TOPIC 5: THE WATER CYCLE AND WATER INSECURITY

hydrological cycle

• also known as the water cycle

• the continuous movement of water on, above and below the surface of the earth

• closed system at a global scale so the total amount of water on earths remains constant

• no external inputs or outputs of water

drivers of the global hydrological cycle

• solar energy: sun heats the water on the earths surface causing it to change state and evaporate into the atmosphere → water is also drawn from the soil by plants and evaporation from leaves and stems by the process of evapotranspiration

• gravitational potential energy: the force that causes precipitation to fall from the sky and water to flor downhill, both on the surface and through the soil, and back to the ocean

stores of the hydrological cycle

• oceans: water stored in liquid form, with only a minute fraction as icebergs

• cryosphere: water found in a largely solid state → some formed from melt water

• terrestrial: water stored in rivers, streams, lakes and groundwater → also known as blue water → visible part of the cycle → green water → water is stored as vegetation, the invisible part of the cycle

• atmosphere: water largely exists as vapour with the carrying capacity directly linked to temperature → can contain liquid water or ice crystals in high altitudes in clouds

fluxes and flows in the hydrological cycle

• flow: movement or transfer of water from one store to another, describing the pathway the water takes → precipitation, surface runoff, infiltration, evaporation, transpiration

• flux: rate of flow of water between different stores → the quantity of water that is transferred per unit of time

relative importance and size of water stores

• size and importance of each star depends on the amount of water flowing between them

• over longer time scales, some stores grow while others shrink

Relative importance and size of water fluxes and flows

• vary spatial and temporally

• largest and most important flux/flow is the evaporation of ocean water into the atmosphere → precipitation and condensation as a result also an important flux

• far smaller and less important is evaporation of water held in surfaces and vegetation in the atmosphere

• evaporation and precipitation arguably most important in establishing overall balance and also influences fluxes on a more local scale, regulating how water is distributed and where it is stored locally

evaporation

• occurs when liquid water changes state into as gas becoming water vapour → normally gains energy from solar radiation → increases amount of water stored in the atmosphere

• scale of flux varies by location and season

• lots of solar energy, large supply of water, warm, dry air → amount of evaporation will be high (vice versa)

condensation

• occurs when water vapour changes state to become a liquid, losing energy to the surroundings

• happens when air containing water vapour cools to its dew point → the temperature at which it will change from gas to liquid

• water droplets can stay in the atmosphere or flow to other subsystems → decreases amount of water stored in atmosphere

• scale demands on amount of water vapour in the atmosphere and temperature → lots of water vapour + large or rapid drop in temperature = levels will be high

precipitation

• the main flow of water from the atmosphere to the ground

• clouds form when warm air cools down, causing the water vapour in it to condense into water droplets, which gather as clouds → when the droplets get big enough, they fall as precipitation

• these water droplets are too small to form clouds on their own and for clouds to form, there have to be tiny particles of other substances such as dust or soot to act as cloud condensation nuclei → give water a surface to condense on → encourages cloud formation rather than dispersal of moist air

• vary seasonally → more rainfall in winter than summer in the uk

• vary spatially → generally higher in the tropics than at the poles

cryospheric processes

• such as accumulation → build up of snow and ice and ablation → melting of snow and ice → balance varies with temperature

• during periods of global cold → inputs greater → water transferred to the cryosphere as snow and less water is transferred away due to melting

• periods of warmer global temperature → scale of ecosphere store reduces as losses due to melting being larger than inputs of snow

• earth is emerging from a glacial period → reached its maximum around 21000 years ago

• climate change is shrinking these ice stores → Antartica, Greenland, artic, alpine glaciers

• variations happen over different time scales → glacial and interglacial periods → or in shorter timescales → annual temperature fluctuations during the winter and summer

residence times

• the average length of time a water molecule will spend in the reservoir or store

• varies greatly from store to store

• calculating residence times important tool for developers and engineers → consult a reservoirs resident time when ovulating how quickly a pollutant will spread through the reservoir → stores with a slower turnover tend to be more easily polluted as the water is in situ for a longer length of time

• may also influence how communities use an aquifer

non-renewable stores of water

• constant circulation and replenishment of stores without any losses means water is generally considered a renewable resource

