Water cycle

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 The water cycle describes the continuous movement of water on, above and below the surface of the Earth.

The total mass of water on Earth and in the atmosphere taking part in the water cycle remains fairly constant over time but the partitioning of the water into the major reservoirs of ice, fresh water, saline water and atmospheric water is variable depending on a wide range of climatic variables.

During the water cycle water moves from one reservoir to another and through different phases, by the physical processes of evaporation, condensation, precipitation, infiltration, runoff, and subsurface flow.

The water cycle involves exchange of energy and change in temperature when water transforms from one phase to another. Also water is purified such that fresh water is provided through evaporation.

The water cycle is driven by heat energy from the sun.

Most water falls back from the atmosphere into the oceans or onto land as rain, where the water flows over the ground as surface runoff.

Main processes of the water cycle.

Precipitation – Condensed water vapor that falls to the surface as rain, snow, hail and sleet.

About 505,000 km3 of water falls as precipitation each year, 398,000 km3 of it over the oceans. The rain on land contains 107,000 km3 of water per year and a snowing about 1,000 km3.

Canopy interception – Precipitation that falls onto plants and evaporates back to the atmosphere rather than falling to the ground.

Snowmelt – Runoff produced by melting snow and ice.

Runoff – The ways water moves across land. This includes both surface runoff and channel runoff.

As it flows, the water may seep into the ground, evaporate into the air, become stored in lakes or reservoirs, or be extracted for agricultural or other human uses.

Infiltration – The flow of water from the surface into the ground.

Once infiltrated, the water becomes soil moisture or groundwater.

Subsurface flow – The flow of water underground,

Subsurface water may return to the surface or eventually seep into the oceans.

Evaporation – The transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the atmosphere.

The source of energy for evaporation is primarily solar radiation.

Total annual evapotranspiration s estimated to approximately 505,000 km3 of water, 434,000 km3 of which evaporates from the oceans.

Sublimation – The state change directly from snow or ice to water vapor.

Deposition – The state change of water vapor directly to ice.

Advection – The movement of water, ice or snow through the atmosphere..

Condensation – The state change of water vapor to liquid water droplets in the air, creating clouds and fog.

Transpiration – The release of water vapor from plants and soil into the air.