Water is an important liquid that sustains life among living things in the planet. Without water, life on earth will cease to exist as it plays a vital role in carrying out the processes of the basic unit of life, i.e., the cell.
Water comes in many forms. It may be in the form of solid (ice), liquid, or as water vapor in its gaseous state. These many forms of water comprise the water cycle which ensures that water is made available to plants and animals and is filtered in the process.
But what basically is the water cycle? What are the processes that make up the water cycle and how is this relevant to the survival of living organisms?
Water Cycle Defined
The water cycle is a never ending cycle of the different forms of water in three major components of the biosphere or areas where life exist. The major processes of the water cycle starting from its liquid state in rivers, lakes and oceans are the following:
Evaporation is the process of conversion of liquid water to gas or water vapor which is invisible to the human eye. This is made possible by the heat coming from the sun that excites the molecules of water causing them to break the bonds (called hydrogen bonds) that hold them together. Once this happens, water molecules are set apart from each other such that they become lighter thus go up to become part of the atmosphere.
2. Evapotranspiration or Transpiration and Perspiration
The liquid form of water may be coming directly from water bodies like rivers, lakes, the seas and oceans. They may, however, originate from both plants and animals. If water originates from plants, this is termed evapotranspiration or simply transpiration (Fig. 1), while in animals evaporation is made possible through perspiration. The latter is seldom described in literature about the water cycle but is likewise an important process of the water cycle considering that there are billions of animals on earth (Did you hear that story about cows causing global warming due to farting?).
Fig. 1. Tropical forests are usually covered with clouds as products of transpiration.
Condensation is the process wherein the molecules of water get attracted to each other due to the cooling effect of the upper sections of the atmosphere. The water molecules become denser and is now visible to the human eye as clouds of varying heights (Fig. 2).
Fig. 2. Clouds of various shapes form due to condensation of water vapor.
The lowest lying clouds contain the densest water vapor and are darker in appearance. The water molecules are now getting heavy and once these molecules reach a critical mass, water vapor once again returns to its liquid state and fall back to earth as precipitation.
Precipitation is the process of water vapor conversion back to its liquid (rain) or solid (snow, hail) state. Precipitation occurs in various parts of the earth; the snow or hail in the cooler regions of the earth or rain in the tropical regions. Precipitation may occur on either land to replenish rivers, ponds or lakes or on large bodies of water like the seas and oceans. There are some areas on earth, however, where precipitation seldom or does not fall at all. These areas are called deserts.
Runoff applies to rain as precipitation. Water in its liquid form travels on land replenishing the river waters, the ponds and the lakes, while some travel all the way down to the sea or ocean.
6. Assimilation or Percolation
Assimilation or percolation occurs when water or melted snow is absorbed by the soil, seeps through the ground and becomes part of the ground water (Fig. 3.). The ground water may be extracted through pipes for human use. Ground water collected in the higher elevations flow underground and are leaked somewhere as spring or connect with lower lying underground water.
Fig. 3. Water under the ground forms part of the water cycle.
All these processes make up the water cycle. The speed by which these processes is influenced by human activities and economic development. Evaporation, for example is sped up by deforestation due to the absence of trees that once covered the land. Cloud seeding can hasten precipitation and percolation prevented by concreted pavements and modern highways. But all through the whole process, the water that was available millions of years ago is the same water that we drink today.
Water Education Foundation
© Patrick A. Regoniel 24 January 2011 What is the water cycle?