Humanity has long paid great attention to water, because it was well known that where there is no water, there is no life. In dry land, grain can lie for many years and grows only in the presence of moisture. Despite the fact that water is the most common substance, it is distributed very unevenly on Earth. On the African continent and in Asia, there are vast areas devoid of water - deserts. The whole country, Algeria, lives on imported water. Water is delivered by ship to some coastal areas and islands in Greece. Sometimes water costs more than wine. According to the United Nations, in 1985, 2.5 billion people around the world lacked clean drinking water.
The surface of the world is 3/4 covered by water - oceans, seas, lakes, glaciers. Water is abundant in the atmosphere and in the Earth's crust. The total free water resources on Earth are 1.4 billion km3 . The main amount of water is contained in the oceans (about 97.6%), and there is 2.14% water in the form of ice on our planet. Water from rivers and lakes is only 0.29% and atmospheric water 0.0005%.
Water is in a constant and active cycle. It is driven by the sun and the main source of water is the ocean. Almost a quarter of all solar energy falling on the Earth is used to evaporate water from the surface of water bodies. Every year, 511,000 km3 of water rises into the atmosphere, including 411,000 km3 from the ocean surface. Approximately 2/3 of the atmospheric water returns in the form of precipitation back to the ocean, and 1/3 falls on land. The annual amount of precipitation is 40 times higher than the water vapour content in the atmosphere. Having fallen out at once, they could form a layer of 1 m thick on the Earth. This water replenishes glaciers, rivers and lakes. In turn, the mainland surface waters flow back into the seas and oceans, dissolving the rocks they meet in their path. The water that moisturises the soil is absorbed by the roots of the plants. Together with water, plants receive dissolved nutrients. In plants, it rises on the stems and returns as steam to the atmosphere through leaves. Mountain glaciers are an important water regulator on land. They give away water mainly in the summer months, when the mountain ice and snow melt particularly intensively. It is worth mentioning that glaciers are the main repository of fresh water on our planet. It is estimated that they contain about 30 million km3 of fresh water, while all rivers - no more than 1.2 thousand km3.
Thus, the water is in constant motion on the Earth. The average time of its stay in the atmosphere is estimated at 10 days, although it changes with the latitude of the area. For the polar latitudes it can reach 15, and in the middle - 7 days. Water change in rivers occurs on average 30 times a year, i.e. every 12 days. Moisture in the soil is renewed within 1 year. Water of flowing lakes is exchanged during tens of years, and non-perflowing lakes - during 200...300 years. World Ocean waters are renewed on average over 3000 years. From these figures, one can get an idea of how long it takes to self-clean water bodies. It is only necessary to keep in mind that if a river flows out of a polluted lake, the time of its self-cleaning is determined by the time of self-cleaning of the lake.
The water cycle is an extremely important process. It provides the land with fresh water, which is constantly renewed. In the process of this cycle, water destroys and dissolves the hard rocks on land and transports them to other places with the formation of sediment. Of course, wind and volcanic eruptions, solar exposure and earthquakes, and later living organisms, have also contributed to the destruction and alteration of the Earth's surface.
Thus, water played an important role in the life of our planet as a vehicle in geological transformations. Groundwater is a constant source of destruction and creativity, forming deposits of mineral resources. However, the period of human observation of geological processes is so short that the changes are practically invisible.