Edexcel A-level Geography - Water cycle

Global Hydrological Cycle

Circulation of water around the earth

How is the Global Hydrological Cycle a closed system?

No external inputs or outputs.

The amount of global water is finite and constant.

Open system

Receives inputs from and transfers outputs of energy and matter to other systems

Sources that drive the Global Hydrological Cycle (2)

Solar energy (in the form of heat)

Gravitational Potential Energy (precipitation falls to the floor/ rivers flow downhill)

What are the stores of the water cycle? (6)

Ocean

Atmosphere

Biosphere

Cryosphere

Groundwater

Surface water

Flows

Transfers of water from one store to another (km^3 per year)

Fluxes

Rates of flow between stores

Flux size of flow of precipitation

486,000 km³/year

Global Water Budget

Takes into account all the water that is held in stores and flows of the global hydrological cycle.

Residence Time

Average time a molecule of water will spend in one of the stores.

Atmosphere Residence Time

10 days

Ocean Residence Time

3,600 years

Ice Cap Residence Time

15,000 years

What is blue water?

Water stored in rivers, streams, lakes and groundwater in liquid form - visible part of the hydrological cycle

What is green water?

Water stored in the soil and vegetation - invisible part of the hydrological cycle

What is grey water?

Relatively clean water waste from baths, sinks and kitchen appliances

What is groundwater?

The water held within permeable rocks (aka aquifer)

What is the cryosphere?

Areas of the earth where water is frozen into snow or ice

What is fossil water?

Ancient, deep groundwater from former pluvial (wetter) periods

What is the biosphere?

The regions of the earths surface and atmosphere occupied by living organisms

What is evaporation?

The change in state of water from a liquid to a gas

What is transpiration?

The diffusion of water from vegetation into the atmosphere, involving a change from a liquid to a gas

What is precipitation?

The movement of water in any form from the atmosphere to the ground

What are flows in the hydrological cycle?

The physical mechanisms that drive the fluxes of water between the stores

Oceans water store

97%

How much of global water is available for humans to use?

3%

Easily Accessible surface water store

1%

What are the three conditions needed for precipitation to form?

Air cooled to saturation point with a relative humidity of 100%

Condensation nuclei, such as dust particles, to facilitate the growth of droplets in clouds

A temperature below dew point

What is dew point?

The temperature at which dew forms - it is a measure of atmospheric pressure

What are condensation nuclei?

Particles in the atmosphere on which droplets of water vapour can collect

Inputs into the global hydrological cycle

Main Input = Precipiation

How can form of inputs have effects on the hydrological cycle?

Precipitation can occur as rain, snow or hail

With snow, entry of water into the drainage system will be delayed.

How can amount of inputs have effects on the hydrological cycle?

This will affect the amount of water in the drainage basin and the fluxes within it.

How can intensity of inputs have effects on the hydrological cycle?

The greater the intensity, the greater the likelihood of flooding.

How can seasonality of inputs have effects on the hydrological cycle?

This is likely to result in the drainage basin system operating at different flow levels at different times of the year.

How can distribution of inputs have effects on the hydrological cycle?

This is significant in very large drainage basins, such as the Nile and the Ganges, where tributaries start in different climate zones.

Flows (7)

Interception

Infiltration

Percolation

Through-flow

Groundwater

Surface Runoff

River/Channel Flow

Interception

The retention of water by plants which is subsequently evaporated or absorbed by the plant

Infiltration

The process by which water soaks into, or is absorbed by, the soil

Percolation

The slow movement of water underground through permeable rock

Throughflow

The lateral transfer of water downslope through the soil

Surface Runoff

The movement of water that is unconfined by a channel across the surface of the ground

River/Channel Flow

The flow of water in rivers or streams

What are percolines

Lines of concentrated water flow between soil horizons to the river channel

What is infiltration capacity?

The maximum rate at which rain can be absorbed by the soil

Types of Rainfall (3)

Convectional

Relief

Frontal

Convectional rainfall

When the ground warms up, evaporation takes place and the heated air rises and then it cools and condenses to form clouds

The rainfall created is often intense and associated with electrical storms and thunder

Frontal rainfall

Formed when warmer moist air meets colder polar air

The warmer air is forced to rise over the denser, colder air, creating low pressure and rain

Orographic rainfall

When warm, moist air is forced to rise over upland areas, causing moisture to condense and create rainfall

What is rain shadow?

A dry area on the downwind side of a mountain.

How do rain shadows form?

When orographic rainfall has occurred over an upland area, the area on the lee side of the hills will receive less rain because the air descends, warms and becomes drier

What are jet streams?

Bands of fast-moving air which determines the direction of weather systems and their speed of movement

Altitude of jet streams

9-16km

Outputs of the Global Hydrological Cycle (3)

Evaporation

Transpiration

Discharge

Outputs of the Global Hydrological Cycle - Evaporation

The process by which moisture is lost directly into the atmosphere from water surfaces, soil and rock.

