The water cycle and Insecurity - Geography Edexcel


Saturation of local hydrological systems

Heavy rainfall can result in the saturation of surface soils, and because more rain keeps coming down, there is not enough time for it to saturate any deeper into the ground. This leads to large amounts of surface run off and flooding.

2015 December - Storm Desmond

Resulted in the normally permeable limestone of Malham cove, to become staurated leading to it becoming the highest water fall in the UK, due to high amounts of surface runoff. 

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Simple Drainage basin (LOCAL)

INPUT - Precipitaion

The land surface characteristics determine what happens next


Local rock types, the quanity of water and the avaliable energy, crontol the througput of water through a local store

OUTPUTS - Evapouration, transpiration, stream flow 

These are the outputs of the system. 

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The global hyrdrological cycle

The HLC is a closed system; no inputs occur form outside, and nothing is lost. 

The nature and state of water changes ll the time, as a result of Solar energy and gravitational  potential energy. 

More evaporation occurs as the global climate warms, therefore there is more moistur ein the atmosphere, leading to larger maounts of precipitaion as the water condenses. 

GPE keeps water moving through the system in a sequence if input, outputs, stores and flows. 

  • At a global scale the system is continuous - outputs govern inputs (nothing is lost or gained)
  • Climactic shifts mean that stores are depleteing, ice is melting without being replenished. 
  • In rming areas ground surfaces dry out - evaporation increases
  • Global air cirulation takes extra vapour to cooler places, leading to more precipitaion 
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Global extremes

Solar engery is concentrated in the tropics - mainly absorbed by the sea.

Sea evaporation produces high levels of rainfall - 74% of rainfall occours at sea

The rest is distributed unevenly (time and space wise) - the seasonal monsoons and droughts of Africa and Asia contrast with temperate climates of Europe. 

Different climactic regions differ in the nature and size of their inputs, transfers and flows of water. 

(See the polar and tropics)

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Polar Hydrology

  • Freeze and thawing, seasonal differences
  • Winter snow insulates the ground 75% of solar radiation is reflected 
  • Permafrost create impermebale surfces
  • lakes and rivers are frozen
  • limited vegetaion cover reduces absorbtion
  • the srpig thaw produces surface run of, increasing evaportaion tenfold
  • the freeszing and thawing cycle causes seasonal release of biogeic gases (caused by plant decompostion) into the atmosphere, as well as carbon and nutirents into the rivers and seas
  • Frontal precipitaion and low humidity 
  • Annual precipiation is less than 200mm

The cryosphere

seasonal thaws bring increased surface saturation and thinning permafrost. If this thaw becomes continuous water will flow away and be lost. This is know as crysosphere loss. 

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Tropical rainforest hydrology

  • few seasonal differences
  • dense vegetaion intercepst and consumes upto 75% of precipitation
  • 50-75% of precipitaion then returns by evapotranspiration (the water lost to the atmosphere from the ground surface, evaporation from the capillary fringe of the groundwater table, and the transpiration of groundwater by plants whose roots tap the capillary fringe of the groundwater table)
  • evapotranspiration cools the air as energy is used during this process
  • Rainforsts generate their own rain; most is recycled within the tropics
  • Less than 25% of rain reaches rivers of other surface water
  • limited ground water
  • Cloud factories
  • Constant high tmeperature
  • Convectional rainfall adn very high humidity 
  • Anual precipitaion over 2000mm
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The global water budget

Oceans loss more water through evaporation than they gain through precipitaion.

For land masses the opposiite is tue

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The drainage basin water cycle

The drainage basin is a subsystem within the Hydrological cycle. Unlike the GHC drainage basins are open systems, with external outputs, therefore amounts of water in the drainage basin vary over time. 

A drainage basin is an area of land drained by a river and its tributaries, also known as a river catchment. 

The boundary of a drainage basin is defined by the watershed - a ridge of high land dividing waters from flowing to different rivers. 

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DB - Inputs


  • Air cooled to saturation point with relative humidity of 100%
  • condensation nuclei, eg. dust particles, to facilitate the growth of droplets in clouds
  • a temperature below the dew point


  • The amount of rainfall - the higher the amount the less variability in DB pattern
  • The type of precipitation - the formation of snow can act as a tempory store and large fluxes of water can be released into the system at one time
  • Seasonality - Monsoon, Mediterranean or continental climates have strong seasonal patterns of precipitation  
  • Intensity - a major impact on flows on or below the surface, over saturation of ground soil
  • Variability
    • Secular variability is long-term: Climate change
    • Periodic variability annual or seasonal etc.
    • Stochastic variability results from random factors, localisation of a thunderstorm.
  • Distribution of rainfall - particularly noticeable in large basins that start in different climatic zones 
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DB - Fluxes 1

Interception - A process by which water is stored in vegetation. 

