Studied by 14 people
What is the global hydrological cycle?
It is the circulation of water around the earth. It is a closed system of linked processes so there are no external inputs or outputs = the amount of global water is finite and constant. What changes = the state in which the water exists (liquid, vapour, ice). The proportions of global water held in each state vary over time with changes in climate.
What is the global hydrological cycle driven by?
The power that drives the global hydrological cycle comes from two sources: Solar energy (in the form of heat) and Gravitational potential energy (causes rivers to flow downhill and precipitation to fall to the ground.)
What are stores? Name the four main stores.
Stores are reservoirs where water is held.
The four main stores are the oceans (largest by far), glaciers and ice sheets (cryosphere, second largest), surface runoff (umbrella terms for land-based stores, including rivers, lakes, groundwater and the moisture held in soils/vegetation) and the atmosphere.
What is the percentage of global freshwater held in freshwater stores?
The cryosphere is the largest, holding 69% of global freshwater. Groundwater holds 30%. Less than 1% is stored in the biosphere.
What are flows/fluxes? Name them.
Flows = the transfers of water from one store to another. (km cubed per year)
Fluxes = the rates of flow between stores. The greatest fluxes occur over the oceans.
Oceans and atmosphere
Evaporation 400,000
Precipitation 370,000
Atmosphere and landmasses
Evaporation 60,000
Precipitation 90,000
Landmasses and oceans
Surface runoff 30,000
Flows: Interception, Infiltration, Percolation, Throughflow, Groundwater flow, Surface runoff, River or channel flow.
What is the global water budget?
Takes into account all the water that is held in stores and flows of the global hydrological cycle. Only 2.5% of it is freshwater; the rest is in oceans. And only 1% of all freshwater is ‘easily accessible surface freshwater'. Nearly 70% is locked up in glaciers and ice sheets.
UK water budget
Seasonal.
Wet season = p > evpt = water surplus (ground stores filled - more direct runoff/discharge = river levels rise.
Dry season = evpt > p = ground stores depleted - water used (plants/humans) + into river + not replaced.
Temperate water budget
Jan-Apr: water surplus, p > evpt, soil water stores full, used efficiently.
Apr-Sept: soil stores start to deplete, evpt increases + p decreases = increased atmospheric pressure, water used - water budget = water deficit, irrigation may be needed.
Sept-Dec: atmospheric pressure/evpt drops, p increases, soil water store recharge.
River regime
Variations of river discharge over a year.
Discharge = volume of water that moves past a certain point in the river.
Physical influences on river regimes
High levels of runoff increase discharge = more water in river = more volume.
Seasonal/climate change influences amount of p in rivers:
Glacial - temp increase = more meltwater + temp decrease = more glacial accumulation.
Evap in summer > winter.
Geology/soil structure influences the speed/amount of infiltration:
Geo: highly porous/permeable = water can percolate = increase base flow.
Geo: lack of joints/cracks/impermeable = increased saturated overland flow.
Soil types: larger air spaces in sandy soil > clay soil. sandy infiltrated quicker. clay
What are the main features of a hydrograph?
Rainfall starts, discharge begins to rise - rising limb.
Peak discharge - some time after the peak rainfall - water takes time to reach the river.
Time interval between peak rainfall + peak discharge = lag time.
Input of rainwater into the river/discharge decreases = falling limb
River's discharge returns to its normal level = base flow.
How does urbanisation affect hydrological processes?
Increases flood risk.
Construction work = removal of the vegetation = exposes the soil
Concrete/tarmac = impermeable, increase surface runoff.
High density of buildings = rain falls on roofs, swiftly fed into drains by gutters/pipes.
Drains/sewers reduce the distance/time rainwater travels before reaching a stream or river channel.
Urban rivers - often channelised w/ embankments guard against flooding. If it occurs = more devastating.
Bridges restrain the discharge of floodwaters, act as local dams, prompting upstream floods.
What is a flashy hydrograph?
Short lag time, high peak, steep rising limb.
Weather: Intense storm exceeds infiltration capacity of the soil, Rapid snowmelt - sudden temp inc, Low evp rates - low temp.
Rock type: impermeable, e.g granite, restrict percolation and enc rapid surface runoff.
