The water cycle and water insecurity (DONE) Flashcards
1
Q
What approach is the global hydrological cycle typically studied as?
A
- The global hydrological cycle is typically studied and understood using a systems approach, which is a central concept for the study of physical geography.
2
Q
What does the systems theory allow us to do?
A
- Systems theory allows us to conceptualise the main water stores and pathways at a global scale, as well as understand the role of local hydrological processes within this wider global system.
- This complex system adjusts and changes as a result of physical and human factors over short and long timescales.
3
Q
What is a system?
A
- A system is any set of interrelated components that are connected together to form a working whole, characterised by inputs, stores, processes (or fiows) and outputs.
- There are two types: closed system and open system.
4
Q
What is a closed system?
A
- A closed system occurs when there is transfer of energy but not matter between the system and its surroundings (the inputs come from within the system).
5
Q
What is an open system?
A
- An open system receives inputs from and transfers outputs of energy and matter to other systems.
6
Q
Why is the hydrological cycle a closed system?
A
- The global hydrological cycle is a closed system because all the water is continually circulated through the stores and there is a constant amount of water in the system.
- The system does not change because there are no gains from or losses to other systems.
7
Q
What drives the global circulation of water in the hydrological cycle?
A
- The global circulation of water is driven by solar energy;
- When heated by the Sun, the water on the Earth’s surface evaporates into the atmosphere.
- When the humid air rises, condensation occurs at the cooler temperatures, forming clouds, and this eventually leads to precipitation and water is returned back to the land and oceans on the Earth’s surface.
8
Q
What energy transfers occur in the hydrological cycle?
A
- On land, gravitational potential energy is converted to kinetic energy as the water moves through the system by plant interception, or over land as surface runoff.
9
Q
How can water flow through soil in the hydrological cycle?
A
- Water also flows through the soil by the processes of infiltration and throughflow.
- Here, it may be stored as soil moisture or, if the bedrock is permeable or porous, will percolate into the rock where it is stored as groundwater.
- Some of this water will return to the oceans via streams and rivers, which may take some time if it is stored in lakes or glaciers en route.
10
Q
What is the largest water store in the global hydrological cycle?
A
- Oceans are the largest:
- 96.9% of water is stored here.
- 1,335,040 x 10^3 Km3
11
Q
Which water store in the hydrological cycle has the largest residence time?
A
- ice caps and glaciers have the largest residence time of 15,000 years.
12
Q
What is the smallest water store in the global hydrological cycle?
A
- Biological water (biosphere).
- 1 x 10^3 Km3
- 0.0001% of water is stored there.
13
Q
Which water store in the hydrological cycle has the smallest residence time?
A
- Biological water (biosphere).
- 1 week residence time.
14
Q
What is the global water budget?
A
- The global water budget is the annual balance of water fiuxes (flows) and the size of the water stores - oceans, atmosphere, biosphere, cryosphere, groundwater and surface water.
15
Q
Why is water generally considered as a renewable resource?
A
- The water stores have different residence times, but the constant circulation, albeit at variable speeds, means that water is generally considered a renewable resource, replenished naturally.
16
Q
What type of water is not considered as a renewable resource?
A
- Fossil water is an exception; this is water that has been contained in an undisturbed space, usually groundwater in an aquifer, for millennia or longer.
17
Q
In what areas is fossil water being extracted an how does this impact the resource?
A
- In arid regions such as the Sahara Desert, the fossil water in these aquifers may be extracted for human purposes (agriculture, industry and consumption), but there is little to no significant recharge, effectively making this type of groundwater a non-renewable resource.
18
Q
What is a drainage basin?
A
- A drainage basin is an area of land that is drained by a river and its tributaries, and separated from neighbouring drainage basins by a ridge of high land called a watershed or divide.
19
Q
What type of system is a drainage basin?
A
- A drainage basin is an open system, so it is linked to other systems by inputs and outputs and involves a number of linked processes and stores.
