The Water-cycle and Nature

water-cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth.

• Since the water cyc. is truly a "cycle," there is no beginning or end.

• Water can change states among liquid, vapor, and ice at various places in the water cycle.

• Although the balance of water on Earth remains fairly constant over time, individual water molecules can come and go


Description

• The sun, which drives the water-cycle, heats water in the oceans.

• Water evaporates as vapor into the air.

• Ice and snow can sublimate directly into water vapor.

• Evapotranspiration is water transpired from plants and evaporated from the soil.

• Rising air currents take the vapor up into the atmosphere where cooler temperatures cause it to condense into clouds.

• Air currents move clouds around the globe, cloud particles collide, grow, and fall out of the sky as precipitation.

• Some precipitation falls as snow and can accumulate as ice caps and glaciers, which can store frozen water for thousands of years.

• Snow packs can thaw and melt, and the melted water flows over land as snowmelt.

• Most precipitation falls back into the oceans or onto land, where the precipitation flows over the ground as surface runoff.

• A portion of runoff enters rivers in valleys in the landscape, with stream flow moving water towards the oceans.

• Runoff and groundwater, are stored as freshwater in lakes.

• Not all runoff flows into rivers.

• Much of it soaks into the ground as infiltration.

• Some water infiltrates deep into the ground and replenishes aquifers, which store huge amounts of freshwater for long periods of time.

• Some infiltration stays close to the land surface and can seep back into surface water bodies (and the ocean) as groundwater discharge.

• Some ground water finds openings in the land surface and emerges as freshwater springs.

• Over time the water of water-cycle reenters the ocean, where water-cycle was started.



Different Processes


Precipitation

Condensed water vapor that falls to the Earth's surface. Most precipitation occurs as rain,
but also includes snow, hail, fog drip, and sleet


Canopy interception

The precipitation that is intercepted by plant foliage and eventually evaporates back to
the atmosphere rather than falling to the ground.


Snowmelt

The runoff produced by melting snow.


Runoff

The variety of ways by which water moves across the land. This includes both surface
runoff and channel runoff. As it flows, the water may infiltrate 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 ground surface into the ground. Once infiltrated, the water becomes
soil moisture or groundwater.


Subsurface Flow

The flow of water underground, in the vadose zone and aquifers. Subsurface water may return to the surface (e.g. as a spring or by being pumped) or eventually seep into the oceans. Water returns to the land surface at lower elevation than where it infiltrated, under the force of gravity or gravity induced pressures. Groundwater tends to move slowly, and is replenished slowly, so it can remain in aquifers for thousands of years.


Evaporation

The transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the overlying atmosphere. The source of energy for evaporation is primarily solar radiation. Evaporation often implicitly includes transpiration from plants, though together they are specifically referred to as evapotranspiration. Total annual evapotranspiration amounts to approximately 505,000 km3 (121,000 cu mi) of water, 434,000 km3 (104,000 cu mi) of which evaporates from the oceans.


Sublimation

The state change directly from solid water (snow or ice) to water vapor.


Advection

The movement of water in solid, liquid, or vapour states — through the atmosphere. Without advection, water that evaporated over the oceans could not precipitate over land.


Condensation

The transformation of water vapor to liquid water droplets in the air, producing clouds and fog.





Go back to environment page


From water-cycle to home page