Small basins - flashy, circular basins - shorter lag time, steep slopes - relief - rapid surface runoff, low infiltration (clay, tarmac)
What is a flat hydrograph?
Long lag time, low peak, gently sloping rising limb.
Weather: Steady rainfall < infiltration capacity of soil, Slow snowmelt, High evap rates = high temp.
Rock type: permeable e.g limestone, allow percolation - limit rapid surface runoff.
Larger basins - more time to reach river, elongated basins, gentle slopes - relief - allow infiltration/percolation, high infiltration (sandy soils).
What is residence time?
This is the average time a molecule of water will spend in one of the stores. Residence times vary from 10 days in the atmosphere to 3,600 years in the oceans and 15,000 years in an ice cap. It is claimed that two water stores, fossil water and cryosphere are non-renewable.
What is Fossil water?
Ancient, deep groundwater made from pluvial (wetter) periods in the geological past.
What is the Cryosphere?
Made up of those areas of the world where water is frozen into snow or ice.
Name the percentages of water in all stores.
97.5% in oceans
2.5% freshwater
69% in ice caps and glaciers
30% in groundwater
1% as easily accessible surface water
52% in lakes
38% as soil moisture
8% as atmospheric water vapour
1% in rivers
1% as accessible water in plants
What is the main input in the hydrological cycle and how do its characteristics have a significant effect on the drainage cycle?
Precipitation. Effects on the drainage cycle:
Form: rain, snow or hail. Clearly, with snow, entry of water into the drainage system will be delayed.
Amount: this will affect the amount of water in the drainage basin and the fluxes within it.
Intensity: the greater the intensity, the greater the likelihood of flooding.
Seasonality: this is likely to result in the drainage basin system operating at different flow levels at different times of the year.
Distribution: this is significant in very large drainage basins, such as the Nile and the Ganges, where tributaries start in different climate zones.
What is the drainage basin?
Area of land drained by a river and its tributaries, sometimes referred to as a river catchment. The boundary = defined by the watershed.
An open system with external inputs and outputs. Since those inputs vary so does the amount of water in the basin. Vary in size - small local stream up, huge river e.g Amazon.
Drainage basins of tributary streams/small rivers nestle w/in the drainage basins of larger rivers.
What affects the drainage basin?
Climate: Mainly impacts the inputs and outputs, influences the type and amount of precipitation overall and the amount of evaporation (i.e. the major inputs and outputs), impacts vegetation type.
Soils: Largely affect the relative importance of the different flows w/in the system (poss most important is surface runoff), determines the amount of infiltration and throughflow, and indirectly, the type of vegetation.
Geology: Largely affects … runoff —//—, can impact on subsurface processes such as percolation and groundwater flow (and, therefore, on aquifers), indirectly affects soil formation.
Relief: Largely affects … runoff —//—, can impact on the amount of precipitation, slopes can affect the amount of runoff.
Vegetation: Largely affects … runoff —//—, presence/ absence of vegetation = major impact on the amount of interception, infiltration and occurrence of overland flow, transpiration rates.
Interception
When precipitation falls on vegetation, it's considered intercepted. Interception loss occurs when this water evaporates. The rate of evaporation depends on factors like temperature, humidity, and sunlight. Different types of vegetation intercept different amounts of water.
Leafdrip
Water may drip from leaves and is influenced by leaf characteristics like shape, the presence of a waxy cuticle, and surface form. Leafdrip can contribute to soil erosion through rainsplash.
Stemflow
It is the water that flows down tree stems, more significant on trees with smooth bark and steep branches. This water can be altered in pH due to pollutants on leaves and branches.
Throughfall
Water that drips off leaves and branches and reaches the ground.
Direct impact
Raindrops carry kinetic energy, and this energy is transmitted to the soil surface, affecting erosion. Accumulated leaf litter can dissipate raindrop energy.
Transpiration
Loss of water through microscopic leaf pores (stomata). Transpiration, along with evaporation, is known as evapotranspiration. Trees transpire more water than crops due to faster removal of humid air by air currents, creating a steeper diffusion gradient that promotes water uptake from the soil.
