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Chemistry is all around us
Copyright 2015
This project has been funded with
support from the European Commission

Educational Packages

Chemistry in Everyday life

Water cycle


The ecosystem is composed of animate and inanimate entities that are in constant interaction with each other. The ecosystem renews and feeds itself only through the material and the energy exchange and cycle among these entities. The energy for the ecosystem is provided from the sun, however, water and other chemical materials for the sustainability of life are present on earth. The presence of materials is limited in the ecosystems and the materials are opt to get exhausted unless they are replaced. It is only through the recycling of the inorganic matter taken during the material and energy cycles into the inanimate environment that the ecosystems can maintain their balance and sustain their presence (for the continuance of life on earth).

Materials such as carbon, water, oxygen, nitrogen, and phosphorous are used in a cyclic manner in the ecosystem for the sustainability of life in nature. Therefore, the living organisms take these materials that they require from the environment, they use them and they give them back to the environment in some way and constantly recycle it. They are renewed and reused by the organisms. As stated by the first law of thermodynamics, matter cannot disappear from the environment but it may only go through chemical changes.

The circulation of the materials required for the continuation of life in the ecosystem between the animate and the inanimate surroundings is called the Ecological Material Cycles, or the Material Cycles. Since these cycles also have biological, chemical and geological components, they are also called the Biogeochemical Cycles. The difference of the material cycles from the energy flow is that they circulate in the ecosystem. The material cycle movements are enabled through the solar energy.

Water Cycle

Water moves in systematic manner between the atmosphere and the land and water on earth through some natural forces and air movements. This is called the water cycle or the hydrological circulation. The water vapor arising into the atmosphere from various resources through solar heating, returns back onto the surface of the earth as precipitation in the forms of rain, snow or hail. A portion of this water joins the underground water reserves while a larger portion accumulates in sources such as lakes and seas.

Water cycle, as all other cycles, displays continuance. Plants also aid in the water cycle through perspiration. All water on earth joins in the cyclic movement of water. Thus, the water evaporating from the seas returns back onto the earth in the form of precipitation and while some accumulates in surface waters, another portion blends in with the underground waters. The final destination for the underground waters is the seas and the oceans.

The water that has been collected there continues the water cycle (the long water cycle). The water evaporating from the seas and the oceans to return back to the surface of the earth in the form of precipitation such as rain, snow or hail without passing through the underground is called the short water cycle. The water rising above through evaporation and perspiration condense in the clouds. Precipitation occurs as a result of this. A portion of the water returning in the form of precipitation is stored in surface waters (lakes and seas). Another portion blends in with the underground water reserves. The water passing through the earth flows towards the seas in the form of the underground water. The water cycle is completed in this manner.

Water molecules move within the lithosphere, hydrosphere and the atmosphere through the activation by the solar energy and gravity. As a result of this movement, the large system called the water cycle is formed. Oceans play an important role in the water cycle. The humidity occurring as a result of the evaporation of the waters from the surface of the earth forms the clouds when it condenses. Precipitation, which is among the atmospheric movements, constitutes a significant portion of this cycle where water is reused over and over again. A single rain droplet evaporates many times in a year and returns back to the earth in the form of precipitation. This way, approximately 4 million km3 of water moves between the lands and the atmosphere.

Water cycle is completed in three phases.

  1. Evaporation: It is the transfer of water to the atmosphere through the evaporation of water as a result of warming and in the form of perspiration in plants. Evaporation on earth takes place as a result of the warming effect of the sun. The evaporation areas for water are the surfaces of the seas, oceans, streams, rainy areas and the soil surface. In addition, the perspiration of plants and other organisms is also among the means of evaporation. Water is released into the atmosphere through evaporation.
  2. Condensation: It is the formation of the clouds when the water vapor cools and becomes saturated in the form of water droplets and ice crystals.
  3. Precipitation: It is the falling of the condensed water droplets in the form of rain drops by further cooling through the effect of gravitation. A portion of the water falling onto the surface of the earth is taken by the plant roots from the depths of the soil and another portion is used by the other organisms.

Most of the water falling onto the surface of the earth through precipitation blend in with the seas, oceans, lakes and streams in the form of surface flows or they reach seas through streams, lakes and underground waters, completing the cycle. .

Precipitation is a significant part of the water cycle, enabling the reuse of water over and over again. A single rain droplet evaporates many times in a year and returns back to the earth in the form of precipitation. This way, approximately 4 million km3 of water moves between the lands and the atmosphere. The type of the layer of soil, the inclination of the land, the vegetation, the position and the condition of the underground and the surface waters all affect the water cycle. The natural vegetation prevents flood damage and enables even distribution of precipitation. As the natural vegetation gets weak, the amount of precipitation decreases and as the amount of precipitation decreases, the mineral cycles that are the source of vitality for living organisms are also broken.

In other words, this process is enabled through the evaporation of the water in the land and the seas by the solar energy into the atmosphere, the formation of water droplets from the water in the vapor form and the recycling of the water to the surface through gravitational forces. Through evaporation of the surface waters, the cycle begins again. The interesting notion in this phenomenon is that a single rain drop evaporates and precipitates approximately 40-42 times in a year and thus, 400 000 km³ water moves between the lands and the atmosphere. However, as a result of this phenomenon, the earth receives on average 1000 mm precipitation per year. Approximately 660 mm of this falls on the land and the rest on the oceans. If there were no hydrological circulation and the humidity in the atmosphere would only condense and precipitate once a year, this amount would have bee 24 mm/year (Figures below). For this reason, hydrological circulation is crucial in terms of water economy. Therefore, this natural process needs not to be artificially altered by people in any way.

The following interesting facts are given regarding this process (The USA Ministry of Agriculture, 1999)
  • Approximately 333.000 km³ of water evaporates from the oceans every year.
  • The amount of water evaporating from the surface of the lands is 63.000 km³.
  • All this water moves between the earth and the atmosphere.
  • However, only 100.000 km³ of water is recycled back in the form of precipitation. The remaining either evaporates as it moves towards the earth in the form of precipitation or binds in solid form in the poles or accumulates hundreds of meters below the earth crust.
  • The average amount of precipitation on land is 600 mm per year and on seas is 340 mm, totaling up to approximately 1000 mm for the whole earth.
  • The distribution of precipitation is extremely different. For instance, the tropical rain forests get approximately more than 10 000 mm precipitation per year whereas most deserts get precipitation below 25 mm.

Advantages of the Water Cycle:

All organisms live on the surface or in a very thin layer of soil very close to the surface of the earth and provide their needs other than the solar energy from the resources that this layer contains. If the water, oxygen and the other materials that are required for the sustainability of life would only have been used once, all would have depleted by now. The arrangement of all processes in nature in the form of cycles enables the renewability of all processes forever. Material cycles enable a constant exchange between the air, water, earth, plants and animals, the reusability of all of the earth’s resources and the sustainability of life.

If water cycle did not exist on earth, the possibility for the organisms to survive would diminish. For instance, approximately 1000 mm of precipitation falls onto earth surface every year. If there were no water cycle, this amount would have been 24 mm. The amount of water that would be held in the form of clouds in the atmosphere only allows for 24 mm of precipitation. For this reason, it is only through water cycle that a single rain droplet evaporates and precipitates for 40-42 times, enabling 1000 mm of precipitation on average.