20
Q
What is precipitation?
A
- Precipitation is any form of water (liquid or solid) falling from the sky.
- Precipitation includes rain, sleet, snow, hail and drizzle.
21
Q
What is the major input into any drainage basin system?
A
- Precipitation is the major input into any drainage basin system.
- The volume of the input varies according to its type and intensity over time and space, linked to the climatic season and associated weather systems.
22
Q
Where are highest precipitation inputs found?
A
- The highest precipitation inputs to drainage basins are found in the tropics, due to the lntertropical Convergence Zone (ITCZ), and in some places during the monsoon seasons.
23
Q
Why does the ITCZ cause large volumes of precipitation?
A
- In the ITCZ, intense solar radiation fuels the convection of warm humid air, resulting in condensation and precipitation (convectional precipitation).
24
Q
Where is the highest average precipitation in the world?
A
- The highest average annual rainfall is in Mawsynram in India, with 11,873 mm of rain per year, mostly during the monsoon season between June
and September.
25
Where is the lowest average precipitation in the world?
- The lowest precipitation inputs are found in stable areas of high atmospheric pressure, such as Quillagua in the Atacama Desert, which is the driest place on Earth, receiving less than 0.2 mm of rain per year.
26
How can continentality influence the distribution of precipitation?
- The distribution of precipitation can be influenced by continentality (distance from the sea).
- Continental interiors, such as the Gobi Desert in Asia or the Alice Springs region in Australia, are far from the moisture of maritime air masses.
27
How can relief impact the distribution of precipitation?
- Relief, such as mountains, and prevailing winds complicate the pattern.
- with high levels of precipitation occurring where prevailing winds are forced to rise over higher altitudes, forming orographic precipitation.
28
What are the physical macros which affect drainage basin flows?
- interception
- infiltration and throughflow
- direct runoff (overland flow)
- percolation and groundwater flow
-
29
What is interception?
- Interception is the process by which raindrops are prevented from falling directly on to the
ground surface by the presence of a layer of vegetation; the leaves, branches and stems of these plants catch the water first.
30
How does vegetation cause interception?
- Leaves, branches
and stems of plants catch the water before it reaches the ground.
- The water later reaches the soil via stem flow (water flowing down the vegetation stems to the ground below) or throughfall, where the
water drips to the ground.
- The undergrowth may intercept again some of the water falling from the canopy (secondary interception), and some of the water will return to the atmosphere
via evapotranspiration (known as interception loss).
31
What is secondary interception?
- This is where the undergrowth may intercept some of the water falling from the canopy.
32
What is interception loss?
- Where water will return to the atmosphere via evapotranspiration during the process of interception.
33
How does precipitation impact the rate of interception?
- Interception is greatest when the precipitation is light and of short duration, as dry leaves and stems have the greatest water storage capacity.
- As vegetation becomes wetter or rainfall intensity increases, more water will drip or flow to the ground and interception effectiveness will decrease.
34
How does vegetation impact rate of interception?
- The vegetation type and cover also influence the amount of interception: denser types of vegetation, such as many coniferous forests, intercept more rainfall than sparser deciduous forests.
- This is especially so in winter, when temperate deciduous trees shed their leaves; and forests intercept more than grasses or crops.
35
What is infiltration and infiltration capacity?
- Infiltration is the movement of water vertically downwards through pores in the soil.
- And the infiltration capacity is the maximum rate at which the soil can absorb precipitation .
36
What factors impact the rate of infiltration?
- The rate of infiltration will depend on a number of factors, especially the amount of water already in the soil - the degree of saturation .
- Once in the soil, water moves both vertically and laterally through it by the process of throughflow - this is a downslope movement under the influence of gravity towards a stream or river.
37
What is direct runoff/overland flow and what are the 2 types?
- Water flowing over the surface of the ground is known as direct runoff or overland flow.
- There are two types: saturated overland flow and infiltration-excess overland flow .
