Figure 2. An average of 1,400mm of water is evaporated from the ocean surface t hat covers 70% of the earth's surface and carried to the atmosphere. Meanwhile, ocean sur-face receives 1,270mm of precipitation. The difference between the tw o (315mm when converted into land area) is carried by a large current in at mos phere (atmospheric general clrcu]ation) to the continent and contributes signif icantly to precipitation. Continents recelve an average of 800mm of precipit at ion, and about 60% of this precipitation returns to the atmosphere through tran spiration from plants and evaporation from denuded areas. The remainlng 40%, i. e. 315mm, flows into the ocean in the form of river water and groundwater.

This circulation of water is the force behind the scenes that makes it possible to use the limited freshwater resources indefinitely. The circulatlon makes isp ossible for many animals and plants to survive and enables the human society to sustain and develop Itself by steadily supplylng freshwater to land. Water circ ulation also helps development of topography by erodlng the continent surface a nd carrying a large volume of soil down-stream. Furthermore, various minerals s alts that are carried from land to ocean contrlb-ute significantly to increasin g biological productivity on coastal waters.