Infiltration
The absorption of water into the soil. Soil porosity, influenced by soil structure and organic matter like decomposed leaf litter, affects the infiltration rate.
Throughflow
Water moves downward due to gravity but may also be deflected laterally by soil particles and impermeable components within the soil.
Overland flow/surface runoff
Water that cannot infiltrate the soil collects on the surface and eventually flows away. This occurs on impermeable surfaces like tarmac or during heavy precipitation when the rate exceeds the infiltration capacity.
Evaporation Rates (Plynlimon)
In the Plynlimon catchment, the total evaporation rates are approximately 15-20% of rainfall from grassland areas, primarily due to transpiration. For forests, this rate is higher at around 30%, primarily attributed to interception of rainfall by trees.
Variation in Interception and Transpiration (Plynlimon)
The importance of interception and transpiration varies across the catchment, with differences from west to east. In regions with lower rainfall, particularly in the south and east, transpiration losses become more significant.
Implications for Water Management (Plynlimon)
These findings have important implications for water supply management. Afforesting grassland-dominated catchments will likely reduce the overall water supply because forests tend to intercept more rainfall than grassland.
Soil Erosion Concerns (Plynlimon)
Afforestation of upland catchments, unlike naturally regenerated forests, can lead to increased soil erosion and sediment loads. This is due to activities like ploughing and ditching, which expose unstable subsoil to erosion in steep channels.
What does research at Plynlimon highlight?
The differences in evaporation rates between grassland and forested areas within the catchment = has significant implications for water supply management and soil erosion concerns associated with afforestation in upland regions.
Splash Erosion
Deforestation leads to an increase in kinetic energy of raindrops, causing splash erosion. This process breaks up soil aggregates and throws soil particles into the air. These soil particles can then either be carried away in surface runoff or block soil pores, resulting in the formation of a sealing crust of very fine silt and clay particles.
Changes in Soil Processes
Deforestation alters processes both at the soil surface and within the soil itself. The death of roots that once bound the soil together, a decrease in organic matter that maintains soil structure, and increased diurnal temperature fluctuations expose the soil to adverse conditions. These changes significantly reduce soil structure and the soil's capacity for water infiltration.
Increased Runoff
With deforestation, surface runoff usually accelerates due to the reduced soil structure and the greater volume of water reaching the soil. This runoff efficiently carries away soil particles loosened by rainsplash.
What is the common assumption about the effect of forest destruction on streams and springs?
The common assumption is that forest destruction leads to streams and springs drying up.
In East Java, what were the initial effects of converting tropical forests into tea, rubber, and cocoa plantations on streamflow?
Initially, streamflow increased for three years, and the increase in water yield was proportional to the biomass removed.
What happened to streamflow in the converted area after those initial three years? (East Java)
Streamflow in the converted area declined but remained higher on average than when the catchment was forested due to significantly reduced evapotranspiration.
What is one potential consequence of converting degraded crop or grasslands to exotic plantation species like eucalyptus? (East Java)
A significant reduction in streamflow.
Why do various studies often show that deforestation leads to a long-term decrease in streamflow?
The decrease in streamflow is usually due to deforestation reducing the soil's infiltration capacity, causing rapid surface runoff.
What can happen during the rainy season as a result of decreased infiltration due to deforestation?
During the rainy season, decreased infiltration can lead to flooding.
What can occur in the dry season because of reduced baseflow resulting from decreased infiltration due to deforestation?
In the dry season, reduced baseflow can lead to a complete stop in streamflow, even though evapotranspiration is significantly reduced.
What are some of the multifaceted impacts of deforestation on hydrology in tropical rainforest ecosystems? What is another example of a tropical basin?
Some impacts include reduced infiltration, increased surface runoff, less precipitation, increased evaporation, less transpiration, more soil erosion, and potential disruptions in water supply.
The Amazon basin contains the world's largest area of tropical rainforest. Deforestation has disrupted the drainage basin cycle in a number of ways, including:
How do humans impact the drainage basin cycle?