Interception loss:

  • Greatest at the start of a storm (especially following a dry period)
  •  The interception capacity varies depending on the type of tree 
    • Coniferous forests with dense needles = greater accumulation of water
    • Contrasts between deciduous forest in summer and winter


  • Decreases over time as rainfall is consistent
  • Higher antecedent soil moisture leads to surface flow 
  • Soil porosity increases infiltration
  • Amount and seasonal changes in vegetation (more significant in forested areas)
  • Raindrop size increases infiltration 
  • Slope angle encourage surface runoff opposed to infiltration
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DB - Fluxes 2 (Flows and Transfers)

Flows and Transfers

Overland flow 

  • Main way that rainwater is transferred to the river 
  • Precipitation flow must exceed infiltration rate
    • Persistently high rainfall over time, torrential rain, meltwater release
    •  Arid or semi-arid regions have little infiltration rate
  • Primary agent of soil erosion 
  • Occurs when depression storage in puddles is exceeded
  • Feature of urban areas


  • Lateral transfer of water through the soil through natural pipes and percolines 
  • Occurs rapidly in porous soils 
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DB - Fluxes 3 (Flows and transfers)


  • Continuation of infiltration 
  • Deep transfer of water into permeable rocks 
    • either through joints or into pores 
  • Likely to be associated with humid vegetated climates

Saturated overland flow

  • Upward movement of the water table into the evaporation zone
  • After a succession of winter storms (UK 2015) the water table rises to the surface in depressions at the base of hillsides 

Base flow

  • Slow transfer of perlocated water through porous rock 
  • Helps maintain a steady level of channel flow in all weather

Channel flow

  • in the river, once the water from transfer processes reaches it
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DB - Outputs 1


  • Moisture is lost to the atmosphere from water surfaces and soil
  • Needs warm, windy, and dry conditions 
  • Temperature, sunlight, humidity and wind speed, size, depth, water quality of the body of water


  • Biological process by which water is lost from plants through stomata and transferred to the atmosphere 
  • Depend on time of year, type and amount of vegetation and the rate of diffusion potential in the atmosphere, and the length of the growing season
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DB - Outputs 2


  • Combined effect of evaporation and transpiration
  • Represents water loss to the atmosphere
    • accounting for the removal of almost 100% of precipitation in arid/semi-arid areas and around 75% in humid areas

Potential Evapotranspiration

  • the water loss that would occur if there was an unlimited supply of water in the soil to be used by vegetation 
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Physical factors influencing drainage basin


  • Influences the type and amount of precipitation overall and the amount of evaporation
  • climate also influences vegetation type


  • Soils determine the amount of infiltration and through flow
  • And indirectly the type of vegetation 


  • Impacts the subsurface processes like perlocation and groundwater flow 
  • Indirectly it affects soil formation


  • Altitude can impact precipitation totals and the amount of surface runoff


  • The presence or absence of vegetation has a major impact on the amount of interception, infiltration and overland flow, as well as transpiration rates
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Human factors influencing drainage basin


  • Cloud seeding - the introduction of silver iodide pellets or ammonium nitrate, to act as condensation nuclei, to attract rain droplets
  • The aim is to increase rainfall in drought-stricken areas 

Evaporation and EVT

  • Changes in global land use through deforestation have huge amounts of influence
  • Increased evaporation potential from artificial reservoirs (mega dams) like Aswan Dam, Egypt
  • The channelisation of rivers in urban areas into conduits cut down surface storage and thus evaporation 
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Physical factors 2


  • Its largely influenced by vegetation and density, therefore deforestation and afforestation have impacts
    • Deforestation - leads to reduction in EVT and increased surface runoff 
      • This results in increased flooding potential and decline of surface storage and a decrease in the lag time between peak rainfall and peak discharge 
      • Speeding up the cycle
      • Research in Nepal - increase in sediment downstream (pg. 12)
    • Aforestation - Should reverse the impact but there is a period of time where there is more runoff
      • This is a result of tractors and planting equipment 
      • which only stops after 30 years, when the trees are fully grown

Infiltration and Soil water

  • Five times greater under forests compared to grassland
  • Farmland means less infiltration and increased soil compaction
  • waterlogging and salinisation are common if there is poor drainage  
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Human impact on ground water

  • irrigation for extensive cereal farming leads to a declining water table
  • Aral Sea

recent reductions in London of water using manufacturing has to lead to less groundwater being abstracted. 

As a result groundwater levels have begun to rise leading to surface water flooding, flooding of cellars and increased leakage into tunnels such as the underground. 

This makes water supplies much more likely to get polluted.