38
When and where does saturated overland flow occur?
- Saturated overland flow occurs when water accumulates in the soil until the water table reaches or ponds on the surface, forcing further rainwater to run off the surface.
- This is particularly common where there are thin soils of moderate permeability.
- Concavities near a stream or riverbank often have high moisture levels and may produce saturated overland flow early in a rainstorm cycle.
39
When does infiltration-excess overland flow occur?
- Infiltration-excess overland flow occurs when the rainfall intensity exceeds the infiltration capacity, so the excess water flows over the ground surface.
- Any surface runoff will quickly deliver water into river channels, increasing the risk of flooding downstream.
40
How does percolation lead to groundwater storage in aquifers?
- When infiltrating water reaches permeable underlying bedrock, it will continue to move slowly downwards into the rock by the process of percolation.
- When the permeable layer lies above an impermeable rock layer the water can percolate no further, creating a saturated zone.
- The water will fill the spaces within the permeable or porous rock, creating groundwater storage and an aquifer (a permeable rock which stores water).
41
How can percolation cause groundwater flow?
- When the water percolates through permeable rock and reaches an impermeable layer, it will fill in cracks and create aquifers which will cause a saturated zone.
- The upper level of this zone is known as the water table.
- Water may then move laterally as groundwater flow if the geological structure allows.
42
How does the porosity of rocks impact the rate of percolation and groundwater flow?
- The rate of percolation and groundwater flow will depend on the permeability
of the rock, which is linked to its porosity or perviousness.
- Porosity relates to the total volume of pore spaces, and is greatest in coarse-grained rocks such as sandstone.
- While pervious rocks such as limestone have joints and bedding planes along which water can flow.
- Therefore percolation and groundwater flow rates will increase with porosity and perviousness.
- Impermeable rocks such as granite, however, will prevent any percolation or water movement through the ground.
43
How is the rate of groundwater flow impacted by the angle of the rock strata?
- The rate of groundwater flow will also increase according to the angle of the rock strata, as a steeper gradient will allow gravity to operate more effectively.
44
What physical factors affect drainage basin outputs?
- Evaporation and transpiration.
| - Channel flow.
45
How does evapotranspiration impact drainage basin outputs?
- Evapotranspiration is the total amount of moisture removed from a drainage basin by processes of evaporation and transpiration; together, these processes can represent a significant output.
- Evaporation is the process by which liquid water is transformed into water vapour (a gas).
- Transpiration is the biological process by which water is drawn upwards from the soil by plants and evaporated through the minute pores, called stomata, in leaves.
46
What factors impact the rate of evapotranspiration?
1. Temperature: the rate of evaporation increases with temperature. The main energy source is solar radiation, so evaporation and temperature will increase with sunshine hours.
2. Wind: will increase the rate of evaporation by reducing the relative humidity and preventing saturation of the air.
3. Vegetation cover: transpiration will increase with increased vegetation cover, which will depend on the type of vegetation and the season. Vegetation with a low albedo, such as dark forests will absorb more solar radiation, increasing evaporation.
4. Soil moisture content: will determine the amount of water available for transpiration. It is dependant on the soil and rock permeability.
47
How can you study evapotranspiration losses using potential and actual evapotranspiration?
- potential evapotranspiration is the amount of evapotranspiration that could take place given unlimited supplies of water in an environment, whereas actual evapotranspiration is the amount of evapotranspiration that takes place given the actual water availability.
48
What is channel flow?
- Channel flow is water that has collected to flow in a rivulet, stream or river, and is another output from the drainage basin system.
- This is essentially the discharge of a river which is the volume of water passing a specific gauging station per unit of time and is measured in cubic metres per second (or cumecs).
49
What is discharge and therefore channel flow dependant on?
- Discharge is dependent on the amount of precipitation falling directly into the channel.
- And also on contributions from drainage basin stores via surface runoff, throughflow or groundwater flow.
50
In general, how can human activities disrupt the drainage basin cycle?
- Human activities can disrupt a drainage basin cycle by changing the speed of processes, creating new stores or by abstracting water.
- Significant disruption is also caused by hard engineering schemes, such as channelisation, to manage river flooding and, to a lesser extent, soft engineering schemes, such as overflow areas.
51
How does cloud seeding disrupt the drainage basin cycle?
- This is the attempt to change the amount or type of precipitation by dispersing substances into the air that serve as cloud condensation nuclei (hygroscopic nuclei).
- New technology and research claims to have produced reliable results that
make cloud seeding a dependable and affordable water-supply practice for many regions, but its effectiveness is still debated.
52
Which places have used cloud seeding and in turn disrupted the drainage basin cycle?
- China used cloud seeding in Beijing just before the 2008 Olympic Games to create rain to clear the air of pollution.
- It is used in the Alpine Meadows ski area in California to improve snow cover, and was used in 2015 in Texas to reduce the impact of drought.
53
How does urbanisation disrupt the drainage basin cycle?
- Urbanisation creates impermeable surfaces that reduce infiltration and increase surface runoff and throughflow through artificial drains; stream and river discharges often increase rapidly as a result.
54
Which places have disrupted the drainage basin cycle through urbanisation?
- Across the UK, urbanisation has increased flood risk in many towns and cities such as Winchester and Maidenhead (2014 floods) and Carlisle, York and Manchester (2015 floods).
55
How does dam construction disrupt the drainage basin cycle?
- Dams increase surface water stores and evaporation and reduce downstream river discharge.
56
Which places have disrupted the drainage basin cycle through the construction of dams?
- Lake Nasser behind the Aswan Dam in Egypt is estimated to have evaporation losses of 10 to 16 billion cubic metres every year.
- This represents
a loss of 20 to 30 per cent of the Egyptian water volume from the River Nile.
57
How does groundwater abstraction disrupt the drainage basin cycle?
- In some locations, groundwater is abstracted
from aquifers faster than it is replaced, causing reduced groundwater flow and a lower water table.
- In other locations, reduced industrial activity or deforestation has increased groundwater storage, increasing the risk of groundwater flooding if the water table reaches the land surface.
58
Which places have disrupted the drainage basin cycle through groundwater abstraction?
- Groundwater is used to irrigate more than 40 per cent of China's farmland and provides about 70 per cent of the drinking water in the dry northern and north-western regions.
- Groundwater extraction is increasing by about 2.5 billion cubic metres per year, and consequently groundwater levels in the arid North China Plain dropped by as much as a metre a year between 1974 and 2000.
- Groundwater rebound has occurred in some of the UK's major conurbations, including London, Birmingham, Nottingham and Liverpool, as a result of reduced abstraction for industry.
59
What are water budgets?
- Water budgets show the annual balance between inputs (precipitation) and outputs (evapotranspiration and channel flow).
- A water budget can be expressed as:
Precipitation (P) =channel discharge (Q) + evapotranspiration (f) ± change in storage (S)
60
What does the equation for water budgets allows us to do?
- This gives a direct comparison of natural water supply and demand.
- Making it possible to identify the time periods when precipitation exceeds evapotranspiration (resulting in a positive water balance), or times when there is a negative water balance (evapotranspiration exceeds precipitation) and an increased drought risk.
61
What does a soil moisture budget graph show and allow us to assess?
- The soil moisture budget graph shows annual changes in precipitation and potential evapotranspiration.
- This allows assessment of the impact on soil moisture availability in different climatic locations.
- This is useful for understanding the vulnerability of terrestrial ecosystems and the challenges for agriculture, especially in the presence of climate change.
62
What are the common features of a soil moisture budget graph?
- Soil moisture surplus
- Soil moisture utilisation
- Maximum annual temperatures
- Soil moisture deficiency
- Soil moisture recharge
- Field capacity
63
What is a soil moisture surplus?
- Where precipitation is greater than potential evapotranspiration and the soil water store is full, so there is a surplus of soil moisture for plant use, runoff into streams and recharging groundwater supplies. The soil is said to be at field capacity.
64
What is soil moisture utilisation?
- Where potential evapotranspiration increases and exceeds precipitation, so there is more water evaporating from the ground surface and being transpired by plants than is falling as rain.
- Water is also drawn up from the soil by capillary action. The water is gradually used up.
65
What is soil moisture deficiency?
- The soil water store has been used up by high rates of evapotranspiration and low precipitation.
- Plants can only survive if they are adapted to periods of drought or are irrigated.
66
What is soil moisture recharge?
- This occurs when potential evapotranspiration decreases so that it is lower than precipitation, and the soil store starts to fill up again.
67
What is field capacity?
- At this point the soil is full of water and cannot hold any more.
68
How long is the Amazon river and how big is its drainage basin?
- The Amazon River is 6,308 km long and drains a basin of nearly 6 million km2.
69
How does the river regime of the amazon vary over the course of a year?
- Its peak discharge is in April-May and lowest discharge is in September which is linked to wet and dry seasons and snowmelt from the Andes.
70
How long is the Yukon river and how big is its drainage basin?
- The Yukon River is 3,540 km long and has a drainage basin area of about 850,000 km2.
71
How does the river regime of the Yukon vary over the course of a year?
- The Yukon is in a tundra climatic area and flows through a mountain range.
- Its peak discharge is in May-June with a dramatic increase due to melting of snow and ice.
- It is low from December to May due to low precipitation and frozen conditions .
72
How was the river regime of the River Nile impacted by the construction of the Aswan Dam?
- The river regime of the River Nile was changed significantly by the construction of the Aswan Dam in 1970.
- The flow of the River Nile below the dam was reduced by about 65 per cent and became regulated between the seasons so that the flood peaks in September were greatly reduced.
- Overall the dam is regarded to have had a severe impact on the river regime.
73
What does a storm hydrograph show?
- A storm hydrograph shows variations in a river's discharge at a specific point over a short period of time (usually before, during and after a storm).
74
How can a storm hydrograph be described?
- The shape of the storm hydrograph changes as a result of physical and human factors, and can be described as either flashy or subdued.
- Flashy hydrographs indicate that there is a rapid increase in discharge, and perhaps a high risk of a sudden flood, whereas subdued is the opposite.
75
What is the normal (base) flow on a storm hydrograph?
- Normal (base) flow is the contribution river flow from long-term storage sources such as permeable rocks, areas of bog and marsh and peat-covered moorlands.
76
What is the rising limb on a storm hydrograph?
- Rising limb is the increase in discharge in response to surface runoff and through- flow from a rainfall event, until peak flow is reached.
77
What is the peak discharge on a storm hydrograph?
- Peak discharge is the maximum discharge by a stream or river in response to a rainfall event.
78
What is the bankfull discharge on a storm hydrograph?
- Bankfull discharge occurs when a river's water level reaches the top of its channel. Any further increase will result in flooding of the surrounding land.
79
What is the falling limb on a storm hydrograph?
- Falling limb is the decline in discharge that occurs after peak flow. This segment is usually less steep than the rising limb because throughflow is being released relatively slowly into the channel.
80
What is the lag time on a storm hydrograph?
- Lagtime is the difference in hours and minutes between the time of maximum precipitation and the time of peak discharge.
81
What is the storm runoff on a storm hydrograph?
- Storm runoff is the part of the river flow derived from the immediate rainfall event. The most rapid transfer of water occurs overland and via throughflow.
82
What is the approach segment on a storm hydrograph?
- Approach segment shows the discharge of the river before the storm (the antecedent flow rate).
83
What factors can affect the discharge of a river and shape of a storm hydrograph?