Outputs of the Global Hydrological Cycle - Transpiration

The biological process by which water is lost from plants through minute pores and transferred to the atmosphere.

Outputs of the Global Hydrological Cycle - Discharge

Flows into another, larger drainage basin, a lake or the sea.

Evapotranspiration

Total amount of water removed from a drainage basin from liquid water to gas and water in the soil taken through plants that is e from the stomata

Drainage Basin

Area drained by a river and its tributaries

Watershed

An elevated area of land that divides and separates neighbouring drainage basins

What are the physical factors affecting drainage basins? (5)

Climate, soils, vegetation, geology, relief

How does climate affect drainage basins?

Impacts inputs and outputs; has a role in influencing the type and amount of precipitation and overall amount of evaporation

How do soils affect drainage basins?

Affect surface runoff; determine the amount of infiltration and throughflow

How does geology affect Drainage Basins?

Geology can impact on subsurface processes such as percolation and groundwater flow

How does relief affect Drainage Basins?

Slopes can affect the amount of runoff

How does vegetation affect Drainage Basins?

The presence or absence of vegetation has a major impact on the amount of interception, infiltration and occurrence of overland flow, as well as transpiration rates

Human Factors Disrupting the Drainage Basin Cycle (3)

River Management, Deforestation, Changing Land Use (Agriculture and Urbanisation), Resevoirs

Human impacts on drainage basin - river management

Construction of storage reservoirs holds back river flows

Abstraction of water for domestic flow and industrial use reduces river flows

Abstraction of groundwater for irrigation lowers water tables

Human impacts on drainage basin - deforestation

Clearance of trees reduces evapotranspiration, but increases infiltration and surface runoff

Human impacts on drainage basin - changing land use: agriculture

​Arable to pastoral = Compaction of soil by livestock increases overland flow

Pastoral to arable = Ploughing increases infiltration by loosening and aerating the soil

Human impacts on drainage basin - changing land use: urbanisation

Urban surfaces (tarmac, tiles, concrete) have increased surface runoff by reducing percolation and infiltration

Drains deliver rainfall more quickly to streams and rivers, increasing chances of flooding.

Human impacts on drainage basin - reservoirs

Reservoirs hold back the flow of water and create new surface stores

Water Budget

Annual balance between inputs and outputs of the water system.

Water Budget Equation

Precipitation = EVT + Runoff +/- Storage

How are water budgets useful on a national/regional scale?

Give an indication of the amount of water that is available for human use (for agriculture, domestic consumption etc.)

How are water budgets useful on local scales?

Inform about available soil water.

This is the amount of water that can be stored in the soil and is available for growing crops.

Valuable to farmers, who can use it to identify when irrigation might be required, and how much is required.

What is potential evapotranspiration?

The water loss that would occur if there was an unlimited supply of water in the soil for use by vegetation

What is evapotranspiration?

The combined effect of evaporation and transpiration

What is a water surplus?

Precipitation > evapotranspiration

What is a water deficit?

Evapotranspiration > precipitation

River Regime

Annual variation in the discharge or flow of a river at a particular point.

What are river regimes measured in?

Measured in cumecs.

What are simple river regimes?

Where the river experiences periods of seasonally high discharge, followed by low discharge

Example of simple river regime?

Rhone

What is a complex river regime?

Where larger rivers cross several different relief and climatic zones, therefore experiencing the effects of different seasonal climatic events

Example of complex river regime?

Mississippi, Ganges

What are the ways in which precipitation can vary?

Type, amount, intensity, seasonality, distribution

Ways precipitation can vary - type

Rain, snow or hail

Snow can act as a temporary store, delays input of water

Ways precipitation can vary - amount

Has a direct impact on drainage discharge - the higher the amount, the less variability

Ways precipitation can vary - seasonality

Different climates due to seasonal changes have different amounts of rainfall having an impact on the physical processes in the drainage basin

What is cloud seeding?

A type of weather modification that aims to change the amount/type of precipitation that falls from clouds

How does cloud seeding work?

By dispersing substances into the air from aircrafts that serve as condensation nuclei

What substances are used in cloud seeding?

Dry ice, silver iodide, sodium chloride

Example of a place that used cloud seeding - Beijing

Beijing prevented precipitation so the 2008 Olympics had clear skies for the opening ceremony

Example of a place that used cloud seeding - Idaho

Idaho Power injects silver iodide into clouds to increase snowfall so when it melts in summer there's a lot of runoff

Benefits of cloud seeding

More precipitation benefits hydropower companies

Wineries use it to protect their vineyards from damage by suppressing hail

Insurance companies (e.g. in Alberta) fund cloud seeding projects to lessen damage claims

Negatives of cloud seeding

Expensive

Unknown long term impact

Use of potentially harmful chemicals

Perennial Channel

River with continual flow all year

Intermittent Channel

River with lack of flow for a few weeks/months a year

Ephemeral Channel

River that only flows for a few hours/days (Wadi)