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Case study - Deforestation is Amazonia

  • Over 20% of the rainforest has been destroyed (accelerating over the past 50 years) 
    • Through a combination of; Cattle ranching, large-scale agriculture (biofuels and soya), and the general development of towns and cities
    • Illegal logging playing a small part too
  • The Amazon contains over 60% of the world's rainforests - the earth's lungs 
    • Removing CO2 as they photosynthesise, acting as carbon sinks
    • Destruction of the forests reduces its capacity 
  • In a forest environment, 75% of intercepted water is returned to the atmosphere through EVT, this reduces to 75% when the forest is cleared
  • This dryer climate can lead to desiccation and further degradation of the rainforests
    • The El Nino southern oscillation can lead to significant drought anyway in Amazonia
  •  The water cycle:
    • As more water runs off into the Amazon drainage system, this exacerbates the possibility of severe flooding and mudslides and leads to aquifer depletion, because less water infiltrates to recharge them
    • Overland flow also leads to soil degradation and erosion as nutrients are washed away
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Water Budgets

  • They are the balance between precipitation, evaporation and surface run-off
  • The water balance varies between continents
    • south America is the best endowed 
    • Africa is the least
  • This is an important distinction because in some places there are severe regional difference 
    • eg. Monsoon, where at certain points there will be a shortage and at others surplus 

In Africa the difference evapotranspiration and precipitation is very small, leading to very little surface run-off entering rivers. 

Water budgets at a regional scale provide information on the availability of water supplies At a local scale they show the annual balance between inputs and outputs, and how this can impact availability.

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River regimes

  • defined as the annual variation in discharge or flow of a river at a particular point 
  • Much of this river flow is not from immediate precipitation or surface runoff
  • but is supplied from groundwater between periods of rain
    • this feeds the system from a base water flow
  • This masks fluctuations in streamflow caused by immediate precipitation

British rivers flowing over chalk show this feature well because they maintain their flow even in very dry conditions, which is a result of base flow from chalk aquifers.

Character of regimes:

  • size of the river and where the measurements have been taken in the basin (large rivers have complex regimes resulting from varied catchments)
  • Type of precipitation: reflect rainfall seasonal maxima (when snow/glaciers melt)
  • Temperatures: evaporation 
  • Geology and overlying soils: water is stored as groundwater in permeable rocks - gradually released into the river
  • Vegetation: wetlands hold water and release it slowly into the system
  • Human: dam building regulates flow
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Storm hydrographs

  • Show the variation of discharge within a short period of time (normally a storm or a group of storms )
  • Prior to the storm, the main supply of water to the river is through groundwater or base flow.
  • as the storm progresses water comes by a number of routes
    • water infiltrates into the soil = throughflow
    • water also flows on the surface = Overland flow
    • Quick flow 
  • The hydrograph records the changing discharge of the rover in response to the specific input of precipitation

Rising limb - The part of the graph where the discharge starts to rise

Peak discharge - The time when the river reaches its highest 

Lag time - the time interval between peak rainfall and peak discharge

Base flow - the normal day-to-day discharge of the river

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Urbanisation and Hydrological processes

  • Buildings = clearing of vegetation, exposing soil and increasing overland flow
    • Disturbed soil increases erodability 
    • Concrete and tarmac are impermeable 
  • High density of buildings means that rain falls onto roofs and is then removed through pipes to drains
  • Drains and sewers are built reducing the distance water must travel before reaching a channel 
  • Urban rivers are channelised with embankments to guard against flooding 
  • Bridges can restrain the free discharge of flood waters and act as local dams for upstream floods 

In extreme weather events, urban areas like Machester, Leeds and York are vulnerable. They must manage flood control problems with a higher, much faster peak discharge, as well as pollution problems from roads as water runs off.

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Types of drought: Meteorological

  • Shortfalls in precipitation as a result of short-term variability or long-term trends
  • They increase the duration of the dry period
  • High temperatures
  • High winds
  • Strong sunshine
  • Low relative humidity 

The causes:

  • Natural variation in atmospheric conditions 
  • desiccation caused by deforestation
  • El Nino ad climate events
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TOD: Agricultural and Hydrological

Agricultural ...Following Meteorological drought

  • Overgrazing can accelerate the onset of agricultural drought
  • The deficiency leads to lack of soil moisture and soil water availability
  • Effects the plant growth and reduces biomass 
  • if the deficit stage is elongated and severe soil moisture budgets can show 


  • associated with reduced stream flow and groundwater levels
  • Due to reduced precipitation and high rates of evaporation 
  • Results in 
    • reduced storage in lakes and reservoirs 
    • salinisation 
    • poor water quality 
  • Threats to wetlands 
  • decreasing water supply for urban areas
  • Rural Northern Brazil - no rivers, water depends on seasonal rainfall
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Famine Drought

The widespread failure of agricultural systems, food shortages over time develop into famines.

