Water for people.pdf - WHO Thailand Digital Repository

8 8 / 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 4.5: Sediment load by basin400(in million tons/year)Changes in sediment yield reflect changes in basin conditions, including climate, soils, erosion rates, vegetation, topography and land use. It is influenced strongly by human actions,such as in the construction of dams and levees (see high sediment load in China and the Amazon basin, where large dams have been implemented), forest harvesting and farmingin drainage basins.Source: Syvitski and Morehead, 1999.particularly with respect to runoff. After a calibration period, landuse was altered in one basin and the differences in the hydrologicalbehaviour of the pair were quantified during the ensuing period.Clear-cutting of forest, the effects of fire, different croppingpractices, grazing and afforestation were among the changesstudied. Coweeta in the United States, Valdai in Russia andJonkershoek in South Africa were among those locations wherethese experiments were launched in the 1920s and 1930s.A large number of studies of this type were started in differentparts of the world in the 1950s and 1960s and the majoritycontributed to the programme on representative and experimentalbasins, which formed a prominent part of the InternationalHydrological Decade (IHD, 1965–74) of UNESCO. Research practicewas strengthened (Toebes and Ouryvaev, 1970) and resultscompared (IAHS/UNESCO, 1970) and as time went on, greaterinterest was given to water quality matters. The results from manyrepresentative and experimental basins were harnessed to theFRIEND Programme, to gain better knowledge of the effects ofhuman activities on the hydrological cycle on a regional scale and toupgrade the assessment of water resources. The European Networkof Experimental and Representative Basins (ERB), which waslaunched in 1986, developed a study of methods of hydrologicalbasin comparison (Robinson, 1993). Maksimovic (1996) provided areview of methods for investigating urban hydrology. At the start ofthe twenty-first century with the IHP broadened towards the social,political and environmental dimensions of water and waterresources, it is fitting that the Hydrology, Environment, Life andPolicy Programme (HELP) is returning to a network of selected riverbasins across the world as the basis for their study.There are some global overviews of the results from thesevarious studies of representative and experimental basins, forexample in Falkenmark and Chapman (1989), but few recentpublications. One reason may be that conditions vary to such anextent that the conclusions from one set of basin studies may nothold in another set sited in a different climatic zone. There is alsothe problem that there are many results for temperate latitudes andrelatively few for other regions of the globe, despite increasedattention to key regions such as the humid tropics (Bonell et al.,1993). Nevertheless, Ellis (1999) summarized the findings of fiftytwostudies mainly concerned with the impact of urban areas on thehydrological cycle. He concluded that: