The water cycle Flashcards
what is meant by energy?
-the power / driving force, e.g. incoming solar radiation ( insolation )
what is meant by a system?
-a set of interacting components connected by flows / transfers of matter or energy that are working together towards some kind of process
what are systems made up of?
-inputs = the movement of matter or energy into a system from the outside, e.g. precipitation
-outputs = the movement of matter or energy out of a system, e.g. surface runoff
-stores = the individual parts of the system that hold or transform matter or energy, e.g. soil, trees, puddles
-flows = the movement of matter or energy from one store to another, e.g. evaporation, throughflow, infiltration
-processes = the physical mechanisms that drive the flows of matter and energy between the stores, e.g. photosynthesis
-boundaries = limits to the system ( e.g. watershed, upper atmosphere )
what are the three types of systems?
-open = have external inputs and outputs of energy and matter exchange at its boundaries
-closed = only have energy as their input and output, matter is contained within the system boundary and so can only cycle between stores
-isolated = have no interactions with anything outside the system boundary and no input or output of energy or matter, which rarely exist in nature
when is a system in equilibrium and dynamic equilibrium?
-when the inputs and outputs of a system are balanced, it is in equilibrium as flows and processes continue to happen but in the same way at all times, so there are no overall changes to the system
-however, in reality there are lots of small variations in the inputs and outputs of a system ( e.g. the amount of precipitation entering a drainage basin system constantly varies ) but the inputs and outputs remain about balanced on average, so the system is said to be in dynamic equilibrium
what happens when one of the elements of a system changes?
-the stores change and the equilibrium is upset, which is called feedback
what are feedback loops?
-active mechanisms in systems that maintain or restore equilibrium, which can be positive or negative
what is a positive feedback loop?
-a cyclical sequence of events that amplifies / increases the change, which moves the system even further away from its previous state of equilibrium
-e.g. greenhouse effect increases, so temperatures rise and thus the rate of evaporation increases, increasing the amount of water vapour ( a GHG ) in the atmosphere, which enhances the greenhouse effect
what is a negative feedback loop?
-a cyclical sequence of events that dampens / decreases the change, which keeps the system closer to its previous state of equilibrium
-e.g. temperatures rise and so the rate of evaporation increases, increasing the amount of water vapour in the atmosphere which causes more clouds to form, and the increased cloud cover reflects more of the sun’s energy back to space, so temperatures fall
what is a cascading system and an example of one?
-a system in which energy and matter are transferred from one subsystem to another, so the output of one subsystem becomes the input for another subsystem
-e.g. the earth, which is also a closed system because energy is input from the sun and output to space, but matter is not input or output
what are the five subsystems of the earth?
-cryosphere = all the parts where it’s cold enough for water to freeze, e.g. glacial landscapes
-lithosphere = the outermost part, which includes the crust and the upper part of the mantle
-biosphere = the part where living things are found, which includes plants, animals, fungi, bacteria, insects etc.
-hydrosphere = all of the water on earth, which may be in liquid form ( e.g. in lakes and rivers ), solid form ( ice stored in the cryosphere ) or gas form ( e.g. water vapour stored in the atmosphere ), and can also be saline ( salty ) or fresh
-atmosphere = the layer of gas between the earth’s surface and space, held in place by gravity
what is the hydrosphere?
-a discontinuous layer of water on or close to the earth’s surface, which includes all liquid and frozen surface water, groundwater held in soil and rock and atmospheric water vapour
what does the hydrosphere contain?
-1.4 sextillion litres of water, 97% of which is saline water in the oceans and less than 3% is freshwater
-of the earth’s freshwater, 69% is frozen in the cryosphere, 30% is groundwater ( water stored underground in the lithosphere ), 0.3% is liquid freshwater on the earth’s surface ( e.g. in lakes and rivers ) and 0.04% is stored as water vapour in the atmosphere
-however, only around 1% of freshwater is accessible to humans
how does water change between its three states?
