The Water Cycle

systems

  • open system: inputs and outputs of both energy and matter.


    the Drainage Basin system


    a drainage basin is an area of land drained by a river.


    the energy comes into and leaves the drainage basin.


    Mass also enters and leaves. ( water and sediment)


    this is an open system


  • closed system: inputs and outputs only as a transfer of energy.


    The global hydrological cycle


    energy from the sun enters and leaves the system.


    the total amount of water in the cycle is always the same; not water enters or exits. As a result you get a closed system.

  • Equilibrium - a state of balance (it remains constant)
  • disequilibrium - imbalance (it changes in size)

Stores and flows

STORES:

Oceans 1400 000, residence time 3,600 years

ice and snow 27300, RT 15,000 years

underground water 22900, RT 10,000 (deep water) 100-200 years for shallow.

lakes 176 RT 2 weeks - 10 years. 50 for larger scale.

soil moisture 16 RT 2-50 WEEKS

atmosphere 13 RT 10 days.

plants and animals / rivers 2

stores measured in 100 km3(cubic kilometres).

  • water stores are not evenly distributed around the globe.
  • Less than 10 countries possess 60% of the world's available fresh water supply.
  • 30% of all freshwater is stored in rocks deep below the ground surface forming vast underground reservoirs called aquifers.
  • these aquifers most commonly form in chalk and sandstone as they are porous and permeable and hence they are unevenly distributed..

FLOWS

  • flows are the physical mechanisms which drive the flux of material between stores. e.g evaporation, precipitation etc..


  • fluxes are measurements of the rate of flow of material between the stores. Because fluxes are a rate, the units are mass per unit time (1000 km3/yr)

Evaporation 336 (over ocean)

precipitation 300 (over ocean)

precipitation 100 (over land)

evaporation and transpiration 64 ( over land)

ocean to land travel 36

Case study

Water

water today exists on earth in three states (liquid water, Gaseous Water vapour, solid ice.


Energy in the form of latent heat is either absorbed or released when water changes state.

  • solid water to water vapour = latent heat of sublimation
  • solid water to liquid water = latent heat of fusion (melting)
  • water vapour to solid water = deposition
  • water vapour to liquid water = condensation
  • liquid water to water vapour = evaporation
  • liquid water to solid water = freezing

condensation:

  • this process occurs when water vapour becomes liquid - it loses energy to the surroundings.
  • If there is lots of vapour in the atmosphere then there will be higher rates of condensation.
  • it occurs when air containing water vapour cools to its dew point e.g when temperatures fall at night due to heat being lost to space.
  • Dew point the atmospheric temperature (varying according to pressure and humidity) below which water droplets begin to condense and dew can form.

Evaporation

  • this process increases the amount of water stored in the atmosphere.
  • the rate of this process depends on:
    • the amount of solar energy
    • surface area and availability of water ( more of this process occurs from a pond than a grassy field)
    • wind speed
      Humidity of the air: the closer the air is to saturation point, the slower the rate.
    • the temperature of the air; warmer air can hold more water vapour than cold air.
  • Ablation is the natural loss of snow or ice from the surface of a glacier, usually by melting but also by calving(breaking off).
  • the area of a glacier where the loss of snow and ice is greater than the amount replaced each year is called the ablation Zone.
  • this will result in a decrease in the volume of the glacier.
  • glaciers may retreat and/or thin.

spacial variation

  • in a tropical environment, the most significant hydrological process would be precipitation.
  • Arctic environment -> Snow/ice
  • desert -> underground water

Clouds and the cryosphere:

condensation and cloud formation occurs in the troposphere 0-10km above sea level.
the troposphere is the lowest portion of the earths atmosphere, and is also where all weather takes place. it contains approximately 99% of its water vapour.

why does it get cooler as you go up?

  • the troposphere is primarily heated from the bottom because the surface is much better at absorbing a wide range of solar radiation as compared to the air.
  • the surface is warmed by the sun and then this energy energy warms up the air above the ground through conduction.
  • this warm air rises upward into the troposphere through convection.
  • when the air cools, it condenses.

Why is there less air pressure as you go up?

  • air pressure refers to the weight of the earth's atmosphere pressing down on everything.
  • the closer to the earth's surface you get the more air is above you, pressing down.
  • if air rises less pressure is exerted on it and it expands. We can see the same effect happening underwater when a scuba diver releases bubbles. they get bigger as they rise.

ADIABATIC EXPANSION

  • the description of what happened to a parcel of air as it rises as air pressure decreases causing an increase in volume and a decrease in temperature.

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Types of clouds and rainfall