The Water Cycle

System

a set of interrelated components working together towards a process

Flow/transfer (& example)

A form of linkage between one store/component and another that involves movement of energy/mass.

• Evaporation/throughflow/stem flow etc.

Input (& example)

The addition of matter and/or energy into a system.

• Precipitation

Output (& example)

matter or energy moving from the system to outside the system/to another system

• Channel flow (not surface runoff)

Store/component (& example)

A part of the system where energy/mass is stored or transformed.

• Puddles/soils/trees etc.

Energy (& example)

power/driving force

• Insolation (incoming solar radiation)

Matter

any substance that can be weighed & takes up space

Models (& why they are used)

• A simplification of something

• Allows relationships between components to be better understood

Open systems

inputs and outputs of energy & matter exchange at its boundaries

Closed system

only energy is inputted and outputted; matter is contained within the system boundary

Isolated systems

does not share matter or energy with their surroundings

Cascading systems

where energy & material are transferred from one subsystem to another

Dynamic

ever-changing

Dynamic equilibrium

where inputs & outputs are equal (stores stay the same)

Positive feedback

where effects of an action are amplified by subsequent knock-on effects

Negative feedback (& example)

where effects of an action are nullified by subsequent knock-on effects

5 global systems

• Atmospheric: interaction of gases

• Lithosphere: interaction of solid/semi-solid/liquid crust

• Biosphere: interaction of biological life

• Hydrosphere: interaction of water

• Cryosphere: interaction of frozen water

How much of Earth’s water consists of freshwater?

3%

How much freshwater consists of ice caps and glaciers?

69%

How much freshwater consists of groundwater?

30%

How much freshwater consists of easily accessible surface freshwater?

1%

How much easily accessible surface freshwater consists of lakes?

52%

How much easily accessible surface freshwater consists of soil moisture?

38%

How much easily accessible surface freshwater consists of water vapour?

8%

How much easily accessible surface freshwater consists of water within living organisms?

1%

How much easily accessible surface freshwater consists of rivers?

1%

How much of the Earth’s water consists of oceanic water?

97%

How much of the Earth’s water consists of atmospheric water?

0.4%

Components of cryospheric water (& definitions)

• Sea ice: water cooled to temperatures below freezing

• Ice caps: land ice < 50,000 km²

• Ice sheets: land ice > 50,000 km² (buildup of snow that doesn’t entirely melt in summer)

• Ice shelves: ice sheets moving out into oceans

• Alpine glaciers: ice found in deep valleys/upland hollows

• Permafrost: permanently frozen soil

Components of terrestrial water (& examples/definitions)

• Surface water: lakes/rivers/wetlands (where there is dominance by vegetation)

• Groundwater: water that collects underground in the pore spaces of rock

• Soil water

• Biological water

How evaporation is caused

Energy from solar radiation hitting the surface of water or land

Saturation point

where no more of something can be absorbed because the capacity is full

Factors affecting evaporation (incl. reasoning)

• Amount of solar energy

• Availability of water

• Humidity of air (closer to saturation point = slower rate of evaporation)

• Air temperature (warmer air can hold more water vapour than colder air)

Transpiration

Where water is transported from the roots of a plant to its leaves and is lost through pores on the leaf surface

How condensation works

• Air cools = less water vapour capacity

• Cooled enough = reach dew point temperature & is saturated

• Via condensation: excess water converts into liquid water

How condensation is caused

When air temperature is reduced to dew point but volume remains constant:

• Occurs when warm moist air passes over a cold surface

• Occurs also when on a clear winter’s night, heat is radiated out to space and the ground gets colder, cooling the air directly in contact with it

Features of the start of Quaternary glaciation

• Started 2.58 million years ago

• Sea levels was approx. 120 m lower than present

• Continental glaciers covered large parts of Europe, North America and Siberia

• Interglacial periods: global ablation (loss of ice mass) > accmulation (gain of ice mass) = present-day hydrological cycle

Drainage basin

An area of land drained by a river and its tributaries

Groundwater flow/percolation (& factors affecting it)

• Horizontal/vertical movement of water within rock under the soil surface

• Controlled by rock saturation: more porous rock e.g. chalk/sandstone = faster rate compared to less porous rock e.g. clay rock

Infiltration (& factors affecting rate)

• Downward movement of water from the surface into soil

• Controlled by soil saturation: more porous soil e.g. course/sandy soils = faster rate compared to less porous soil e.g. clay soil

• Animals creating burrows

Interception storage (& factors affecting it)

Precipitation that falls on vegetation surfaces/man-made cover which is stored

1. Precipitation levels

2. Density of vegetation in drainage basin

Overland flow

Tendency of water to flow horizontally across land surfaces (where rainfall has exceeded infiltration capacity)

Run-off

All water that enters a river channel and eventually flows out of the drainage basin

Stemflow

Precipitation intercepted by the canopy that reaches the ground via flowing down stems/stalks/tree trunk