• however, some stores are non-renewable as they are not replaced in short period of time

• fossil water: water that has been contained in an undisturbed place, salt groundwater in an aquifer for a millennia or longer such as the Sahara desert → may be extracted for human purposes such as agriculture industry and consumption → little no significant recharge

• ablation → melting of glaciers, due to climate change, is reducing storage of water as ice → not replaced → cryosphere losses

drainage basin

• catchment area from which a river (and tributaries) gets its water

• boundary marked by a ride of high land called the watershed

• can be any size

• subsystem within the global hydrological system and is an open system

• input —> precipitation

• output —> ocean: streamflow, discharge —> atmosphere: evaporation, transpiration

• series of stores that hold water, linked by flow or transfers

• amount in drainage basin varies over time

parts of a drainage basin: watershed, catchment area, source, confluence, tributary, mouth

• watershed: ridge of highland surrounding a drainage basin that marks the boundary between the drainage basin

• catchment area: area within the drainage basin

• source: beginning or the start of a river

• mouth: point where the river connects to the sea

• confluence: pain where two rivers or stream join

• tributary: stream or small river which joins a larger stream or river

drainage basin hydrological cycle components: inputs and outputs

• input: precipitation - any form of moisture falling from the atmosphere and can be in the form of snow, rain, hail, skeet, dew or frost

• output: evaporation - physical process y which moisture is lost directly into the atmosphere from water surfaces and soil as water vapour

• output: transpiration - biological process by which water Is lost from a plant through stomata in its leaves and transferred to the atmosphere

• output: evapotranspiration - combined effect of evaporation and transpiration representing the most important aspect of water loss to the atmosphere

drainage basin hydrological cycle components: stores

• surface storage: rainfall that is temporarily retained and does not immediately add to the streams flow such as rain that sits on ponds

• channel storage: the storage of water in streams or rivers

• water table: marks the boundary between unsaturated and saturated zones within the earth and can fluctuate based on rainfall amounts and other factors

• groundwater storage: water held below the water table in aquifers

• depression storage: storage of water in hollows and holes in the ground to form puddles

• soil moisture storage: the water that is held in the spaces between soil particles

• atmosphere: water is stored as droplets in clouds

• oceans and seas: water is stored in liquid form in the oceans and seas

• snow and ice: water is stored in solid form as snow and ice in ice sheets and glaciers

drainage basin hydrological cycle components: flows

• infiltration: process whereby water soaks into or is absorbed by the soil and is a vertical movement - capacity depends on soil texture, vegetation cover, existing soil moisture and time —> rate of infiltration (infiltration capacity) depends on antecedent moisture conditions and the soils porosity

• channel flow: water that flows along the river itself and is fed by three transfer processes: groundwater flow, overland flow and through flow

• direct runoff: encompasses any water that reached the river channel quickly, could include overland flow, routes such as artificial pipes and fast subsurface pathway —> tends to be quite fast

• through flow: the lateral transfer of water down slope through the soil via natural pipes (line of roots or soil weaknesses) and percolines (lines of concentrated water flow between soil horizons to the river channel) —> while slower than direct overland flow, this shallow transfer can occur quite rapidly in porous, sandy soils

• percolation: water seeps down through the soil into the water table via permeable rocks (those with joints (pervious) or those that are porous)

• groundwater flow: water flowing slowly below the water table through permeable rock; this feeds into rivers through riverbanks and riverbeds and this is called baseflow —> slow transfer of water through rocks but in limestone areas where there are extensive underground channels the flow can be fastest

interception flows

• interception loss: vegetation surfaces catches falling precipitation (this is called interception), some of this water will evaporate and become an output of the drainage basin so interception loss is water evaporated form the plants surface

• through fall: precipitation which drips of leaves and does reach the ground

• stemflow: intercepted precipitation which runs down the stems of plants to reach the ground

• vegetation slows down and reduces water transfer

overland flows

• overland flow: movement of a sheet of water across the ground

• infiltration-excess occurs when rainfall intensity is so great that not all water can infiltrate, irrespective of how dry or wet the soil is

• saturation-excess occurs when rainfall continues for a long time and thus, the entire soil becomes saturated and overland flow begins