-solid to liquid = melting, latent heat is absorbed
-liquid to solid = freezing, latent heat is released
-liquid to gas = evaporation, latent heat is absorbed
-gas to liquid = condensation, latent heat is released
-gas to solid = deposition, latent heat is released
-solid to gas = sublimation, latent heat is absorbed
what is the hydrological cycle?
-the continuous movement of water on, above and below the earth’s surface
-it is a closed system as there are no inputs or outputs of water
what factors ( i.e. flows ) drive the change in magnitude of water stores?
-evaporation
-condensation
-cloud formation and precipitation
-cryospheric processes
what is evaporation?
-the process by which liquid water changes state to become water vapour ( a gas ), which requires energy in the form of latent heat and cools the surroundings
-this increases the amount of water stored in the atmosphere
what does the rate of evaporation depend upon?
-the magnitude of the evaporation flow varies by location and season, e.g. if there is lots of solar radiation, a large supply of water and warm, dry air, the amount of evaporation will be high because the closer the air is to saturation point, the slower the rate of evaporation, and warmer air can hold more water vapour than cold air
-however, if there is not much solar radiation to be absorbed, little available liquid water and cool air that is already nearly saturated ( unable to absorb any more water vapour ), evaporation will be low
what is condensation and when does it happen?
-the process by which water vapour changes state to become a liquid, which releases latent heat and warms the surroundings
-it happens when air containing water vapour cools to its dew point ( the temperature at which it will change from a gas to a liquid ), e.g. when temperatures fall at night due to heat being lost to space, because as air cools it is able to hold less water
-the water molecules need something to condense on, such as tiny particles ( e.g. smoke, salt, dust ) that are collectively called condensation nuclei, or surfaces ( e.g. leaves, grass, stems, windows ) that are below the dew point temperature
-if the surface is below freezing point, water vapour sublimates and changes directly from gas to solid in the form of hoar frost
-water droplets can stay in the atmosphere or flow to other subsystems, e.g. when water vapour condenses, it can form dew on leaves and other surfaces, which decreases the amount of water stored in the atmosphere and increases the amount stored in the biosphere
what does the rate of condensation depend upon?
-the magnitude of the condensation flow depends on the amount of water vapour in the atmosphere and the temperature, e.g. if there is lots of water vapour in the air and a large or rapid drop in temperature, condensation will be high
what is cloud formation and precipitation?
-essential parts of the water cycle, e.g. precipitation is 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, and when the droplets get big enough, they fall as precipitation
-for clouds to form, there have to be tiny particles of other substances ( e.g. dust or soot ) to act as cloud condensation nuclei which give water a surface to condense on since water droplets caused by condensation are too small to form clouds on their own
-cloud formation and precipitation can vary seasonally ( e.g. in the UK, there is normally more rainfall in winter than in summer ) and by location ( e.g. precipitation is generally higher in the tropics than at the poles )
what are cryospheric processes?
-processes that change the amount of water stored as ice in the cryosphere, such as accumulation ( the build-up of ice mass ) and ablation ( the loss of ice mass )
-the balance of accumulation and ablation varies with temperature
-during periods of global cold, inputs into the cryosphere are greater than outputs as water is transferred to it as snow, and less water is transferred away due to melting, whereas during periods of warmer global temperatures, the magnitude of the cryosphere store reduces as losses due to melting are larger than the inputs of snow
-the earth is emerging from a glacial period that reached its maximum 21,000 years ago, so there are still extensive stores of ice on land in Antarctica and Greenland, numerous alpine glaciers and a large volume of sea ice in the Arctic and Antarctic
-as well as the changes in global temperature that occur over thousands of years, variations can also occur over shorter timescales, e.g. annual temperature fluctuations mean that more snow falls in the winter than in summer
what is a drainage basin?
-an area of land drained by a river and its tributaries, including water found on the surface, in the soil and in near-surface geology ( also called the river’s catchment area )
-it is the area that supplies a river with its supply of water
-it can be viewed as an open, local hydrological cycle which has inputs, outputs, flows and stores
what are the features of drainage basins?
