THE WATER CYCLE

The water cycle describes the existence and movement of water on, in, and above the Earth. The water cycle has been working for billions of years and all life on Earth depends on it continuing to work and the Earth would be a pretty stale place without it.

The hydrologic cycle begins with the evaporation of water from the surface of the ocean. As moist air is lifted, it cools and water vapor condenses to form clouds. Moisture is transported around the globe until it returns to the surface as precipitation and once the water reaches the ground, one of two processes may occur; some of the water may evaporate back into the atmosphere or  the water may penetrate the surface and become groundwater. Groundwater either seeps its way to into the oceans, rivers, and streams, or is released back into the atmosphere through transpiration. The balance of water that remains on the earth's surface is runoff, which empties into lakes, rivers and streams and is carried back to the oceans, where the cycle begins again.

Water cycle as a process follows the following stages to accomplish its journey though they are not universal since it can start at any stage and back to that initial stage.

Evaporation is the process by which water changes from a liquid to a gas or vapor. Evaporation is the primary pathway that water moves from the liquid state back into the water cycle as atmospheric water vapor. It can happen when liquids are cold or when they are warm. It happens more often with warmer liquids. Furthermore, the process is known as evaporation when occur in the water bodies while it called transpiration when the plant loses its water via stomata. This is the discharge of water vapor from the leaves of plants into the atmosphere. It is a process that the eye cannot see, even though the amounts of moisture involved it significant. The term ‘evapotranspiration’ is used when there is also evaporation of moisture from the soil, together with that from the leaves of the plants. (Shiklomanov I.A., 2003)

Condensation is the process of change of water from its gaseous form (water vapor) into liquid water. Condensation generally occurs in the atmosphere when warm air raises, cools and loses its capacity to hold water vapor. As a result, excess water vapor condenses to form cloud droplets. The upward motions that generate clouds can be produced by convection in unstable air, convergence associated with cyclones, lifting of air by fronts and lifting over elevated topography such as mountains. (Bigg, 2003)

The definition of precipitation is any form of water - liquid or solid - falling from the sky. It includes rain, sleet, snow, hail and drizzle plus a few less common occurrences such as ice pellets, diamond dust and freezing rain. Precipitation is the primary mechanism for transporting water from the atmosphere to the surface of the earth.  Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor, so that the water condenses and "precipitates. Precipitation is a major component of the water cycle, and is responsible for depositing the fresh water on the planet.

Runoff is the movement of land water to the oceans, chiefly in the form of rivers, lakes, and streams. Runoff consists of precipitation that neither evaporates, transpires nor penetrates the surface to become groundwater. Even the smallest streams are connected to larger rivers that carry billions of gallons of water into oceans worldwide. It is precipitation that did not get (infiltrated) absorbed into the soil, or did not evaporate, and therefore made its way from the ground surface into places that water collect. The process always depends on the underlying factors which abundantly include nature of the slope, the underlying soil type of the area, the amount of water and gravitational force. (Shiklomanov, 1998)

Infiltration is the process by which precipitation or water soaks into subsurface soils and moves into rocks through cracks and pore spaces. Water can be absorbed by the soil and may stay in the soil for long time until it gradually gets evaporated. If there is a lot of vegetative cover (green plants) the infiltrated water can also be absorbed by plant roots and later transpired. Infiltration occurs at the upper layers of the ground but may also continue further downwards into the water table. The rate of infiltration depends on factors such as the amount of precipitation, the type of soils, the amount of vegetative cover over the area, pre-saturation levels, the topography of the land as well as the levels of evapotranspiration in that region. (Curry & Peter, 1999)

The water cycle is simply the complete journey that water makes in its life, from one place to the other, and from one state to the other. As the word cycle suggests, there is no starting point. This means that we can begin at any point and follow its path until it gets to where we started again. Therefore, following this stage, the water back to the initial state to start the same journey since this process is endless process.

The whole process of water cycle from the first stage to the last one  can best be illustrated using the following diagram;

REFERENCES

 

Bigg, G. R. (2003). The oceans and climate. Cambridge, New York: Cambidge University Press.

Curry, J. A. (1999). Thermodynamics of atmospheres and oceans. . San Diego: Academic Press.

Shiklomanov I.A,. (1998). Man’s Impact on River Runoff, Leningrad, Gidrometeoizd. Methodological approaches to assessment and analysis of the effect of different human activities on the , 432.

Kuchment, L. S. (2004). THE HYDROLOGICAL CYCLE AND HUMAN IMPACT ON IT, in Water management resources. Oxford: EOLSS Publisher.

Shiklomanov I.A., R. J. (2003). World Water Resources at the Beginning of the 21st Century. Cambridge: Cambridge University Press.