• this is a rapid and fast transfer

physical factors within drainage basins: climate change

• inputs: amount of precipitation and seasonal patterns —> in some climates (monsoon, mediterranean, continental climates) strong seasonal patterns of rainfall or snow will have a major impact on when inputs are received —> if precipiation falls as snow, it can act as a temporary store and then a large flux of water may be released into the system when experiencing rapid thawing

• flows: more infiltration-excess overland flow when precipitation is greater or more intense; it is difficult for rain to infiltrate if the rain is too intense —> convectional thunderstorms are short, heavy bursts of precipitation and are confined to a small area and can cause a sudden rise in the channel flow from increased direct runoff —> passing of a depression will give a longer period of steady rainfall extending all over a drainage basin which allows time for infiltration —> if the ground is frozen, water flows over the surface and cannot infiltrate —> warmer weather results in more leaves, causing more interception and evapotranspiration and so less water reaches the ground, so infiltration and percolation are reduce

• outputs: evaporation and transpiration is greater when hotter —> cold temperatures slow or prevent evapotranspiration

physical factors within drainage basins: vegetation

• inputs: vegetation transpires and so if there’s lots of it, higher transpiration rates which will increase local rainfall

• flows: large forests intercept lots of rain, slowing infiltration, direct runoff and through flow —> contains roots that text net and break up the soil which create cracks and fissures in the soil, allowing for more rapid infiltration and increased infiltration capacity

• outputs: extensive tree cover increases evapotranspiration and reduces channel flow

physical factors within drainage basins: soils

• flows: more spaces such as sandy soils allow more water to infiltrate, reducing overland flow but increasing through flow —> some soils, pipes develop as water flows along the lines of roots or burrows increasing through flow rates —> compacted soils inhibit infiltration leading to greater amounts of overland flow

• output: clay soils reduce infiltration and so increase evaporation from the ground and runoff —> saturated soil creates more overland flow removing water more quickly form the drainage basin —> deeper soil able to store more water so this will reduce overland flow and output of water from the system

physical factors within drainage basins: geology

• flows: porous describes the physical structure of the rock where interconnected voids or pores within it structure which allows the rock to be permeable making it easier for water to flow through, the more permeable it is —> tend to led to less overland flow and increased through flow and groundwater flow

• output: impermeable rocks such as granite prevent infiltration and percolation, producing more overland flow and greater number of streams leading to water moving out of the system more quickly

physical factors within drainage basins: relief

• inputs: orographic (relief) rainfall created on high ground —> shallow slopes prove infiltration as water has time to penetrate into the ground

• flows: very steep slopes tend to encourage overland flow and reduce infiltration

• output: steeper slopes move the water out of the drainage basin quickly reducing the amount of output through evaporation

human factors: water storage reservoirs

• dams increase surface water stores and evaporation and makes access to fresh water easier for humans

• reduces downstream river discharge

• lake Nasser behind the Aswan Dam in Egypt estimated to have evaporation losses of 10 - 16 cubic metres every year representing a loss of 20-30% of the Egyptian water volume from the river nile

human factors: water abstraction

• reduces groundwater levels and changes river flows

• growth of global population results in increased water demand

• where precipitation levels sufficient, water can come from rivers themselves which will lower the levels of water in channels

• where precipitation levels are low, alternative supply is groundwater - this supply of water in porous rocks underground is known as an aquifer

• excessive abstraction - water taken too quickly and does not recharge naturally, is unsustainable and can lead to the depletion of groundwater stores

• in some locations, reduced industrial activity has increased groundwater rebound increasing the risk of groundwater flooding if the water table reaches the land surface

• climate change may lead to a reduction in annual rainfall, less water available per person and this would lead to a need to extract more from groundwater stores which is unsustainable

human factors: water abstraction case studies

• China - groundwater used to irrigate 40% of farmland and to provide 70% of drinking water in northern and northwestern regions with extraction increasing by 2.5bn m³/year and consequently, groundwater levels in arid north china plan dropping by as much as a metre per year

• Aral Sea - began shrinking in the 1960s when soviet irrigation schemes for cotton growth took water form the Syr Darya and Amu daryl rivers greatly reducing the amount of water reaching the sea; by 1994, levels fallen by 16m, surface area declined by 50%, volume declined by 75% and salinity levels increased by 300%