River Management
Construction of storage reservoirs holds back river flows
Abstraction of water for domestic flow and industrial use reduces river flows
Abstraction of groundwater for irrigation lowers water tables
Deforestation
Clearance of trees reduces evapotranspiration, but increases infiltration and surface runoff
Changing land use - agriculture
Arable to pastoral: compaction of soil by livestock increases overland flow
Pastoral to arable: ploughing increases infiltration by loosening and aerating the soil
Changing land use - urbanisation (covered in a later section)
Urban surfaces (tarmac, tiles, concrete) speed surface runoff by reducing percolation and infiltration
Drains deliver rainfall more quickly to streams and rivers, increasing chances of flooding.
What components of the basin are most affected by humans?
Most affected by humans :
evaporation and evapotranspiration
interception
infiltration
groundwater
surface runoff
Aridity
A lack of water characterized by small amounts of precipitation occurring infrequently and unreliably, often in the form of heavy downpours.
Deserts
Areas receiving less than 250mm of rainfall per year.
Water balance
The relationship between input of water as precipitation, output of moisture resulting from evapotranspiration, and changes in water held in the ground.
Potential evapotranspiration
The volume of water that could be lost through evaporation and transpiration in an area.
Aridity index
A measure of aridity based on the relationship between precipitation and potential evapotranspiration.
Semi-arid
Areas with moderate aridity, making up 14% of the globe.
Atmospheric high pressure
A factor associated with aridity, resulting in low atmospheric humidities and limited cloud formation or rain.
Rainshadow
The effect produced by tall mountain ranges, where the leeward slopes experience an arid climate due to descending air and lack of condensation.
Cold ocean currents
Upwelling of cold sea water along western coasts, reducing the amount of water that air can hold and limiting precipitation.
Continentality
The effect of air masses moving over continents, losing moisture as precipitation and resulting in low rainfall in the center of continents.
Arid areas
Regions with very low precipitation and limited water availability.
Primary causes of desertification
Socioeconomic factors such as lack of land ownership, insecurity of tenure, and the expansion of cash crop agriculture at the expense of small subsistence farmers.
Secondary causes of desertification
Factors related to climate, soil, water, biology, and human activities that contribute to the process of desertification.
Overgrazing
The activity of allowing herd sizes to exceed the carrying capacity of an area, leading to detrimental effects on vegetation, soil quality, and animal health.
Overcultivation
The practice of intensive cultivation of land, often to produce more food or cash crops, resulting in soil exhaustion, loss of nutrients, and reduced crop yields.
Vegetation clearance
The removal of vegetation, either for land preparation or fuelwood, which lowers the water table, removes sources of shade and food, and increases the burning of dung as a fuel source.
Salinisation and waterlogging
Problems associated with poorly drained irrigation schemes in arid areas, leading to the rise of groundwater levels and increased salt concentration in the soil.
Waterlogging
The saturation of soil with water, leading to impaired plant growth and the formation of an upper salt crust.
Nutrients
Substances essential for plant growth and development, typically found in soil and absorbed by plant roots.
Erosion
The process of wearing away or removal of soil and rock by wind, water, or other natural agents.
Salinisation
The accumulation of salts in soil, often resulting from irrigation practices, which can negatively impact plant growth.
Desertification
The process by which fertile land becomes desert, typically due to factors such as climate change, deforestation, and unsustainable agricultural practices.
Drought
A prolonged period of abnormally low rainfall, leading to water scarcity and adverse effects on agriculture, ecosystems, and human populations.
Meteorological drought
A type of drought characterized by significantly lower rainfall than expected for a particular region and time of year.
Agricultural drought
A type of drought that causes at least partial crop failure due to a prolonged period of dry weather.
Physiological drought
A condition in which plants growing in salt-rich soils struggle to absorb water, leading to water loss and difficulty in growth.
Climate change
Long-term shifts in weather patterns and average temperatures, often attributed to human activities and the enhanced greenhouse effect.
Evapotranspiration
The combined process of evaporation from the Earth's surface and transpiration from plants, contributing to the water cycle.
Greenhouse effect
The trapping of heat in the Earth's atmosphere by certain gases, such as carbon dioxide and methane, leading to global warming.
Climatic uncertainty
The unpredictability of future climate conditions, including variations in rainfall and temperature, which can impact arid areas and their ecosystems.
Physical factors causing drought: ITCZ
The Intertropical Convergence Zone, ITCZ, is a band of low pressure around the Earth which generally lies near the equator.
At the equator, there is low pressure, and the warm moist air rises, cools and condenses to form clouds which leads to precipitation. The air moves North and South of the equator, where there is low pressure before cool dry air sinks resulting in high pressure near the surface. This prevents the formation of rain clouds, leading to the formation of deserts.
July - shifts North (max -25 degrees) = wet season.
January - shifts South (max -20 degrees) = dry season.
Blocking Anticyclones
Anticyclones = areas of intense high pressure where air molecules descend to the earth’s surface.
Occur in both winter and summer with varying effects, characterised by:
low wind speeds - weak pressure
stable conditions - no clouds.
Absence of clouds = air molecules descend through the troposphere they warm meaning condensation does not occur.
Blocking anticyclones: Areas of high pressure can sometimes be very slow moving, preventing other, faster-moving pressure systems from moving into a region e.g block the passage of the Polar Front Jet Stream across the British Isles, = mid-latitudes pass North or South of the British Isles = long periods of stable weather in the UK.
Blocking Anticyclones and The European Drought 2018
Blocking anticyclones caused the jet stream to remain further north than usual, causing a stationary high-pressure weather system = dry conditions = severe water scarcity = reduced crop yields, forest fires, and low water levels in rivers and reservoirs. Sweden - 11 wildfires in Arctic circle, UK Amber heat warning.
Blocking Anticyclones and the ITCZ.
Sometimes the descending part of the Hadley cell (high pressure) can block the movement of the ITCZ preventing it from reaching as far north, causing drought e.g. in the Sahel. = aka Blocking anticyclone.
Blocking anticyclones - Sahel
Typical postion of the ITCZ during rain years on 21 June = Middle of the West Sahel region (covering areas of Nigeria, Burkina Faso, Guinea etc). Just enough rain to avoid a drought from strong south-westerly winds.
Drought years, the ITCZ isn’t as north as it should be - instead on 21 June = barely covers coastline of the West Sahel region, weak south-westerly winds.
ENSO cycles and Meterological droughts
ENSO cycle is a scientific term to describe the variations in temperature between the ocean and atmosphere in the Equatorial Pacific.
El Niño = warm phase.
La Niña = cold phase.
Can have impacts on global weather and climate for 9-12 months (sometimes years)
La Niña events every 2-7 years on avg.
El Niño events every 3-7 years on avg.
Normal vs ENSO
Non-El niño year, the trade winds blow from east to west along the equator.
The air pushes the warm water westerwards
Thermocline, upwelling,
Warm, moist air rises, cools and condenses, forming rain clouds
El Niño event
The trade wind pattern is disrupted - it may slacken or even reverse and this has a knock-on effect on the ocean currents
Air circulation loop reversed
Cool water normally found along the coast of Peru is replaced by warmer water.
At the same time, the area of warmer water further west, near Australia and Indonesia, is replaced by cooler water.
Usually last for 18 months.
Trigger very dry conditions throughout the world, usually in the second year. E.g, the monsoon rains in India and South East Asia often fail.
La Niña
Sometimes follow an El Niño event. Build-up of cooler-than-usual subsurface water in the tropical part of the Pacific = severe drought conditions, esp W coast of S.America. V strong air circulation + v warm water moving e-w., cold water is dragged further across the Pacific Ocean. Jet streams follow the boundaries between hot and cold air.
What are the meteorological causes of flooding?
Intense storms = flash flooding (short lag time), e.g semi-arid areas, more common in mountainous areas
Prolonged, heavy rain, e.g Asian monsoon and passage of deep depressions across the UK
Rapid snowmelt - particularly warm spring e.g plains of Siberia.
What is tidal flooding?
Often a result of storm surges / high river flows meet particularly high spring tides in estuaries.
Storm surge = very low air pressure which raises the height of the high-tide sea. Strong onshore winds drive the 'raised' sea to coast, breaching coastal defences and flooding.
What human activities exacerbate flood risk?
Urbanisation, land use.
Impermeable - tarmac
Use of groundwater for arable crops
Dams
Streams channelled into culverts to aid rapid drainage of farmland
Ploughing compacts soil + grazing animals trample soil
Deforestation
Pasture replacing grasslands - less infiltration.