-watershed = the edge of surrounding highland that separates different drainage basin systems ( boundary )
-source = where the river starts in highland areas
-confluence = the point where a stream meets a river
-tributary = a smaller stream that will flow into the main channel
-mouth = the point where the river meets the sea
what is the input to the drainage basin system?
-precipitation = any water that falls to the surface of the earth from the atmosphere, including rain, snow, hail, dew and frost
what are the three types of rainfall?
-frontal ( due to other air masses ) = warm air is less dense than cool air, so when the two meet, the warm air is forced up above the cool air and cools down adiabatically as it rises
-relief / orographic ( due to topography ) = when warm air meets mountains, it is forced to rise, causing it to cool and condense at higher altitudes
-convective ( due to convection ) = when the sun heats up the ground, moisture on the ground evaporates and rises up in a column of warm air, which cools and condenses as it gets higher before falling as rain
what are the outputs of the drainage basin system?
-evaporation
-transpiration
-evapotranspiration
-river discharge / river flow
what is transpiration?
-the loss of water from vegetation through pores ( stomata ) on their surfaces, i.e. evaporation from within leaves
what is evapotranspiration?
-the total output of water from the drainage basin directly back into the atmosphere, including evaporation and transpiration
what is river discharge?
-the volume of water passing a measuring point in a given time, measured in m3/sec or cumecs
how is river discharge calculated?
-cross-sectional area of the river ( width x depth ) in m2 x velocity at the measuring point in m/sec
what are the flows in the drainage basin system and their relative rates of movement?
-infiltration = moderate
-overland flow / surface runoff = fast
-throughfall = slow
-stemflow = fast
-throughflow = moderate
-percolation = slow
-groundwater flow = usually slow but variable
-baseflow = usually slow but variable
-interflow = moderate
-channel / stream flow = fast
what is infiltration?
-the downward movement of water from the surface into soil
-infiltration capacity = the maximum rate at which a soil is capable of absorbing water
-if precipitation falls at a greater rate than the infiltration capacity, overland flow will occur as the soil will be saturated ( i.e. have reached its maximum capacity )
-infiltration will be reduced when the slope gradient increases as water flows over the surface quicker than it can infiltrate
what are infiltration rates controlled by?
-gravity
-capillary action
-soil porosity which is controlled by its texture, structure and organic content, e.g. coarse textured soils have larger pores and fissures than fine-grained soils and therefore allow for more water flow
-vegetation cover increases infiltration rates as they slow the rate of flow which allows infiltration to occur, and their roots penetrate / break up the soil as do burrowing organisms e.g. worms, increasing potential pore spaces and channels
what is overland flow?
-the tendency of water to flow horizontally across land surfaces as sheetflow ( over the whole surface ) or in small channels / rills when rainfall has exceeded the infiltration capacity of the soil and all surface stores are full to overflowing
-water can only build up on the surface after a long period of rain, an intense rainstorm or on an impermeable surface ( either man-made or natural, e.g. an impacted footpath or frozen surface )
what is throughfall?
-the precipitation that reaches the ground directly through gaps in the vegetation canopy and drips from leaves, twigs and stems when the canopy-surface rainwater storage exceeds its capacity
what is stemflow?
-the intercepted precipitation that reaches the ground by flowing down stems, stalks or tree boles
what is throughflow?
-the lateral movement of water downhill through the upper soil, along lines of seepage called percaline, into streams or rivers
-the more vegetated an area, the faster the rate of throughflow because it is aided by root channels in the soil
what is percolation?
-the downward movement of water through the soil into porous rock or rock fractures in the water table
-the rate is dependent on the fractures that may be present in the rock and the permeability of the rock
what is groundwater flow?
-the slow movement of water through permeable rock below the water table, which ensures that there is water in rivers even after long periods of dry weather
-water flows slowly through most rocks, but rocks that are highly permeable with lots of joints ( gaps that water can get through ) can have faster groundwater flow, e.g. limestone
what is baseflow?
-groundwater flow that feeds into rivers through river banks and beds
what is interflow?