- Drainage basin size
- Drainage basin shape
- Drainage relief
- Soil type
- Rock type
- Drainage density
- Natural vegetation
- Land use
- Precipitation intensity
- Precipitation duration
- Snowfall
- Evapotranspiration
84
How can drainage basin size impact shape of a storm hydrograph?
- Small basins: water will reach the channel rapidly,as it has a shorter distance to travel.
- Large basins: water will take longer to reach the channel as it has a greater distance to travel.
85
How can drainage basin shape impact shape of a storm hydrograph?
- Circular basins: it will take less time for the water to reach the channel, as all the extremities are equidistant from the channel.
- Elongated basins: water will take a long time to reach the channel from the extremities of the drainage basin.
86
How can drainage basin relief impact shape of a storm hydrograph?
- Steep slopes: water flows rapidly downhill and reaches the channel quickly.
- Gentle slopes: water can infiltrate into the ground and travel slowly to the channel through the soil and rock.
87
How can soil type impact shape of a storm hydrograph?
- Claysoils and thin soils: clay soils have a low porosity and the grains swell when they absorb water, so water infiltrates slowly. Thin soil becomes saturated quickly.
- Sandy soils and thick soils: sandy soils have a high porosity, so the water can infiltrate. Deep soils allow more infiltration.
88
How can rock type impact shape of a storm hydrograph?
- Impermeable rocks: water cannot percolate into the rock, increasing surface runoff to rivers.
- Permeable rocks:water percolates through pore spaces and fissures into the groundwater store.
89
How can drainage density impact shape of a storm hydrograph?
- High drainage density: a large number of surface streams per km2 means the storm water willreach the main channel rapidly.
- Low drainage density: a small number of surface streams per km2 means the water travels slowly through the soil and rocks to the river.
90
How can natural vegetation impact shape of a storm hydrograph?
- Thin grass: intercepts little
water and there is little loss by evapotranspiration, so more water reaches the channel rapidly.
- Forest and woodland: intercepts
water and has high rates of evapotranspiration, so less water reaches the channel, and more slowly.
91
How can land use impact shape of a storm hydrograph?
- Urban: urban surfaces have more hard surfaces such as roads, and drains that carry the water rapidly and directly to the river.
- Rural: vegetated surfaces intercept water and allow infiltration so water travels slowly to the river channel.
92
How can precipitation intensity impact shape of a storm hydrograph?
- High intensity: when rain falls faster than the infiltration capacity, surface runoff occurs and transports the water rapidlyto the channel.
- Low intensity: water can infiltrate into the soil and then travel slowly through the soil to the river channel.
93
How can precipitation duration impact shape of a storm hydrograph?
- Prolonged: the water table rises and the soil becomes saturated, causing surface runoff,which travels rapidly to the riverchannel.
- Short duration: most of the water infiltrates into the soil and travels slowly through the soil into the rocks before reaching the channel.
94
How can snowfall impact shape of a storm hydrograph?
- Fast snowmelt: meltwater cannot infiltrate into the frozen ground, so it flows rapidly over the surface into the river channel.
- Slow snowmelt: the ground thaws with the snow, so the meltwater can infiltrate into the soil and rocks before reaching the channel.
95
How can evapotranspiration impact shape of a storm hydrograph?
- Low rates of evapotranspiration: fewer losses from the drainage basin system will increase discharge into the riverchannel.
- High rates: high evapotranspiration losses willreduce discharge into the riverchannel.
96
How does the Natural Drought Mitigation Centre in the USA describe a drought?
- The National Drought Mitigation Centre in the USA describes drought as an 'insidious hazard of nature'.
- Implying that it develops gradually (it has a slow onset), with harmful impacts that vary geographically
97
What does the United Nations state about the definitions of a drought?
- The United Nations states that international agreement on an objective definition
of drought has not yet been achieved.
- Different definitions are used around the world; some quantify the rainfall deficit over a period of time, while others measure impacts such as reservoir levels or crop losses.