How does river mismanagement affect flood risk? (human)
channelisation: improve river discharge, reduce the flood risk. But displaces the river downstream. Locations may be overwhelmed by inc discharge
dams: block flow of sediment down a river, reservoir fills up with silt; downstream = inc river bed erosion
river embankments: protect from floods of a given magnitude but fail when a flood exceeds capacity = scale of flooding is greater.
Socio-economic impacts of flooding.
death and injury
spread of water-borne diseases
trauma
damage to property, particularly housing
disruption of transport and communications
interruption of water and energy supplies
destruction of crops and loss of supplies
disturbance of everyday life, including work
Environmental impacts of flooding.
Some positives.
recharged groundwater stores
increased connectivity between aquatic habitats
soil replenishment
for many flooding triggers breeding, migration and dispersal
The UK Floods
Notably summer of 2007 + winter of 2015-16.
Severe floods as a result of prolonged heavy rainfall, but at different times of the year.
2016 floods: large areas of the UK received 2x average amount of rainfall for that time of year. Carlisle + Cockermouth in Cumbria some of worst-hit places.
Apparent inadequacy of flood protection measures. Following were singled out for blame:
budget cuts on flood defences
EU Directive puts env conservation before regular dredging of rivers
poor land management = blocked ditches
global warming
How does climate change affect inputs and outputs in the hydrological cycle?
Precipitation:
A warmer atmosphere has a greater water-holding capacity
Mode of precip may be more important than the amount.
Increases in rainfall intensity
More areas of precip in tropics and high latitudes
Fewer areas of precip between 10° + 30° n/s of Equator
Inc length/frequency of heatwaves = inc drought occurrence
Climate warming, more precipitation in northern regions is falling as rain rather than snow
Evaporation and Evapotranspiration:
Evap over large areas of Asia and N.America appears to inc
Transpiration linked to veg changes → linked to changes in soil moisture and precip.
How does climate change affect stores in the hydrological cycle?
Surface runoff and stream flow
More low flows (droughts) and high flows (floods)
Increased runoff and reduced infiltration
Groundwater flow
Uncertain, because of abstraction by humans
How does climate change affect flows in the hydrological cycle?
Reservoir, lake and wetland storage
Changes in wetland storage x linked to climate change
Storage is decr as temp incr
Soil moisture
Little change = higher precip and evap cancel out
Uncertain, it depends on many factors, climate is only one
Precip is inc = likely soil moisture will also inc
Permafrost
Deepening of the active layer = releasing more groundwater
Methane released - thawed lakes may accelerate change
Snow
Decr length of snow-cover season
Spring melt starting earlier
Decreasing temporary store
Glacier ice
Strong evidence of glacier retreat + thinning since 1970s
Less accumulation = more precip as rain
Decreasing store
Oceans
More data on surface temp needed
Ocean warming = more evap
Ocean warming may = more cyclones
Storage capacity inc by meltwater
Rising sea level
What factors lead to diminishing water supply and increased uncertainty?
Inc in annual tempe = greater evap from surface water and reservoirs in summer, spring discharge may increase
Greater rates of EVPT, desiccation of forest stores
Impact of oscillations, e.g. ENSO, is leading to increasingly unreliable patterns of rainfall, e.g. less predictable monsoons
More frequent cyclone and monsoon events threaten water supplies intermittently
Inc intensity + frequency of droughts = global warming and oscillation - issue for rainfed agriculturalists
Depleted aquifers = groundwater issues
Decreasing rainfall - global warming
Loss of snow/glaciers as a store threatens many communities in mountain areas, e.g. Himalayas
Water security
The ability to access sufficient quantities of clean water to maintain adequate standards of food and goods production, proper sanitation, and sustainable health care.
What are the three dimensions of water security?
Availability (sufficient supply to meet demand)
Access (water is affordable, institutions (water companies) are reliable, and management is effective)
Utilisation (adequate infrastructure to use available water resources)
What is Water stress?
When the water demand exceeds the available amount during a certain period or when poor quality restricts its use.
It occurs when renewable water resources are between 1,000 m³ and 1,700 m³ per capita per year.
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