-water flowing downhill through permeable rock above the water table
what are the stores in the drainage basin system and their relative lengths of storage?
-interception storage = short-term
-vegetation storage = mid-term
-surface storage = variable
-soil storage = mid-term
-channel storage = short-term
-groundwater storage = long-term
what is interception storage?
-the precipitation that falls on the vegetation surfaces ( canopy ) or human-made cover and is temporarily stored before evaporating or falling from the leaves as throughfall
what is interception storage impacted by?
-the density of the vegetation cover as the more vegetated an area, the more interception that will take place, reducing the amount of water available for overland flow which leads to a reduction in soil erosion and flooding
-the shape and size of the leaves, e.g. bigger leaves will intercept more precipitation and those with drip tips will only provide a short-term store as water will quickly flow off the leaves
what is vegetation storage?
-water that has been taken up by plants and is contained in them at any one time
what is surface storage?
-water stored in puddles ( depression storage ), ponds and lakes
what is soil storage?
-moisture stored in the soil which is utilised by plants
what is channel storage?
-water held in a river or stream channel
what is groundwater storage?
-water stored in the ground, either in the soil ( soil moisture ) or in pore spaces in the rocks
what is the water table?
-the top surface of the zone of soil or rock where all the pores are full of water ( i.e. the zone of saturation )
what is meant by an individual storm?
-a rainfall period separated by dry intervals of at least 24 hours
what is meant by an individual rainfall event?
-a rainfall period separated by dry intervals of at least 4 hours ( Hamilton and Rowe, 1949 )
what is meant by the water balance / budget?
-the balance between inputs ( precipitation ) and outputs ( run-off and evapotranspiration ) in a drainage basin
-it can be shown using the formula: precipitation ( P ) = discharge ( Q ) + evapotranspiration ( E ) +/- changes in storage ( S )
what are the features of soil water budget graphs?
-soil moisture recharge = precipitation is greater than potential evapotranspiration so the soil water store is being filled
-soil moisture utilisation = the soil water store is being used by plants so evapotranspiration increases and soil moisture decreases
-soil moisture deficit = potential evapotranspiration exceeds precipitation so plants that don’t have adaptations to survive dry conditions wilt and die
-soil moisture surplus = precipitation is greater than potential evapotranspiration so the soil is saturated and overland flow can occur, leading to floods
-potential evapotranspiration = the amount of water that could be evaporated or transpired from an area if there was sufficient water available
-precipitation = the input to the drainage basin
what are the seasonal changes in the UK’s water balance?
-in wet seasons, precipitation exceeds evapotranspiration which creates a water surplus, meaning the ground stores fill with water and there is more surface runoff and higher discharge, so river levels rise
-in drier seasons, precipitation is lower than evapotranspiration which means that ground stores are depleted as some water is used ( e.g. by plants and humans ) and some flows into the river channel but isn’t replaced by precipitation
-therefore, at the end of a dry season, there is a deficit ( shortage ) of water in the ground before the ground stores are recharged in the next wet season ( i.e. autumn )
how does climate affect global water budgets?
-equatorial regions = positive water balance because the warm/wet climate due to high temperatures and low pressure means that there is lots of both precipitation and evapotranspiration, leading to water surplus and a high potential for flooding when rainfall exceeds evapotranspiration and soil moisture utilisation in between these periods but no deficit, so rivers and plants have a source of water all year round
-temperate desert regions = negative water balance because the warm/dry climate due to high temperatures and pressure means that there is more evapotranspiration than precipitation, leading to water deficit for most of the year, water recharge and then surplus during the rainy season, followed by a short period of utilisation and then further deficit
-polar regions = positive water balance because the cold/dry climate due to low temperatures and high pressure means that there is very little precipitation and evapotranspiration, but precipitation just about exceeds evapotranspiration, leading to water surplus for most of the year
why does water recirculate between the earth’s surface and the atmosphere more over land than over the ocean?
