(2) The Global Water Cycle

the water cycle

the constant recycling of water between land, ocean and atmosphere

stores of water

most of earth's water is stored as saline (salt) water in oceans

freshwater stores = ice sheets (Antarctica and Greenland)

groundwater store

rivers, lakes, and the atmosphere contain remarkably small amounts of the global water stores

transfers of water

precipitation
... transfers water from the atmosphere to the Earth's surface

evaporation
... moves water back to the atmosphere

infiltration
... the transfer of water into the ground


percolation
... the transfer of water through rocks as groundwater flow

the global hydrological cycle

= a closed system

the global distribution and size of major stores of water

lithosphere, hydrosphere, cryosphere, atmosphere


energy comes from solar energy

sources and sinks = oceans, lakes, rivers, groundwater stores, atmosphere

the makeup of the world's water stores

of the total water on the planet....

97.5% is in oceans

2.5% is freshwater

divisions of freshwater sources

permafrost = 0.8%

groundwater = 30.1%

surface water (river/lakes/wetlands) = 0.4%

glaciers = 68.7%

oceanic water (hydro-spheric)

... the ocean dominates the global water store with approx. 1,350,000,000 km3 to an average depth of 3,700m (96.6% in total)

oceanographers claim that only 5% of the ocean has been explored

oceanic pH

oceanic water contains dissolved salts from rock minerals, pH = alkaline (8.25)

this pH has fallen by 0.1 during the last 250 years due to ocean acidification

Cryospheric water

includes glacial ice, both land based and sea ice, as well as permafrost

glacial ice has many forms such as ice sheets, valley glaciers, and ice shelves

Greenland and Antarctic ice sheets contain 99% of the cyrospheric ice on earth

where do glaciers form?

glaciers form in climate zones where snow (which falls in colder months) does not entirely melt in the summer, the snow accumulates and is compressed under its own weight to form glacial ice

Terrestrial water (hydrospheric and lithospheric)

Surface water = rivers, lakes, and wetlands

- rivers act as a store and transfer water

- Amazon River has a discharge of 209,000 m3/s which is larger than the next 7 largest rivers combined

... rivers make up 0.0002% of global water

Groundwater

Soil water

Biological water

groundwater

defined as water stored in subsurface geology in pore spaces no deeper than 4,000m

(water does no exist in the earth's crust, but has been discovered at depths of 13km)

soil water

= bound to the humus in soil by electrical charge

biological water

= stored in all living biomass

60% of humans is water

Groundwater and Aquifers

most groundwater is contained within 100m of the surface

this can be found in the permanently saturated zone within solid rocks (the phreatic zone)

water table varies seasonally throughout the aeration zone

aquifers

= significant underground reservoirs of water

just over 30% of all freshwater is stored in rocks below the ground surface in aquifers

commonly form in chalk and sandstone rocks as are porous and permeable

water tables - image is chatgpt so might not be accurate

underground surface areas beneath soil or rock which are saturated with water

upper level of saturated rock = phreatic zone
... rises and falls in response to groundwater flow, water abstraction and recharge

aeration zone = immediately below land surface

aquicludes = impermeable barriers

the phreatic zone

permanently saturated zone within solid rocks below the water table

upper levels of of phreatic zone are saturated with water

the role of aquifers + groundwater in the water cycle

1. groundwater recharge of aquifers happen naturally from infiltration, river seepage, and artificial methods

2. underground flow of water directed by hydrostatic pressure and presence of impermeable layers of rock known as aquicludes

3. aquicludes force water to edge of aquifer where it leaks out of rock as a spring

the aeration zone

zone immediately below the land surface where the pores contain both water and air, but are not totally saturated with water

Plant roots can capture the moisture passing through this zone, but it cannot provide water for wells

(also known as the unsaturated zone)

soil water budget

The capacity of soil to store and transfer water

The balance of soil water that involves the amount of precipitation, evapotranspiration, and water storage and loss.

e.g., clay can store water easily but has limited water transfer

e.g., porous sandy soils hold little moisture as water is easily transferred through pores

aquicludes

the impermeable layers that are barriers to groundwater flow

atmospheric water

holds around 0.4% of global total
(12,900 km3 of water vapour)

water exists in all 3 states:
water vapour
water droplets
ice

the role of the atmosphere in the water cycle

water vapour in atmosphere absorbs, scatters and reflects incoming solar radiation
... maintains warmer atmospheric temperature through natural greenhouse effect that is needed for life on earth


evaporation and condensation are key processes

(rate of processes depends on temp. and humidity)

Processes of change in the water cycle

1. Global Atmospheric circulation system (large scale)

2. Climate change (global + regional scale)

3. Hydrological cycle and Drainage Basins (medium + small scale, spatial change)

4. Cryospheric processes (long term, temporal change)

the global atmospheric circulation systems in the water cycle

= the large-scale movement of air by which heat is distributed on the Earth's surface

The wind belts and the jet streams girdling the planet are steered by 3 convection cells:
1. the Hadley cell

2. the ferrel cell

3. the polar cell

The Hadley Cell

Convection Currents that cycle between the equator, 30 degrees North and South.

The Ferrel Cell

30 to 60 degrees.

