International Hydrological Programme
Asia and the Pacific
The Asian continent has an area of 43.5 million km2 occupying one third of the land surface of the globe and supports a population of 3,445 million people.
It is a continent of great contrasts – contrasts in relief, climate, water resources, population density and standard of living, for example. There are also contrasts in the hydrological network: countries fringing the Pacific and Indian Oceans, such as Japan and Malaysia, have networks with high levels of capability and they contrast with those towards the centre of the continent where networks are generally deficient.
Asia’s geology and relief are very complicated and the climate is extremely varied, the monsoon dominating the south and east. Climatic differences are intensified by high mountain chains and plateaux, disrupting the pattern of precipitation that, in general, decreases from south to north and from east to west. High rates of evaporation occur across the southern half of the continent with areas of desert in the west and centre.
Some of the world’s largest rivers drain Asia to the Arctic, Pacific and Indian Oceans: the Ganges and the Brahmaputra (India); Yangtze (China); Yenisey, Lena, Ob, Amur (Russian Federation); and Mekong (South-East Asia) for example, but there are large areas draining to the Aral and Caspian Seas and further areas of inland drainage in western China.
The mean annual runoff from Asia for the period 1921 to 1985 is estimated to be 13,500 km3, about half of which originated in South-East Asia, in contrast to the Arabian Peninsula with an estimated 7 km3.
There are large aquifers and many lakes in Asia, such as Lake Baikal in the Russian Federation. China, India, Russia and Pakistan have a large number of reservoirs, primarily used for irrigation.
The continent faces serious flood problems and sedimentation problems, particularly in China, as well as pollution of surface water and groundwater in densely populated areas. However, the water problems of the basin of the Aral Sea are the most acute.
Australia is the smallest continent with an area of 7.6 million km2 and a population approaching 20 million. However, the islands of Tasmania and New Guinea, those of New Zealand and those comprising the remainder of Oceania add a further 1.27 million km2 and some 10 million additional people.
Australia and New Zealand are developed countries with advanced hydrological networks but they are less advanced over the rest of the region.
Australia is a large ancient plateau, raised along its eastern fringe, but the structure and geology of much of the remainder of Oceania is more varied and recent.
Australia is the driest continent with a mean annual precipitation of 200 to 300 mm over much of the country, with totals rising to 1,200 mm and more along the eastern fringe and to 1,000 mm in the south-west corner. By way of contrast, many of the islands have much higher rainfalls: for example along the west coast of New Zealand’s South Island 5,000 mm a year is recorded. Evaporation rates are high over Australia and over the rest of the region.
The rivers of Oceania are short and fast-flowing and produce an average annual runoff of about 2,000 km3. The average runoff from Australia is only some 350 km3 a year: except for the Murray Darling, most of the rivers are short and drain the eastern coast.
There are considerable quantities of groundwater, but there are problems of salinity, some induced by irrigation.
There are relatively few lakes in Australia and many of them are ephemeral.
