco in Venezuela)together account for 27 percent (see table 4.7).These figures are derived from the forthcoming study byShiklomanov who collected and analysed flow records from the worldhydrological network divided between twenty-six homogeneous andcomparable regions covering the globe (Shiklomanov, 1998a). The2,500 most suitable records were selected from this network andadjusted to the period 1921 to 1985. This adjustment was necessarybecause although a few of the records were for longer periods ofobservation, many were for shorter periods, many records had gaps andsome had to be estimated from precipitation totals. From this study, theaverage total flow per year from the land surface to the ocean wasestimated to be 42,800 km 3 with slight variations from year to year.There are groups of wet years and dry years, but no trend over thesixty-five-year period. In terms of water resources the variability fromone year to the next is very important but this variability is masked bythe averaged data. This distorted view pertains particularly to arid andsemi-arid regions, where the coefficient of variation (C v) of annualdischarges is in excess of 0.7 and where the driest years can experiencea discharge of less than 10 percent of the long-term average. For wetregions (with average annual rainfall greater than 1,000 mm), annualvariability is benign and the coefficients of variation are smaller (typicallybetween 0.15 and 0.3) and the driest year is rarely less than 40 percentof the long-term average. So, where river flows are lowest around theworld, the year-to-year variability is highest. Smaller rivers show greaterannual variability than larger rivers. Runoff is unevenly distributedthrough the year for most regions of the globe with 60 to 70 percentoccurring in the spring and early summer and 2 to 10 percent in thethree driest months. For example, in Russia and Canada between 55to 70 percent of the runoff occurs between May and August, while47 to 65 percent of the runoff in India and China is between July andSeptember. Floods contribute a large proportion of the flows duringthese periods when they transport the major part of the annual loadof sediment and of materials in solution (see table 4.8). A number ofsevere floods have occurred in recent years, for example those on the