Throughfall (& when it occurs)

• Precipitation that reaches the ground directly through gaps in plant canopy

• Precipitation that drips off leaves/twigs/branches (occurs when canopy surface exceeds storage capacity)

Throughflow

Movement of water down-slope through subsoil due to gravity (enhanced by underlying impermeable rock due to increased horizontal motion)

Water balance

Balance between inputs (precipitation) & outputs (run-off & evapotranspiration & soil/ change in groundwater storage)

Watershed

High land that separates drainage basins (boundary)

How surface storage is created

• When rainfall intensity > infiltration rate (& has reached infiltration capacity)

• Soil = saturated

Water table

The underground boundary between the saturated & unsaturated zone

Potential evapotranspiration

Amount of water that could be evaporated or transpired (or both) if there was sufficient water available

Soil moisture graph (6 stages)

Discharge (equation)

volume x velocity

Why hydrographs are important

• Assessing management of water resources (incl. irrigation provision)

• Whether water-related structures required (e.g. reservoirs/bridges/flood banks/urban drainage schemes/sewage treatment works)

• Whether flood warning/alleviation schemes required

• Whether hydroelectric power is suitable

• Ecological health & recreational value of wetlands

River regime

The variability in its discharge annually in response to precipitation, temperature, evapotranspiration and drainage basin characteristics

Physical factors affecting hydrograph steepness (not covered in GCSE)

• Length & size of drainage basin

• Drainage density (i.e. lots of surface streams acting as tributaries to main river)

• If already saturated

• Extreme weather (surfaces baked hard vs frozen)

• Type of precipitation (snow vs rain)fc

Human factors affecting hydrograph steepness (not covered in GCSE)

• Deforestation = increased soil erosion = channel sedimentation = reduced bankfull capacity

• Creating furrows that run directly downslope

• Ploughing wet soils = creates (impermeable) plough pans which inhibit percolation

• Abstraction = reduced base flow

Severe Storms e.g. Storm Desmond, UK (2015 - 2016)

1. Increased precipitation (341.4mm in 24h) > saturated ground > increased SR > greater discharge/flood risk

2. Increased prec. > higher velocity of SR (due to upland gradient) > increased FR

3. Increased surface storage > more water for human use > water surplus

4. Increased speed of SR > increased soil erosion (from upland areas) > silting up river > reduced river capacity > increased FR

Droughts e.g. Californian Drought (2012 - 2017)

1. *Reduced prec. [deficit: 100% - 180% of annual rainfall] > increased soil capacity > reduced discharge > reduced FR

2. Reduced prec. > drier soils > more soil erosion etc. (however hard/baked soils > decreased infiltration rate > increased SR etc.)

3. *Reduced prec. > reduced surface storage > less water for human use > water deficit

El Niño (CS Detail only)

• Cause: reduced trade winds & reduced upwelling of cold water > increased ocean temperatures

Consequences:

• Floods: Peru/Ecuador

• Droughts: India/Indonesia

• Hot: Canada

• Cold: South US

Deforestation: The Amazon/Indonesian Rainforest

• Reduced interception storage > increased SR

• Reduced ETP > reduced precipitation > drier soils > increased soil erosion > increased soil transferred to rivers > reduced discharge capacity of river

• However, reduced ETP > reduced precipitation > reduced SR

• Positive feedback: reduced ETP > reduced precipitation > reduced water storage in system > reduced ETP (etc.)

CS Detail (Amazon):

• 25% of world’s river discharge

• Rapid deforestation (<20% deforested, <40% deforested & degraded) in last 50 years

CS Detail (Indonesia):

• Reduced prec. over Indonesia > reduced prec. of summer monsoon season (which travels north) > water shortages across South Asia

Eutrophication

Excessive richness of nutrients of a body of water, causing a dense growth of plant life.

Soil Drainage in the UK

1. Soil compaction > reduced infiltration > increased (artificial) throughflow > increased discharge

2. Drier soils > increased soil erosion > increased soil transferred to rivers (via overland flow) > reduced discharge capacity of river

3. Increased soil erosion > increased eutrophication > reduced water quality

Water Abstraction from the Chalk of Southern England

• Reduced groundwater storage = reduced discharge

• Lowers water table

CS Detail:

• South-east England’s main water supply: big aquifer under London

• Recharged by upland areas: during winter when ETP low

• London expansion: decreased recharge effectiveness (urbanisation) = increased discharge

Water Abstraction in the London Basin (CS Detail only)

• Historical source which uses wells/boreholes (machine-drilled wells)

• Peak (1960s): water table dropped to 88m below sea-level

• Therefore increased risk of saline intrusion > water unusable

• Abstraction: decreased discharge in River Thames

• Factory relocation: water table rose 3m/year

• Central/East London (since 2000): groundwater levels decreased by 5-7m

• Monitored by General Aquifer Research Department & Development team to manage abstraction