The Water Cycle – exchange of water between the ocean and atmosphere

The water cycle, or hydrologic cycle, refers to the exchange of water between the ocean and atmosphere. On the Earth, the oceans, which cover 71 per cent of the surface, are the major reservoir of the water? The oceans lose water by evaporation and gain water by precipitation.

This is where the process becomes intriguing. The oceans only gain about 90 per cent of the water that the evaporated. The remainder is transferred to the land and returns to the oceans as runoff. The total amount of water involved in the cycle, about 0.04 per cent of the Earth water, represents all of the water that appears in the atmosphere (some of which appears as rain), the rivers, lakes, wetlands, soil moisture, and groundwater.

At any one time, under present climatic conditions, 97.5 per cent of the water on the planet occurs in the oceans. The remaining 2.5 per cent is divided among the following categories. About 80 per cent is in the ice caps and glaciers and about 20 per cent is ground-water.

Lakes contain about 0.7 per cent of the water soil moisture holds about 0.44 per cent of the water, and the atmosphere and streams contain about 0.05 and 0.005 per cent respectively of the non-oceans water. Table 8.1 lists the approxi it illustrates the time, percentage, and amount of each reservoir; mate residence relatively small portion of total water involved in the land part of the cycle.

The land area occupied by open water also provides perspective of the water cycle. Lakes occupy about 2.5millionkm²1.7 per cent of the surface area of the continents of the Earty (Czaya 1981). They contain about 241,000 km³ of water. The major lakes of the Earth i.e., Lake Baikal of Asia the Great Lakes of North America, and the Rift Valley lakes of Africa contain almost three- quarters of the freshwater.

All of the remaining lakes, ponds, and wetlands share the remaining portion. Streams on the other hand occupy less than 0.1 per cent of the area of the land (Hynes 1970). Annually, rivers, which occupy about one-seventeenth of the land area of lakes, discharge about 1,000 km³ of water as stored in lakes.

Annually, streams, with only a fraction of the depth of lakes, handle about 2.6 times more volume of water per occupied area in lakes. That in essence describes the streams-a large volume of water flowing among a narrow ribbon of band.