Air at 30 degrees stable so moves towards the poles towards unstable air, or sub-polar lows

The Polar Cell

A convection current in the atmosphere, formed by air that rises at 60 degrees N and 60 degrees S

sinks at the poles, 90 degrees N and 90 degrees S

global atmospheric circulation + he water cycle

= the driving force behind cloud formation and precipitation


the presence of the 3 interconnected cells show the rising and falling of air in latitudinal zones

ITCZ

Mid-latitude cloud formation

local weather/precipitation

ITCZ - GACM

at the equator, high temperatures result in high rates of evaporation. The warm moist air rises, cools and condenses to form towering banks of cloud and heavy rainfall in Low Pressure Zones
= Inter-Tropical Convergence Zone (ITCZ)

... changes seasonally moving north and south as sun influences the spatial and temporal changes in transfers and stores magnitudes of water

Mid-latitude cloud formation - GACM

cloud formation is mostly driven by convergence of warm air from the tropics and cold air from the Arctic

the boundary of these two air masses (the polar front) results in rising air and cloud/rain formation

strong upper winds from the jet stream drive these unstable weather systems across mid-latitudes

= largely changeable conditions in UK + why we experience storms

Local precipitation - GACM

formation of thunderstorms from intense convective activity

varies in both time and space of water cycle transfer processes

Climate Change in the process of change

last ice age: 1/3 of earth was covered by glaciers and ice sheets
... water was "locked up" in snow and ice
... sea levels were a lot lower

warmer periods, sea levels started to rise as ice melted
... change in cryospheric and hydrospheric stores

global warming = rising temperatures
... sea levels rising as glacial ice melts (eustatic)

Drainage Basin and hydrological cycle processes of change

Medium/Small scale
spatial change

dependent on the type and rate of processes present

also dependent on the stores
e.g., soil water budget, the geology and permeability of rocks and soils

may also change temporally
e.g., seasonal changes
(melting and freezing of sea ice, permafrost soils)

relief of land
e.g., hillslope scale

... by isolating a section of the global hydrological cycle you create an open system within a larger closed system

the hillslope water cycle - process of change at the local scale

dependent on inputs, outputs, flows, and stores

amount of water stores can hold is often dependent on many factors operating over relatively short timescales or the magnitude of processes

Deforestation - removal of trees reduces interception and infiltration, overland flow increases

Storms - intense rainfall increases the amount of rainfall reaching the ground and increases the magnitude of stores

Seasonal Changes - winter snowfalls and frozen ground interrupt the water transfers and affect the magnitude of stores

Urbanisation - if the slope is developed (e.g., housing) impermeable surfaces will reduce infiltration. Trees will probably be cut down as well. Water will flow quickly through pipes to nearby river channels

Farming - ditches drain the land and encourages water to flow quickly to rivers. irrigation increases the amount of water on the ground

cryospheric processes in change

2nd largest store of water = in ice

(99% of ice is in Antarctic and Greenland ice sheets)

snow falling on glaciers and ice sheets become compressed and enter long term storage, forming layers of glacial ice

shorter timescale: snow accumulates during winter adds to the mass of a glacier or ice sheet. In summer, melting occurs or ice calves

glacial equilibrium line = marks the altitude where annual accumulation and melting are equal
... recent decades, climate has warmed so equilibrium line has moved to higher altitudes

--> most glaciers in the world are now shrinking or retreating

cryospheric processes

i.e., glaciation

the processes that affect the total mass of ice at any scale from local patches of frozen ground to global ice amounts

... include formation and loss of glacial ice

how does glacial ice form

about 70% of the worlds freshwater is stored and transported as glacial ice

snow falls in winter, doesn't fully melt in summer and accumulates, compresses to form glacial ice. Over thousands of years glacial ice layers build up to form glaciers

glacial ice is blue as no air is trapped inside of it
(snow is white due to being composed of 90% trapped air)

snowflake - granular snow - firn - glacial ice

how alpine glaciers work as a system

- Overall, inputs throughout the year are less than the outputs, causes ablation

- Ablation zone means that the ice is melting away

- Outputs of evaporation and sublimation (solid to gas) dominate

- High outputs of meltwater in summer

- Big chunks of ice may break off due to sublimation, creating ice bergs or ice shelves


(glacier = temporary cryospheric store)

ablation zone

area or zone where previous snow from the previous winter melts, exposing bare ice

glacier mass balance

the difference between the amount of snow and ice accumulation and the amount of ablation occurring in a glacier over one year

winter = positive mass balance
... the accumulation of ice is greater than the ablation of ice through outputs

summer = negative mass balance
... the accumulation of ice is less than the ablation of ice through outputs

impact of climate change on glacier mass balance

Climate change will cause an earlier shift to negative mass balance and possibly a permanent decrease in the duration of a positive mass balance, until there is no positive balance left

melting of ice on sea levels - processes of change

the total melting of all the polar ice sheets could reuslt in a 60m rise in sea level

... adds water to ocean store that it cannot manage

positive feedback of sea level rise causing more ice calved, leading to increase in melting ice sheets which increases sea level rise

ice calves

where ice breaks away

example of cryopshere + glacier mass balance

Storbreen Glacier, Norway

mass balance is now largely negative
(changes from 1990 - 2014)

Types of rainfall

relief, frontal, convectional

relief rainfall

stage 1:
warm wet air is forced to rise over high land

stage 2:
as the air rises it cools and condenses
... clouds form an precipitation occurs

stage 3:
... the drier air descends and warms

stage 4:
any moisture in the air evaporates (e.g., clouds)

convectional rainfall

stage 1: the sun heats the ground and warm air rises

stage 2:
as the air rises it cools and condenses
... clouds form an precipitation occurs

stage 3:
large cumulonimbus clouds are formed

stage 4:
heavy rain storms occur
... these usually include thunder and lightening

frontal rain

stage 1:
an area of warm air meets an area of cold air

stage 2:
the warm air is forced over the cold air

stage 3:
where they converge, the warm air cools and water vapour condenses

stage 4:
clouds form and precipitation occurs