 They have severe social, economic and environmental impacts.

These humanitarian crises depend on international solutions.

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El Nino-Southern Oscillation

El Niño happens when the seawater temperature rises in surface waters of the tropical Pacific Ocean. Every two to five years the Pacific Ocean has this event. A weak, warm current starts around Christmas along with the coast of Ecuador and Peru. It lasts for only a few weeks to a month or more. Every three to seven years, an El Niño event may last for many months. This can change the weather and have important effects around the world. Australia and Southeast Asia can have drought but the deserts of Peru have very heavy rainfall. East Africa can have both. In a La Nina event, the weather patterns are reversed.

The Southern Oscillation was discovered by Sir Gilbert Walker in 1923.[1] It is a "seesaw" of atmospheric pressure between the Pacific and Indian Oceans. There is an inverse relationship between the air pressure measured at two sites: Darwin, Australia, in the Indian Ocean and the island of Tahiti in the South Pacific. The Southern Oscillation Index (SOI) is the difference in sea-level pressure measured at Tahiti and Darwin. The Cold Tongue (CT) Index measures how much the average sea-surface temperature in the central and eastern Pacific near the equator varies from the annual cycle. The two measurements are anti-correlated so that a negative SOI is usually together with an unusually warm ocean wind known as El Nino.

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Case study - Human influences on drought

Human activities do not cause drought, but they do make them more severe.

Positive feedback loop.

  • Sahel is a semi-arid region on the fringe of the Sahara 
  • It contains some of the worlds poorest developing countries
  • High variability of rainfall at climate scales 
    • Seasonally - drought sensitive, transitional climate zone
    • Annually - warm sea surface temperatures in tropical seas favour strong convectional uplift over the ocean that weakens West African monsoon.
  • Ethiopian-Eritrean drought 
    •  increased by socio-economic conditions due to environmental degradation caused by overgrazing, deforestation for firewood, high levels of rural poverty
    • Ethiopian and Eritrean were also at war
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Case study - Drought in Australia

Two main types

  • Serious deficiency 
  • Severe deficiency 

Reasons why?

  • Low, highly variable rainfall (climate is dominated by sub-tropic high pressure)
  • Droughts vary - some are intense, some last for years, some are localised 
  • Linked to El Nino
  • There has been a shift in rain patterns making the East (where most people live) drier compared to NW

The Big Dry

- associated with longer-term climate change 

- warmer, drier climate

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Wetland functions

  • Supporting 
  • Primary production ar a very high level
  • Nutrient recycling 
  • food chain support
  • life support system of the carbon cycle


  • flood control 
  • groundwater recharge and discharge
  • Protecting land from erosion
  • water purification


  • Fuelwood, peat, fisheries, mammals and birds - Tourism

Cultural - Aesthetic, recreational value and cultural heritage

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Wetland destruction

  • Drought has major effects on wetland
  • limited precipitation = less interception because vegetation will deteriorate 
    • thus less infiltration and perlocation to ground stores (aquifers)
    •  Leading to fall in the water table 
  • Evaporation will continue and transpiration rates will decrease = Less functional wetlands 
  • Desiccation may also accelerate due to wildfires
  • In Europe and USA large areas of wetland have been destroyed for agriculture and urban development

Water transfer schemes affecting wetland 

  • Jonglei Canal project - white Nile diverted from Sudd swamp to dry land in south Sudan 
  • Okavango delta - degradation for cattle rearing
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Physical causes of flooding

  • Meteorological or long-term climactic causes
  • Prolonged heavy rain (low-pressure systems or depressions)
    • autumn or winter
  • Intense seasonal monsoon rainfall causes flooding
    • 70% of rainfall in 100 days
    • Low lying plains of large rivers in southern Asia at risk
    • Bangladesh over half of the country is <12.5m above sea level
  • Excessive rainfall across larger river basins associated with tropical cyclones (Southern Africa)
  • Snow and ice melt in higher latitudes 
    • Spring melt causes extensive flooding (Asia and America)
    • quick transition from winter to spring causes upper to melt while lower reaches remain frozen 
    • flood water is held up by ice dams
  • Glacial outburst floods occur as ice dams melt (Himalayas)
    • draining of glacial lakes 
    • landslides and earthquake-induced dam failure
  • Estuarine areas with high river flows meet storm surges - only partly climatological
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Secondary physical causes of flooding

River basins 

Vegetation - higher cover means more interception, storage and evapotranspiration. Increasing lag time.

Slope - Steeper slopes = more surface runoff

Rock type - permeable rock = greater infiltration and ground storage 

Drainage density - Where density figure is low there are longer lag time and less risk of flooding

Soil depth - Deeper soil absorbs more water = less surface run off

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