Hydrologic Cycle

Hydrologic cycle is a cycle in which water is continually moving within and above the earth. There are six major components of this cycle: evapotranspiration, condensation, precipitation, infiltration, percolation and runoff. Concepts and components of the hydrologic cycle are as follows:


Evapotranspiration is the combined result of two processes: evaporation and transpiration. Evapotranspiration uses a larger portion of precipitation than the other processes linked with the hydrologic cycle.

Evaporation is the process of returning damp to the atmosphere. Water on any surface, especially the surfaces of mud holes, ponds, streams, rivers, lakes, and oceans, is warmed by the sun's heat until it reaches the point at which water turns into the vapor, or gaseous, form. The water vapor then rises into the atmosphere.

Transpiration is the process by which plants go back moisture to the air. Plants take up water through their roots and then lose some of the water through pores in their leaves. As hot air passes over the surface of the leaves, the moisture absorbs the heat and evaporates into the air.


The cooling of water steam until it becomes a liquid. As the dew point is reached, water vapor forms tiny visible water droplets. When these droplets form in the sky and other atmospheric conditions are present, clouds will form. As the droplets collide, they merge and form larger droplets and eventually, precipitation will occur.


It is moisture that falls from the atmosphere as rain, snow, sleet, or hail. Precipitation varies in amount, intensity, and form by season and geographic location. These factors force whether water will flow into streams or penetrate into the ground. In most parts of the world, records are kept of snow and rainfall. This allows scientists to decide average rainfalls for a location as well as categorize rainstorms based on duration, intensity and average return period. This information is crucial for crop management as well as the engineering design of water control structures and flood control.


Infiltration is the entry of water into the earth surface. Infiltration constitutes the only source of water to maintain the growth of plants and it helps to sustain the ground water supply to wells, springs and streams. The rate of infiltration is influenced by the physical character of the soil, soil cover (i.e. plants), water content of the soil, soil temperature and rainfall intensity. The terms infiltration and percolation are often used interchangeably.


The descending movement of water through soil and rock is called Percolation. Percolation occurs under the root zone. Ground water percolates through the soil much as water fills a sponge, and moves from space to space along fractures in rock, through sand and gravel, or through channels in formations such as cavernous limestone. The terms infiltration and percolation are often used interchangeably.


The movement of water, frequently from precipitation, across the earth's surface towards river channels, lakes, oceans, or depressions or low points in the earth's surface is Runoff. The individuality that affects the rate of runoff includes precipitation period and intensity as well as the ground's slope, soil type and ground cover.

Human Impact on water Cycle

The earth's water supply is constant, but man is able to changing the series of this fundamental supply. Population increases, rising living standards and industrial and economic growth have placed greater stress on the natural environment. Man activities can cause inequity in the hydrologic equation and can affect the quantity and quality of natural water resources available to current and future generations.

Water used by households, industries, and farms have increased. People demand clean water at reasonable costs, yet the amount of fresh water is limited and the easily accessible sources have been developed. As the population increases, so will our need to withdraw more water from rivers, lakes and aquifers, threatening local resources and future water supplies. A larger population will not only use more water but will discharge more wastewater. Domestic, agricultural, and industrial wastes, including the intensive use of pesticides, herbicides and fertilizers, after overload water supplies with hazardous chemicals and bacteria. Also, poor irrigation practices raise soil salinity and evaporation rates. These factors contribute to a reduction in the availability of potable water, putting even greater pressure on existing water resources.

Large cities and urban sprawl particularly affect local climate and hydrology. Urbanization is accompanied by accelerated drainage of water through road drains and city sewer systems, which even increases the magnitude of urban flood events. This alters the rates of infiltration, evaporation, and transpiration that would otherwise occur in a natural setting. The replenishing of ground water aquifers does not occur or occurs at a slower rate.

Together, these various effects determine the amount of water in the system and can result in extremely negative consequences for river watersheds, lake levels, aquifers, and the environment as a whole. Therefore, it is vital to learn about and protect our water resources.

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