International Hydrological Programme
Europe
Occupying an area of 10.46 million km2, Europe is one of the world’s most densely populated and developed regions. It has a dense hydrological network containing a number of stations with 200 years or more of records. This network is best developed in the west and least in the east of the continent.
Precipitation, in general, increases westwards to the Atlantic rim from about 400 mm in Russia and Poland to over 4,000 mm a year in Norway and Scotland. It also increases with altitude such as in the Alps. However, parts of the south, Spain and Italy, for example, receive low rainfall amounts and experience high rates of evaporation that cause water resource problems.
Although there are some large rivers, such as the Volga (250 km3), the Danube (225 km3) and the Rhine (86 km3), the majority are relatively small, many with steep courses. The mean annual runoff from Europe, for the period of 1921 to 1985, is estimated at 2,900 km3 per year, most from northern Europe and least from the southern part of eastern Europe. The number of lakes and reservoirs is large and there are extensive aquifers.
Over the last 200 years, industry, energy generation, agriculture and urban development have changed the pattern of runoff from the continent and altered its quality characteristics. Many rivers and lakes are badly polluted by discharges of untreated sewage, mine wastes and agricultural effluents.
North America
North America, including its adjacent islands to the north and south, has an area of 24.25 million km2 and a population of some 450 million, more than half living in the highly developed United States and Canada.
These countries have the most advanced hydrological networks of the world, with routine use made of radar and satellite data.
The relatively simple structure of the continent, with high mountains in the western third and vast plains extending to the east towards lower mountains, allows the arctic and the tropics to influence the climate through their weather extremes – the hurricanes, for example, which track across the south.
Precipitation roughly follows the same pattern as the relief. Along the Pacific rim rainfall can reach 3,000 mm and more at higher altitudes; considerable variations occur within the western mountains and plateaux, while annual totals of between 500 and 1,500 mm occur eastwards. To the south there are very dry areas shared by Mexico and the United States. The north is dominated by the Great Lakes and a large number of others, while there are many reservoirs and extensive aquifers over much of the region.
The Mississippi-Missouri is the principal river system, followed by the St. Lawrence, Mackenzie, Columbia and Colorado. These rivers and the multitude of smaller ones carried an annual average of runoff of about 7,900 km3 to the surrounding seas over the period from 1921 to 1985. Throughout the nineteenth and twentieth centuries, human activities considerably changed the natural pattern of runoff in the majority of drainage basins and the position of the water table in many aquifers. The flow in most rivers is regulated, while numerous abstractions and discharges are made for a variety of different purposes. Large interbasin transfers take place in Canada to assist power generation. Agriculture, particularly irrigated agriculture in the west, causes resource problems and pollution arises from industry and mine wastes. There have been serious flood problems in the Mississippi basin in recent years and there are recurring floods in countries fringing the Caribbean in the wake of hurricanes.
