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Water Cycle Processes

Factors affecting the magnitude of stores in water cycle

The water cycle is variable over time and over space.  It does not operate in exactly the same way in different parts of the world for example.  Although the component parts are similar, the stores and flows of the water cycle would vary significantly for tropical humid climates and for those in the Arctic Tundra for example.  There are several processes (that you need to understand) that drive change in the magnitude of these stores over time and space, including:
1. Evaporation,
2. Condensation,
3. Cloud formation,
4. Causes of precipitation and
5. Cryospheric processes

All of these can vary and the exam board expect you to understand how these things change at hill slope, drainage basin and global scales, and with reference to varying timescales involved.

WATER STORES RESIDENCE TIMES

Water Stores Residence Times

1 Source: Freeze, R.A. and Cherry, J.A, 1979. P.5, Groundwater, Prentice-Hall


Stores within the Water Cycle have different residence times. Residence time is defined as the amount of water in a store divided by either the rate of addition of water to the store or the rate of loss from it. 2
stores such as Ice caps and sheets have incredibly long residence times.  Water gets “locked up” in these stores and has to travel through the cryospheric system or wait for changing climatic conditions before it is released again.  The oceans also store water for long periods of time, water can be transferred to incredible depths and remains in the oceanic store for long periods of time.  Surface sea and ocean water is recycled between stores more rapidly.  In the atmosphere the residence time of water vapour relative to total evaporation is only about 10 days. Water is rapidly moved in and out of this store via various processes in the water cycle. Lakes, rivers, ice, and groundwaters have residence times lying between these two extremes and are highly variable. 

Phase changes of water as matter
It is useful to consider the change in, and transfer between water stores via a simple diagram shown below.  The relative size of the stores (the circles) and processes (the arrows) can be adjusted to reflect different places and different periods in time.
 

Phase change water diagramSource 3

The processes


Latent heat of fusion
• Melting: the substance changes from a solid to a liquid (in this case ice to water), extra energy needed
• Freezing: the substance changes from a liquid to a solid (in this case water to ice), loss of energy
Latent heat of vaporisation
• Vaporisation: the substance changes from a liquid into a vapour (in this case water to water vapour), extra energy needed
• Condensation: the substance changes from a vapour to a liquid (in this case water vapour to water), loss of energy to atmosphere
Latent heat of sublimation
• Sublimation: The substance has a 2-phase change (in this case from ice to gas), extra energy needed
• Deposition: The substance has a 2-phase change and loses energy (gas to ice in deposition)
Essentially, the diagram shows which processes move water from each of its states of matter: gas, liquid water and ice (the solid form).  If water is transformed from ice to liquid water, ice to gas or liquid water to gas additional energy is required.  If water is transformed in the opposite direction, i.e. gas to liquid water or ice, liquid water to ice the energy is lost.

These processes can vary from place to place, the 2 diagrams below are representations of how the stores and flows might adjust for cold and tropical environments.

Antarctica phase change Equatorial phase change water

The Major processes in the water cycle.

1. Evaporation
Evaporation is the process of turning from a liquid to a gas.  Evaporation occurs when energy from the sun hits the surface of the water/land and causes liquid to change from liquid to gas.
Rates of evaporation depend on:
• Amount of solar energy
• Availability of water
• Humidity of the air - The more humid the air the closer to saturation point the air is so less evaporation will occur
• Temperature of the air – Warmer air can hold more water than cold air.
Humidity
Humidity measures how much water vapor is in a parcel of air at any given time.  Absolute humidity is the mass of water vapour in an air mass measured in grams per cubic metre (g/m3).  This can also be expressed in a slightly different way by talking about specific humidity, which is measured in grams per kilogram of air.  Warmer air can hold more water vapour than cooler air, and this is a vital concept to understand when discussing the weather.  The warmer the air gets the more and more water vapour that air can hold.  This means that humidity can vary from place to place and time to time.  Obviously, globally the air in the Tropics can hold much more water vapour than higher latitudes because the air is warmer throughout the year.  For the UK, the air can hold more water vapour as a gas in summer than it can in cooler winter months.  In addition, there are diurnal or daily variations, as temperatures rise during the day and fall at night.
 

Relative humidity is therefore important, this is the amount of water vapour in the air at a given temperature compared to how much the air could possibly hold at that temperature.  If the air has 100% relative humidity it is said to be SATURATED and therefore holds as much water vapour as it can give the temperature. 5 This is known as the dew point and any change in pressure or temperature will mean that water vapour is condensed into water droplets or ice crystals if the air is cool enough.


Humidity Graph

2. Condensation
Condensation is the conversion of a vapour or gas into a liquid. Water that exists as vapour in the atmosphere is converted into droplets during this process.  If the water vapour converts directly into a sold form as ice crystals this is known as sublimation. 
Condensation occurs because air is either;
• cooled or
• there is a fall in pressure. 
As air cools or if there is a fall in pressure it is able to hold less water vapour.  The dew point is the temperature at which water vapour in the air turns into liquid water.
Air can cool by conduction – the ground loses heat rapidly and this chills the air above, this can result in frosts, dew and mist.  Air can also cool by uplift through the atmosphere caused by relief features, fronts or convective uplift, as explained below.  A final method of cooling is by advection, where warm moist air moves over a cooler surface, for example, where air warmed over land passes over cold ocean currents as it does on the West coast of South America.