• London - reductions in water-using manufacturing activity led to less groundwater abstraction but also groundwater levels have began to rise; this is called groundwater rebound. this lets to surface water flooding, flooding of cellars and basements as well as increased leakage into tunnels such as in the London Underground

human factors: changing land use

• infiltration is up to 5X greater under forests when compared to grasslands

• conversion to farmland reduces interception, increases soil compaction and more overland flow

• changing from natural landscapes to urbanised landscapes increases impermeable surfaces leading to an increase in overland flows and a reduction in infiltration - can lead to flooding as rive levels rise quickly leading to a short lag time and high peak discharge

• UK - Winchester and maidenhead (2014 floods) and Carlisle, York and Manchester (2015 floods), River Severn, Tewkesbury 2007

human factors: deforestation

• removal of trees leads to a reduction in evapotranspiration and increase in overland flow which increases flooding potential

• leads to a decline of surface storage and a decrease in lag time between peak rainfall and peak discharge

• speeds up the cycle

amazon rainforest water cycle

• dense and multiple canopies means rates of interception and evapotranspiration are high causing high humidity and heavy local convectional rainfall (self-sustaining cycle)

• evaporation is important in sustaining regional rainfall in areas around the periphery of the tropical rainforests

• humans activities are disrupting this process

• precipitation: crucial in maintaining forest and regional water cycle and its high levels of rainfall due its location

• overland flow: amazon river and its tributaries transport vast quantities of water to the Atlantic ocean

• evapotranspiration: forests major contributor transferring moisture form the oil and vegetation into the atmosphere

• deforestation and changing land use: disrupted water cycle, affecting regional precipitation patterns and evapotranspiration rates

amazon rainforest water cycle: deforestation

• driven by agricultural expansion, general development towards towns and roads, legal and illegal logging

• disrupts water cycle by reducing amount of water returned to atmosphere via evapotranspiration leading to a decrease in atmospheric moisture and reduction in precipitation in the region

• in a forest environment 75% of intercepted water is returned to the atmosphere via evapotranspiration, reducing to 25% when the forest is cleared so areas become drier, increase drought risk, drier climate, desiccation and further forest degradation

• removal of trees can also increase overland flow, so water flows more quickly rather than being absorbed, increasing flooding during wet season and decrease water availability during dry season

amazon rainforest water cycle: deforestation impacts on water storage

• Amazon rainforest acts as a natural water store, holding water in trees, soils and wetlands and acts asa filter for rainfall, slowing down water flow and reduce flood risk

• fast movement of water out of the system can lead to aquifer depletion; less water infiltrates into the ground to recharge the water table

• deforestation decreases capacity for water storage, increases speed at which water moves through the basin, leading to increased overland flow; can result in more intense flooding as water reaches river more quickly and in larger volumes (rainy seasons)

• tree roots bind the soil together, preventing erosion but when deforestation occurs, soil becomes vulnerable to erosion by wind and rain, degrading soil quality and reducing lands ability to support vegetation and increase sediment load in rivers, negatively affecting aquatic ecosystems

amazon rainforest water cycle: new water storage reservoirs

• large scale hydroelectric dams built on rivers in the Amazon, such as Belo Monte Dam, creates new water storage within the drainage basin which disrupts the natural flow of water by regulating river discharge which can affect local water availability and ecosystems

• dams alter water flow patterns, may lead to changes in sedimentation impacting aquatic life and reduces natural replenishment of groundwater

the water budget

• shows the balance between inputs and outputs in a hydrological system and how it impacts soil water availability

• equation allows a calculation of water supply during a year and identification of time periods when thermal not be nought to meet natural and human needs (P = Q+E±S)

• water balance of a location can be shown using a water budget graph

• negative budget - outputs greater than inputs, leading to a negative balance of a water in a system; water is lost over time and in deficit

• positive balance - surplus of water

water budget model in a drainage basin

water budget and climate type

• latitude is a key factor influencing water budgets affecting climate type and seasonality

• as you move further away from the equator seasons become more pronounced (tropical regions have no seasons)

• tropical regions: low pressure belts: climates that are consistently hot and wet with positive water balance

