The Hydrologic Cycle

The circuit of water movement from the atmosphere to the Earth and back to the atmosphere through various stages or processes is known as hydrologic cycle (Figure). The processes involved are: precipitation, interception, runoff, infiltration, percolation, storage, evaporation and transpiration, etc.

                                           Figure: The Hydrologic Cycle

The energy from the Sun stimulates evaporation—the conversion of liquid water into water vapor. The water vapor moves up into the atmosphere, eventually forming clouds which return to the earth as rain or other precipitation.

About 500,000 km³ of water are evaporated from the earth’s surfaces and vegetation each year, some 110,000 km³ of which falls as rain or snow on the continents. Some of the water falling on land runs off the surface of the soil, and some infiltrates the soil and drains into the groundwater. Both the surface runoff and groundwater seepage enters streams and rivers (about 40,000 km³) that, in turn, flow into oceans which balances the same quantity of water that is transferred annually (by evaporation and subsequent drifting by wind ) in clouds from the oceans to the continents.  

Throughout the hydrological cycle, many processes act to purify water. Evaporation and subsequent precipitation act as a natural distillation process that removes impurities that dissolved in water. In evaporation, molecules of water vapor enter the atmosphere, leaving behind salts and other contaminants and thus creating purified fresh water. Water flowing aboveground through streams and lakes and below ground in aquifers is naturally filtered and purified by chemical and biological processes. Thus, the hydrologic cycle can be viewed as a cycle of natural renewal of water quality.

The redistribution of heat that results from the massive evaporation, precipitation, and transport of water is a major factor in keeping the world temperatures relatively constant and making world habitable.

Influence of Human Activities on Hydrologic Cycle

We have been intervening in the Earth’s current water cycle in three major ways.

First, we withdraw large quantities fresh water from streams, lakes, and underground sources. Heavy withdrawals have led to groundwater depletion or intrusion of ocean salt water into underground water supplies.

Second, we clear vegetation from land for agriculture, mining, road and building construction, and other activities and sometimes cover the land with buildings, concrete, or asphalt. This increases runoff, reduces infiltration that recharges groundwater supplies, increases the risk of flooding, and accelerates soil erosion and land slides.

Third, we modify water quality by adding nutrients and other pollutants. This overload of plant nutrients can change or impair natural ecological processes that purify water.