3. Cloud formation
Cloud formation and condensation require “dirty” air, as it is the pollen and dust particles around which water droplets form.  As the air reaches saturation point or Dew point, water droplets form around pollen or dust particles (condensation nuclei).  These water droplets are tiny, and coalesce or collide to form larger droplets.  In the case of sub zero temperatures, it is ice crystals that form and join together in the ice crystal mechanism proposed by Bergeron and Findeisen. 6 When the water droplets or ice crystals in clouds grow to a certain size, gravity causes them to fall because of their own weight.  There are different types of cloud dependent upon height and composition. 7

 

4. Causes of precipitation
Precipitation is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation include drizzle, rain, sleet, snow, graupel and hail. Precipitation is the main input into the drainage basin system.  There are 3 basic types, all involve the uplift of air, a subsequent cooling and fall in air pressure, condensation follows and then either coalescence of droplets or fusing of ice crystals.


Frontal Rainfall
This occurs where warmer air meets colder and the former is forced to rise. The warm air is rising, so it cools initially at the Dry Adiabatic Lapse Rate (9.8°C per 1000m ascent) and then at the MORE VARIABLE Saturated Adiabatic Lapse Rate as latent heat is released during condensation. This eventually results in cloud formation and eventually rain (once the droplets have collided enough to be big enough to fall).


 

Relief rainfall
Relief rainfall is a major method of precipitation formation in the UK and relates to the precipitation that is created as air masses are pushed up and over mountainous or upland areas.  Relief rainfall occurs where moist air is forced to rise over a physical barrier such as a mountain range. For example, warm air is carried to the West coast of Britain by our prevailing (dominant) winds, the South Westerlies. This air encounters the high land on the coast of Ireland, then the Lake District and the Pennines and it is forced to rise above this barrier. As it rises, the warm air cools with height at a rate of 9.8°C per 100om (the DALR). As the air cools water vapour condenses to form clouds and eventually it rains over Britain's highland areas. As the air descends to the East coast of Britain or the Lee slope it warms slightly and there is less rainfall. This results in a rain shadow on Britain’s east coast.  It is for this reason that the West coast of Britain is wetter than the East, Blackpool receives 950mm of rainfall per year, the Pennines 2000mm+, and Newcastle 700mm. 
 

Convectional rainfall
This is where the Sun's energy hots the surface of the planet which heats the air above.  This air then has extra energy and rises upwards in thermals.  The heat energy also causes rapid evaporation and evapotranspiration so the air that is rising is humid. The air cools as it rises, and water vapour will condense once dew point is reached. Large quantities of water vapour and fast condensation causes storm clouds.  It often results in towering cumulonimbus clouds.  This type of rainfall is common in hot tropical areas but is also known in the UK in summer.

5. Cryospheric processes
Cryospheric process are those processes that affect the total mass of ice at any scale from local patches of frozen ground to global ice amounts. They have a direct impact on the major stores of water, they lock up water as ice from the hydrosphere lowering sea levels, or release water during melting in warm periods rising sea levels. 
The Crysophere includes: seasonal snow, frozen ground, sea ice, glaciers, ice caps, and ice sheets. It contains 1.8% of all water on Earth but nearly 70% of the freshwater.

The formation of ice.
Ice forms from the compression of falling snow.  As layer upon layer of snow is added it can exert a pressure on the snow at the base, compressing that snow and forcing air out of it.  This will slowly form a denser substance called Névé and eventually ice.  Melting and refreezing of previously fallen snow can also assist in this process.


Within an ice mass we need to consider 2 variables;
1. Accumulation is the build-up of ice mass
2. Ablation is the loss of ice mass
Glacier Mass Balance
Where accumulation and ablation are equal the glacier is said to be in a steady state.  This rarely happens, and 2 other states exist;
1. When accumulation is greater than ablation leading to growth in ice mass and potential glacial advance down the valley
2. When ablation is greater than accumulation leading to a loss of ice mass and the potential retreat of the glacier up valley.
Over a year accumulation tends to be within the colder months and ablation within the warmer months.

Ice movement
It is hard to understand that Ice moves given that it is a solid, but it can and does.  Ice can move at extraordinary speeds, and glaciers in surge conditions are known to move at up to 300m a day.  The Franz Joseph glacier in New Zealand has been known to surge in the past.  Ice can move in many ways and this is determined by the glacial mass balance and the temperature and precipitation regime of the area the glacier is found within.  The fact that ice moves is important.  It shifts ice into more temperate zones where it can melt and become part of the atmosphere or hydrosphere via meltwater.  It also moves ice to the edges of ice sheets by the sea where the ice can calve off to become ice bergs and melt into the oceans.

Glacial System

 

 

1 Source: Freeze, R.A. and Cherry, J.A, 1979. P.5, Groundwater, Prentice-Hall – available at http://hydrogeologistswithoutborders.org/wordpress/original-groundwater-by-freeze-and-cherry-1979-now-available-online/
2 https://www.britannica.com/science/hydrosphere/The-water-cycle#ref238299 accessed 7th October 2018
3 https://waterknowledge.colostate.edu/hydrology/water-cycle/ accessed 7th of October 2018
4 http://www.antarcticglaciers.org/glaciers-and-climate/estimating-glacier-contribution-to-sea-level-rise/ accessed 7th October 2018
5 https://en.wikipedia.org/wiki/Dew_point accessed 7th October 2018
https://en.wikipedia.org/wiki/Wegener%E2%80%93Bergeron%E2%80%93Findeisen_process  Accessed 7th October 2018
7 https://scied.ucar.edu/webweather/clouds/cloud-types  accessed 7th October 2018

Page written by Rob Gamesby, 7th October 2018

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