• desert environments: high pressure belts: negative water balance

• polar environments: high pressure belts: cold and dry

• temperate environments: low pressure belts: cool, wet and have a positive water balance

global atmospheric circulation system

• important in determining the water budget for an area

• positive water balance in climates like temperate and tropical as they are in low pressure areas where river flowing form these zones are vital ins applying zones of deficit (Nile and Egypt)

water budget graphs: temperate climate

• four stages of water balance in a cool temperate environment where sol moisture deficit does occur

water budget graphs helps highlight river basin management challenges

• periods when water drought or deficiency is likely

• periods where flooding is likely

• when irrigation is likely needed

• longer term changes in the storage capacity of the drainage basin

• need for water transfer system

• places with similar annual rainfall can still experience different water issues (effective rainfall is the amount f precipitation remaining after evaporation)

river regime and discharge

• annual variation in discharge or flow of a river at a particular point or gauging station

• discharge of a river is the volume of water that moves past a certain point in the river channel per second; measured in cumecs

• river regimes can vary in river basins due to impact of climate, geology and soils

reasons for variation in river regimes: climate

• amount, pattern and intensity of precipitation

• regimes often reflect seasonal maxima or when snow fields/glaciers melt

• temperature determines rate of evapotranspiration; evaporation higher in the summer as temperatures are warmer as will transpiration as plants will have leaves and be growing

• temperatures below freezing lead to a suspension of channel flow

reasons for variation in river regimes: geology and soils

• geology, overlying soils, permeability and porosity

• porous or permeable rocks act as aquifers; groundwater storage so water released slowly leading to a very steady regime with fairly consent supple from base flow

• impermeable geology can lead to higher levels of saturated overland flow and therefore, quick response regime with peaks following periods of heavy rain

• some soil types can retain water more easily such as sand soils (larger air spaces between grains than clay soils); water can infiltrate quickly leading to a steady regime

• deep soils can also store more water leading to a steady regime

reasons for variation in river regimes: other factors

• human activities such as dam building for energy or irrigation which regulates the flow

• amount and type of vegetation cover such as wetlands can hold water and release it very slowly into the system

• size of the river and where measurements are taken in the basin

• many large rivers have very complex regimes as they cross many climate types

contrasting complex river regimes - The Yukon, the Amazon, and the Indus

• the yukon river (Alaska, Canada); low discharge in winter where precipitation falls as snow while it increases rapidly in spring and summer due to snowmelt. the soils are also prone to permafrost, making them impermeable for most of the year so overland flow happens quickly once surface snowmelt occurs

• the amazon river (South America); more gradual rise in discharge though there is a wet and dry season, the difference is not as profound because of high levels of rainfall in all months which infiltrate the ground quickly due to thin soils and sedimentary rocks

• the Indus River (south and Central Asia): fed by snowmelt from the Himalayas in spring and by regular precipitation in the monsoon months; the dry season from October to march means flows at a lower level than Yukon as its water deficit for 5-6 months; mainly made up if igneous and metamorphic rocks which are relatively hard and impeccable do river discharge can increase quickly during monsoon season

what is a storm hydrograph

• a hydrographic is a line graph that plot changes in a rivers discharge over time which can record river flow on a daily, monthly, annual or individual storm event basis

• a storm hydrographic records an individual storm event by plotting the hourly discharge of a river showing how quickly the rivers discharge increases in response to the precipitation event

hydrograph: ‘flashy’ river: rock type, soils, relief

• rock type: impermeable rocks (granite) restrict percolation and encourage rapid direct overland flow; peak discharge increases as more water reaches the river faster making the hydrographic flashier

• soils: clay soils have low porosity and grains swell when absorbing water so water infiltration is slow; thin soil becomes saturated quickly leading to more overland flow

• relief: water flows rapidly down steep slops and reaches channel quickly, reducing lag time as relief promotes overland flow but also means less time to infiltrate the soil so overland flow and peak discharge is greater

hydrograph: ‘flashy’ river: basin size, shape, drainage density

• basin size: in small basins, water reaches the channel rapidly due to shorter distance of travel so lag time will be shorter

• shape: in circular basins it will take less time for the water to reach the channel as extremities are equidistant from the channel