-EVT from vegetation means that lots of rainfall can be recycled as vapour, leading to cloud formation and precipitation above land
-over the ocean, the moisture is spread out by strong prevailing winds and ocean currents, dispersing the vapour over large distances and reducing the likelihood of local recirculation
-land surfaces heat up and cool down more rapidly than the ocean ( due to a lower specific heat capacity ), resulting in higher daytime temperatures which increases the rate of EVT as lots of latent heat is available
-humidity is generally lower over land than over the moist ocean which promotes more rapid EVT since less humid air is further away from saturation point and so can hold more water vapour
what is the temporal variation in water recirculation?
-during summer, increased solar radiation and longer daylight hours leads to higher surface temperatures and vegetation is in full leaf, increasing the rate of EVT and causing more water recirculation between the earth’s surface and the atmosphere than during winter which experiences more precipitation than EVT
-lower temperatures at night decrease the rate of EVT, resulting in a daily cycle of water recirculation which peaks during the afternoon
-variable weather conditions can cause short-term fluctuations in water recirculation as heavy cloud cover and precipitation decrease the amount of solar radiation reaching the earth’s surface, reducing the rate of EVT and water recirculation
how is river flow studied and why is it important to do so?
-by measuring the discharge of a river
-discharge is the combined result of the many climatological and geographical factors which interact within a drainage basin, so knowledge of it is very important in the assessment and management of water resources, the design of water-related structures, flood warning and alleviation schemes, the development of HEP etc.
-also, keeping records of river flows is key to identifying hydrological trends which will help in the management of future flood and drought episodes
what is meant by a river regime?
-the variation in river discharge over the year in response to precipitation, temperature, evapotranspiration and drainage basin characteristics
how does climate affect river regime?
-climates that consist of high, consistent rainfall generally have rivers with higher base flows and more sustained discharge throughout the year
-during wet seasons, river discharge will be much higher than in dry periods due to the increase in precipitation and decrease in EVT which requires latent heat provided by the sun
-in areas with winter snowfall, river discharge peaks in spring and early summer when temperatures rise as snow and ice melt to become water which enters the river channel via one of the various flows
-climate influences vegetation which in turn affects EVT, infiltration rates and the amount of overland, groundwater, throughflow etc. so water can recirculate quicker in warm, wet regions and doesn’t reach the river channel quickly, reducing river discharge
what is meant by a storm ( flood ) hydrograph?
-a graph of the rainfall for the drainage basin of a river and the discharge of the same river over the time period when the normal flow is affected by a storm event
-their purpose is to track the progress of a storm in a drainage basin and they are important because they can predict how a river might respond to a future rainstorm and assess the likelihood of flood, which can help in managing the river ( e.g. building embankments or floodplain zoning )
what are the features of storm hydrographs?
-baseflow = the level of groundwater and throughflow
-stormflow = discharge resulting from storm precipitation involving overland, groundwater and throughflow
-peak discharge = the highest point on the graph, when the river discharge is at its greatest
-lag time = the delay between peak rainfall and peak discharge, which happens because it takes time for the rainwater to flow into the river
-rising limb = the part of the graph up to peak discharge that represents the discharge increasing
-falling limb = the part of the graph after peak discharge that represents the discharge decreasing
-bankfull discharge = the maximum discharge that a river channel is capable of carrying without flooding
what is the difference between a flashy and subdued hydrograph?
-flashy = high peak discharge, short lag time, steep rising and falling limb, higher flood risk
-subdued = low peak discharge, long lag time, gentle rising and falling limb, lower flood risk
what physical factors contribute to a flashy hydrograph and why?
