The water cycle, or the hydrologic cycle, refers to the continuous circulation of water within the Earth’s hydrosphere. Water moves into and from the various reservoirs on, over, and under the surface of the Earth, and in the process transforms into its various phases of solid (ice), liquid (water), and gas (vapor), with the total mass of water remaining fairly constant. The physical processes of evaporation, condensation, sublimation, precipitation, transpiration, and runoff are responsible for sustaining the water cycle. Heat energy is also exchanged during the cycle, with this store and release of heat affecting climates worldwide. The water cycle is highly crucial to maintaining life on Earth, as it replenishes the world’s freshwater resources and moderates extremes in climate.
Physical Processes Involved in the Water Cycle
Various physical processes act in coordination in order to keep the water cycle active at all points of time. Evaporation involves the vaporization of water from the liquid phase, triggered by the heat energy source from solar radiation. Evaporation can also be extended to include transpiration, a process of water loss from plants' leaves, and perspiration, which is similar water loss from animals under conditions of heat stress. 90% of atmospheric water is contributed to by evaporation, and 10% by transpiration from plants. Condensation and advection refers to the transformation of evaporated water vapors into liquid water droplets suspended in the air as clouds or fog, and the movements of such condensed atmospheric water between ocean and land. The condensed water reaches the land and ocean surface by the process of precipitation. Some precipitation is subjected to canopy interception, wherein vegetation on land absorbs the water before it reaches land. Water that reaches the land is subjected to runoff (or melt water in the case of snow), and it moves across the land towards the oceans in the form of rivers draining into the seas. In the process the water also gets evaporated, seeps underground (in a process known as infiltration wherein water collects in the soil or even deeper in underground aquifers) or is extracted by plants' root systems or other means. The subsurface flow of water occurs below the ground, and even this might end up in the oceans or surface on land via springs. A small amount of sublimation, involving the direct conversion of solid ice to water vapor, is also seen occurring in nature.
Water Reservoirs and Residence Times
Water reservoirs represent a region where part of the water participating in the water cycle is stored for a certain period of time. Oceans are the largest water reservoirs on the planet, storing nearly 97% of the waters of the hydrosphere, while the ice caps and glaciers store another 2%. Underground water reservoirs, rivers, lakes, ponds, and streams store small percentages of the total water of the hydrosphere as well, while the water content found in living organisms represents the smallest of all reservoirs. Another important term associated with the water cycle is the "residence time", which is represented by the following mathematical formula:
Residence Time = Volume of reservoir/(The rate water is added to the reservoir or leaves the reservoir)
The residence time thus represents the average time a water molecule spends in the reservoir, and can be expressed either positively or negatively depending upon whether the reservoir is experiencing net losses or net gains of water. For example, groundwater can reside for 10,000 years under the Earth’s surface before leaving the underground reservoir, while atmospheric water exists for a maximum of 10 days in the atmosphere before precipitating as rain or snow.
Importance of the Water Cycle and the Impact of Human Activities
Global climates are affected by, and extremely sensitive to, changes in the water cycle's patterns, as the cycle allows for the exchange of heat and moisture between landmasses and water bodies. Evaporation of water leads to the cooling of the environment, while condensation warms the environment by releasing heat energy. The physical geography of the Earth is also highly influenced by the water cycle, as the meltdown of glaciers and runoff from rivers carves out valleys, peaks, canyons, lakes, and other landforms seen on Earth. Recently, the water cycle of the planet has intensified, and the rates of evaporation and precipitation have greatly increased. Human activities, such as the damming of rivers and streams, the extraction of surface and underground water for irrigation and other purposes, and extensive deforestation, have adversely affected the functioning of the earth's water cycle. Global warming has further impacted the hydrosphere by triggering the melting of polar ice caps, which are now losing more water by evaporation, snow-melt and runoffs than they are gaining water by precipitation. This threatens to raise the oceans' water levels and flood coastal cities around the world.