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The Water Cycle


The water cycle is also known as the hydrological cycle or the hydrologic cycle, and can be defined as;

the continuous movement of water on, above and below the surface of the Earth.”

It is essentially a series of processes by which water is evaporated from the sea and eventually condenses and precipitates over the land, before returning to the oceans via various pathways.

The amount or mass of water on Earth remains fairly constant over time but the location of that water into the major reservoirs of saline water, ice, fresh water, and atmospheric water is variable and depends on a wide range of climatic variables. Water on Earth moves from one reservoir or store to another, such as from river to ocean, or from the ocean to the atmosphere, by the physical processes of evaporation, condensation, precipitation, infiltration, surface runoff, and subsurface flow. Movement between these stores often involves the water changing form between liquid, solid (ice) and vapour.
The water cycle also involves the exchange of energy, which leads to temperature changes. When water evaporates, it takes up energy from its surroundings and cools the environment. When it condenses, it releases energy and warms the environment. These heat exchanges influence climate. 1


The Water Cycle

The Water Cycle by Ehud Tal [CC BY-SA 4.0  (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons 2


The water cycle starts with water evaporating from the ocean, this warm moist air rises in thermals where it cools as it rises through the troposphere (at the dry adiabatic lapse rate of 9.7°C per 1000m).  As the air rises and cools it loses its capacity to hold water vapour as its relative humidity rises, and condensation occurs releasing latent heat.  This forms clouds at the dew point, and these clouds are blown inland. 
Relief features can force the clouds higher, water droplets collide with one another and get bigger, and eventually the droplets are big enough to fall to earth as precipitation.  This precipitation can be stored on the surface as snow or ice, or can be intercepted by trees and vegetation.  On the trees or vegetation the water can be dripped off leaves, flow down the vegetation as stem flow or be taken up and lost as transpiration. 


The combined losses of water through transpiration and evaporation are known as EVAPOTRANSPIRATION. It could also fall straight into the ground where it can percolate into the soil then infiltrate into the rock underneath if the soil and rock are permeable.  If the rock is not permeable or the soil stores are full then surface runoff will occur.  This water will then work its way through the soil (soil flow) or rock (through flow) or over the land and into streams and rivers.  These small tributary streams join together at confluences and the river will grow in size and strength.

These processes are variable over time and space.  Not all parts of the world have precipitation all year for example, some have noticeable dry seasons.

1 - https://en.wikipedia.org/wiki/Water_cycle accessed 23rd September 2018

2 - https://en.wikipedia.org/wiki/Water_cycle#/media/File:Diagram_of_the_Water_Cycle.jpg accessed 7th October 2018

Written 07/10/2018 - by Robert Gamesby

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