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Abstract
The water cycle is an important biogeochemical cycle that involves the movement of water on, below, and above the Earths surface. It comprises several processes that include evaporation, evapotranspiration, condensation, precipitation, and infiltration. The cycle is affected adversely by human activities such as agriculture, deforestation, and abstraction of water from rivers and lakes. These activities lower the water table, increase soil salinity, and lower the amount of groundwater.
Introduction
The water (hydrological) cycle refers to the process by which water moves below, above, and on the surface of the earth through processes that include evaporation, precipitation, condensation, infiltration, and surface runoff (Nelson, 2003). It is one of the most vital biogeochemical cycles that are affected by changes in climatic conditions. The water cycle is driven by changes in temperature that alter the state of water at any given time, and as a result, facilitate its continuous movement.
The water cycle
The cycle can be effectively described starting with the process of evaporation. Energy from the sun heats water in oceans, seas, rivers, and dams causing its movement into the atmosphere as water vapor. The same process occurs in plants through a process called evapotranspiration (Nelson, 2003). As water vapor rises higher into the atmosphere, it changes from gas to liquid through a process known as condensation.
Concentration of water droplets in the atmosphere leads to formation of clouds that are moved about by air currents. Movement of air currents causes collision of cloud particles in the atmosphere, and as a result, clouds fall down to Earth as precipitation in the form of rain and snow. Precipitation that falls as rain flows back into water bodies while some seeps into the ground through the process of infiltration (Nelson, 2003).
The water that soaks into the ground replenishes aquifers that hold huge amounts of water. When snow reaches the Earth surface, it accumulates and forms glaciers and ice caps that comprise water in the solid state. When temperatures rise, frozen water melts and flows back into water bodies as surface runoff (Shiklomanov, 2009). A small amount of infiltration seeps back into the land surface as groundwater discharge and flows into streams and rivers.
On the other hand, some groundwater finds its way to the Earth surface as freshwater springs. Plants absorb some groundwater and use it in processes such as photosynthesis. This water is later released to the atmosphere through evapotranspiration (Shiklomanov, 2009). The process is repeated as weather patterns change.
Impact of human activity on water cycle
Humans interrupt water pathways through withdrawals and discharges that have great impact on the water cycle. Human activities such as deforestation, agriculture, construction of dams, urbanization, removal of groundwater from wells, and water abstraction from rivers and lakes has affected the water cycle significantly. Deforestation and agriculture reduce the amount of water vapor released by trees and vegetation into the atmosphere thus resulting in low amounts of rain (Nelson, 2003).
In addition, loss of ground cover makes land drier and causes increased run-off and leaching. Removal of groundwater lowers the amount of groundwater available and lowers the water table (Shiklomanov, 2009). In that regard, human activity affects both the quality and quantity of water. Over-extraction of groundwater for domestic and industrial uses leads to subsidence and alters the water cycle. Agricultural activities such as irrigation reduce water stores and increase the salinity of soil (Shiklomanov, 2009).
References
Nelson, R. (2003). The Water Cycle. New York, NY: Lerner Publications.
Shiklomanov, I. A. (2009). Hydrological Cycle. New York, NY: EOLSS Publications.
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