• drainage density: high drainage density means storm water will reach the main channel rapidly, leading to shorter lag time and flashier hydrograph

hydrograph: ‘flashy’ river: vegetation and human factors

• vegetation: thin grass intercepts little water and loss by evapotranspiration so more water reaches the channel rapidly leading to a short lag time; low density deciduous in winter means low levels of interception and more rapid movement, leading to a short lag time

• urbanisation: produces impermeable concrete and tarmac surfaces which carries water directly to the river; gutters and drains allow for. a quick route, shorter lag times and higher peak discharge but can be managed by channel straightening which reduces flood risk but moves the risk further downstream

• land use change: deforestation reduces interception, bare fields increase overland flow, deciduous trees leads to less interception and more overland flow in winter, ploughed arable land leads to less interception and more overland flow and shorter lag time

hydrograph: ‘flat/subdued’ river: rock type, soils, relief

• rock type: permeable rocks, such as sandstone, allows water to percolate through pore spaces and fissures into the groundwater store limiting rapid overland flow, increases lag time and reduces peak discharge

• soils: sandy soils have high porosity allowing for water infiltration and increasing the rate of infiltration; deep soils allow for more infiltration, increases lag time and reduces peak discharge

• relief: low and gentle slopes allow for water to infiltrate and percolate into the ground, travelling slowly to the channel through soil and rock, increasing lag time and reducing peak discharge

hydrograph: ‘flat/subdued’ river: basin size, shape, drainage density

• basin size: larger basins catch more precipitation so have a higher discharge but water takes longer to reach the channel as it as a greater distance

• shape: elongated basins means water will take longer to reach the channel form the extremities within leading to a subdued hydrograph

• drainage density: low drainage density means water is more likely to enter the ground and moves slowly through the basin, leading to a longer lag time and subdued hydrograph

hydrograph: ‘flat/subdued’ river: vegetation and human factors

• vegetation: forest and woodland intercepts water and has high rates of evapotranspiration so less, and more slowly, water reaches the channel; dense, delicious in summer means high levels of interception and slow passage through the system so more water is lost to evaporation from negation surfaces leading to long lag time and lower peak discharge

• urbanisation: low population density and few artificial impermeable surfaces increases infiltration; abstraction of water reduces groundwater levels promotes infiltration and percolation when rain falls; water management through dams and reservoirs regulates flow downstream and makes hydrograph more subdued

• land use change: contour ploughing encourages infiltration and reforestation increases interception, infilitration and evapotranspiration; planted fields reduce direct runoff

role of players in managing land use

• urbanisation has a major impair on the hydrological cycle

• can manage the catchment by developing appropriate land use such as forests and moorlands in the upper areas

• managing development in the lower part of the catchiest by land use zoning and limiting building on the floodplains

• however decision makers and planners must also defend high value properties and installations against agreed flood recurrence levels

• comparatively low cost strategies to lower flood risk such as semi-permeable surfaces for car parts, high level wiring systems in homes, government developing affordable insurance and tightened building regulations

synoptic charts, isobars and occluded fronts

• the closer the isobars are together = associated with flooding, Lowe pressure, depressions and stronger winds

• the farther apart the isobars are = associated with drought, high pressure, anticyclones and weaker weeds

• occluded front forms when cold front of depression catches up with warm front, lifting the warm air between the fronts into a narrow wedge above the surface = warm, moist air rises, cool air condenses to form precipitation

drought

• shortage of water over an extended period of time

• referred to as creeping hazards due to its long onset period: means its difficult to determine whether a drought has began or if its just a dry period that will end shortly

standard drought evapotranspiration index (SPEI) and EDO/GDO

• climate index used to determine the onset, duration and magnitude of drought conditions compared to historical baselines

• uses real time global databases to track active drought events

• organisation like the European drought observatory (EDO) and global drought observatory (GDO) use large multi-soruce datasets to monitor trends and use early warnings

• bid data sets include satellite observations —> tracks soil moisture deficits, precipitation anomalies and vegetation stress by looking at how healthy and green plants look: in-situ data —> ground based weather stations providing real time local data: hydrological and meteorological weathering —> used to forecast river flow, groundwater depletion and weather trends

short term causes of droughts: blocking anticyclones

• mid-latitude blocking anticyclones found at latitudes of temperate zones or from about 60 degrees north or south of the equator