-high drainage density = many tributaries to the main river, increasing speed of drainage and decreasing lag time as the water travels fastest as streamflow
-circular basin = all points in the basin are roughly the same distance from the point of discharge measurement, so lots of water will reach the measuring point at the same time, decreasing lag time
-small basin = precipitation has less distance to travel, so it reaches the main channel more quickly, decreasing lag time
-large basin = they catch more precipitation, so they have a higher peak discharge than smaller basins
-steep-sided basin = water flows more quickly on the steep slopes, whether as throughflow or overland flow, decreasing lag time, and the water has less time to infiltrate the soil, so runoff is higher
-prior rainfall = increased surface runoff as ground is saturated and infiltration capacity has been reached, decreasing lag time and increasing peak discharge
-impermeable soil and underlying geology = decreased infiltration and percolation, so decreased groundwater and throughflow, decreasing lag time and increasing peak discharge as surface runoff is increased
-less dense vegetation cover = less interception by trees means less water stored on leaves ( which usually slows the movement of rainwater to the ground ) and less output from EVT, decreasing lag time and increasing peak discharge
-intense rainfall = heavy storms with a lot of water entering the basin over a short time make the soil more likely to reach its infiltration capacity which results in more surface runoff, decreasing lag time and increasing peak discharge
-rain rather than snow or hail = snow takes time to melt before the water enters the river channel so rainfall decreases lag time and increases flood risk
-low temperatures = less EVT so greater peak discharge
what human factors contribute to a flashy hydrograph and why?
-deforestation = reduced interception rates allows water to hit the surface directly and the lack of roots reduces the infiltration rate into the soil, both of which result in rapid overland flow, and deforestation increases soil erosion, which leads to sedimentation of the channel and reduces the bankfull capacity of a river, increasing flood risk
-agriculture = ploughing breaks up the topsoil and allows greater infiltration, subduing hydrographs, but furrows can be created that run directly downslope and act as small stream channels, leading to greater surface flows which decreases lag time and increases peak discharge
-urbanisation = most settlements are designed to transfer water as quickly as possible away from human activity to the nearest river, but they lead to more impermeable surfaces, less vegetation, road camber and drainage systems, which results in more surface runoff and greater flood risk as interception, EVT and infiltration are reduced
what are the natural drivers of change in the water cycle over time?
-storm events
-seasonal changes
how do rainfall and storm events drive change in the water cycle over time?
-intense storms generate more precipitation and greater peak discharges than light rain showers
-the larger input of water causes flows ( e.g. runoff ) and stores ( e.g. groundwater ) to increase in size
-some flows ( e.g. infiltration ) may not be able to occur rapidly enough for the size of the input, increasing runoff
how do seasonal changes drive change in the water cycle over time?
-the size of inputs, flows and stores in the water cycle varies with the season, e.g. in the UK, summer is normally drier than winter
-during the winter, temperatures may drop below 0°C, causing water to freeze, which can reduce the size of flows through drainage systems, while the store of frozen water grows ( i.e. the cryosphere )
-when temperatures increase again, flows and inputs can be much larger as the ice melts and EVT increases
-most plants show seasonal variation, e.g. deciduous trees lose their leaves in winter which reduces interception along with stemflow and throughfall, increasing the speed at which water reaches the river channel as infiltration and runoff increases
-however, more water is lost through EVT before it reaches the river channel during summer as temperatures are higher and deciduous trees have their leaves, reducing runoff and peak discharge
what are the human drivers of change in the water cycle over time?
-farming practices ( ploughing, crops, livestock, irrigation and soil drainage )
-land use changes ( deforestation and urbanisation )
-water abstraction
how do farming practices drive change in the water cycle over time?
-ploughing breaks up the topsoil so that more water can infiltrate, reducing the amount of surface runoff
-arable farming ( crops ) increases infiltration and interception compared to bare ground, reducing runoff, and EVT increases, which can increase rainfall
-pastoral farming ( livestock, such as cattle ) causes compaction of the soil, decreasing porosity and infiltration, and increasing runoff
-irrigation ( artificially watering the land ) can increase runoff if some of the water can’t infiltrate, and groundwater or river levels can fall if water is extracted for irrigation
how does soil drainage drive change in the water cycle over time?
-drain tiles in fields remove excess water from the soil profile, increasing the productivity of the field and helping to improve the efficiency of growers as air can enter the soil for root cell respiration, it is easier to achieve greater root penetration, the soil can be more easily warmed which increases the likelihood of improved seed germination, and heavy machinery and animals can work on / graze the land without compacting the soil
-however, soil drainage increases the speed of throughflow in the soil, which can increase the likelihood of flooding and the variability of river regime, and the dry topsoil can be subject to wind erosion if not properly protected, which impacts water quality because farmers compensate for the loss of nitrates by using more fertiliser which washes into local watercourses where it causes eutrophication
how does deforestation drive change in the water cycle over time?