• anticyclones are areas of sinking air which result in high pressure and result in periods of settled and calm weather

• can block path of depressions by slowing down bad weather or forcing it round the outside of the pressure system and are called blocking anticyclones

• part of the earths surface beneath a blocking anticyclone experiences the same kind of weather for a long period of time

• blocking highs can cause long severe winters as well as have led to prolonged droughts

short term causes of droughts: jet streams

• the jet stream: fast-flowing, high altitude winds, acts as a steering current for the low pressure system that brings in rain that meanders in a wave like pattern

• its position directly affects which regions receive rainfall

• forms at the boundary between cold dense solar air and warm less saturated subtropical air

• greater the temperature difference, the stronger the jet stream

• when positioned in the north of a region, generally leads to drier and more settled weather to the south of it: rain bearing low pressure systems are steered to the north, away from the area

long term causes of drought: global atmospheric circulation (GAC)

• descending air (high pressure belts) between the Hadley and ferrel cells and between polar cells creates hot and cold deserts respectively

• areas adjacent are also influenced, creating semi-arid climates where droughts are common

long term causes of drought: the ITCZ (intertropical convergence zone)

• a low presser belt which encircles the globe around the equator, bringing in heavy precipitation to this part of the globe

• occurs between the two Hadley cells, where warm tropical air (trade winds) flows towards the equator from north and south and meets

• moves north and south of the equator seasonally causing and alternating wet season when it arrives and a dry season when it leaves in the areas immediately north and south of the equator: climate of savanna regions

medium term causes of droughts: ENSO cycles (El Niño southern oscillation): normal conditions

• in the pacific involves an eats to west air flow known as the easterly trade winds

• these winds drive warm water west towards Australia and Sia, allowing cold, nutrient rich water to up well off the wet coast of South America

• cold ocean surface temperatures off the west coast of South America leads to subsiding air, high pressure and dry hot conditions under clear skies

• conversely, warm surface ocean temperatures lead to rising air and low pressure over northern austral and Indonesia menage hot, humid conditions and plenty of rainfall

medium term causes of droughts: ENSO cycles (El Niño southern oscillation): El Niño conditions

• global temperature increases by up to 0.2 degrees

• usually appears every 3-7 years, trade winds weaken significantly

• allows warm surface water to flow back towards the eastern pacific, so less cold water reaches the ocean surface

• warmer ocean heats the air above it, causing air to rise and wetter and warmer weather in an area that is normally dry (coastal Peru and Chile)

• over northern Australia and indonesia, the cooler the normal surface ocean temperatures cause subsiding air leading to dry weather

medium term causes of droughts: ENSO cycles (El Niño southern oscillation): La Niña conditions

• global temperatures decrease by up to 0.2 degrees

• usually allows a particularly strong El Niño phase and can persist for months to several years

• in the pacific, weather conditions are an intense version of normal conditions: areas which are normally dry suffer drought and areas normally wet will suffer through intense rainfall and flooding

• low pressure over the western Pacific becomes even low and so rainfall further increases over south east Asia and Australia

• high pressure over the western pacific even higher, countries like Chile and Peru become even wetter

• due to increased pressure difference between the two places, trade winds strengthen, helping explain why this leads to a more intense version of the normal situation

what causes ENSO

• the cycle is natural (happening for at least 300 years)

• scientists know what happens during the cycle but not why they happen in the first place

• cause seems to be a complex interaction between the ocean and atmosphere, which swings between warm El Niño phases and cold La Niña

• early signs of an El Niño pattern forming (changes to ocean temperature and pressure patterns) can be picked up by satellites but the start of either vent can not be predicted

• el nino and la niña events last about 12 months and occur every 2-7 years

impact of climate change on longer term trends of droughts

• research on impact of climate change on blocking anticyclones and jet streams ongoing, but there does seem to be evidence that it does have an impact: blocking anticyclones might stay in place longer leaded to increased risks of prolonged extreme weather

• ENSO cycles not caused by global warming but may be affected by it

• since the GAC is powered by temperature differences, climate change likely as a profound on this and on the movement of the ITCZ