-on a local level, although precipitation stays the same, the EVT is lower because the replacement vegetation has smaller leaves and roots and is less dense, so overland flow and throughflow occur due to the increase in infiltration, which increases river flow and the likelihood of flooding
-where deforestation is extensive, positive feedback can occur in the basin hydrological system as EVT is low so much of the water leaves the area in the river channel rather than being recycled continuously, which means that there is less water vapour available in the atmosphere for precipitation and so precipitation levels fall, reducing the river flow
how does urbanisation drive change in the water cycle over time?
-construction of new buildings and roads creates an impermeable layer over the land, preventing infiltration and reducing EVT, which massively increases runoff, resulting in water passing through the system much more rapidly and making flooding more likely
what is meant by water abstraction?
-the process of taking water from a natural source ( rivers, lakes, groundwater aquifers ) for human use, e.g. drinking, irrigation and industry
what is meant by an aquifer and what are the two types?
-a permeable rock which stores / transfers water
-unconfined = where porous rock is open to surface water and is directly recharged by precipitation
-confined = where there are thick layers / beds of rock over the aquifer ( known as the confining beds )
how does water abstraction drive change in the water cycle over time?
-more water is abstracted to meet demand in areas where there is a water deficit, e.g. those with low rainfall and high population density, and/or intensive agricultural or industrial activity, which reduces the amount of water in stores such as lakes, rivers, reservoirs and groundwater
-during dry seasons, even more water is abstracted from stores ( especially groundwater and reservoirs ) for consumption and irrigation, so stores are depleted further
-however, if water is being pumped from beneath the ground faster than it is being replenished through rainfall, the result is sinking water tables, empty wells, higher pumping costs and, in coastal areas, the intrusion of saltwater from the sea which degrades the groundwater ( e.g. along the Mediterranean coastlines of Italy, Spain and Turkey, where the demands of tourist resorts are the major cause of over-abstraction )
what is the impact of water abstraction from the chalk of southern England?
-the water within the chalk aquifer is replenished by rainfall that lands on the exposed chalk hills of the North and South Downs and the Chilterns
-normally recharge takes place during the winter months when potential EVT is low and soil moisture deficits are negligible
-groundwater amounts vary seasonally, with levels rising from autumn through winter into spring
-during the summer months, potential EVT generally exceeds rainfall, soil moisture deficits build up, and little percolation takes place
-in summer, water still leaves the chalk from springs but chalk streams can have intermittent sections
-multiple drier winters depletes the aquifer, reducing the flow in chalk streams and potentially drying up rivers altogether
-therefore, over-abstraction depletes high quality groundwater which can be abstracted for public supply, has an economic impact on local communities due to the inability to fish, enjoy river views or undergo other recreational activities, and results in a huge, irreversible reduction in biodiversity
what is the impact of water abstraction in the London basin?
-precipitation percolates through permeable chalk where it is stored and released naturally at springs
-during the 19th and early 20th century, the increase in industrialisation resulted in the chalk aquifer being increasingly exploited
-at the peak of abstraction in the 1960s, groundwater levels beneath central London had dropped to 88m below sea level, creating a large depression in the water table
-by the early 1990s, groundwater levels began to recover by 3m/year as deindustrialisation caused a reduction in abstraction, which posed the threat of rising groundwater to structures such as London Underground and building foundations
-by 2000, the rise in groundwater that the GARDIT strategy was designed to prevent had largely been achieved
-between 2000 and 2018, groundwater levels have risen across north London due to limited abstraction before levelling off in recent years, but have fallen in east, south-west and south-central London due to increased abstraction
-in east London, there is a risk of saline intrusion as saline river water can enter the chalk aquifer when groundwater levels near the river are lower than the water level in the River Thames