98
What is the UN's general definition of a drought?
- The UN provides a general definition: 'drought is defined as an extended period - a season, a year, or several years - of deficient rainfall relative to the statistical multi- year average for a region'.
99
What are 4 different types of drought?
- Meteorological
- Hydrological
- Agricultural
- Socio-economic
- The first three types measure drought as a physical phenomenon.
- Whereas the last type measures drought in terms of supply and demand for human use (domestic, farming, industry).
100
How much land area across the world is exposed to a level of drought?
- Approximately 38 per cent of the world's land area has some level of drought exposure.
- This land area covers 70 per cent of both the total global population and agricultural land.
101
How much land area is exposed to sever drought and how much of the population does this cover?
- The areas of most severe drought exposure include 10 per cent of the land surface and 18 per cent of the global population .
102
What is the main physical cause of drought in regions of the world?
- The physical causes of drought in regions of the world can be largely explained by the global atmospheric circulation system.
103
What is a meteorological drought?
- Occurs when long-term precipitation is much lower than normal, but there is no consensus over the minimum amount of precipitation that turns a dry spell into an official drought.
- It is region-specific since the atmospheric conditions that result in deficiencies of precipitation are highly variable between climate types.
104
What is an agricultural drought?
- Occurs when there is insufficient soil moisture to meet the needs of a particular crop at a particular time.
- It is caused by factors such as precipitation shortages, differences between actual and potential evapotranspiration, soil water deficits and reduced groundwater or reservoir levels.
- A deficit of rainfall over cropped areas during critical periods of the growth cycle can result in crop failures or underdeveloped crops with greatly depleted yields.
- Agricultural drought is typically evident after a meteorological drought but before a hydrological drought.
105
What is a hydrological drought?
- Occurs when there are deficiencies in surface and subsurface water supplies as measured in rivers, reservoirs, lakes and groundwater.
- It originates with a deficiency of precipitation but is usually out of phase with or after the occurrence of meteorological and agricultural droughts, as it takes longer for precipitation deficiencies to reach some of the components of the hydrological system such as soil moisture, stream flow and groundwater or reservoir levels.
106
What is a socio-economic drought?
- Occurs when the water demand for social and economic purposes (such as crop irrigation or hydro-electric power) exceeds water availability.
- This could be the result of a weather related shortfall In water supply or overuse of the available water supplies.
- It differs from the other types of drought because its occurrence depends on temporal and spatial variations in supply and demand.
107
Describe the general processes taking place in the global atmospheric circulation system.
1. Intense solar radiation at the Equator warms
the air, which rises and starts convection. The air cools as it rises and water vapour condenses to form clouds and rain.
2. The subtropical high-pressure zone is created where air that had risen at the Equator has cooled and sinks to form a belt of high air pressure and hot, dry conditions.
3. The air returns to ground level at the Equator, creating trade winds.
4. The trade winds meet at the ITCZ where the warmed air rises. The position of the ITCZ moves with the seasons and this movement causes alternating wet and dry seasons in the tropics .
5. The warm air moving from the subtropics to the mid-latitudes meets cold polar air at the polar front, where the warm, less dense air rises, causing condensation and rainfall.
6. The warmer air rises into the polar front jet stream and is transferred at high altitude towards the poles, where it cools and sinks. This creates a movement of air at ground level back towards the equator.
108
How does the ITCZ move with the seasons?
- In the northern hemisphere
summer (June-August), the ITCZ is north of the Equator.
- In December to February, the southern hemisphere is tilted towards the Sun and therefore the ITCZ is south of the Equator.
109
What is the intertropical convergence zone (ITCZ)?
- The ITCZ is a belt of low atmospheric pressure located around the Equator.
- It moves north or south of the Equator seasonally.
- For example it is further north during the northern hemisphere summer.
110
How does the movement of the ITCZ cause alternating wet and dry seasons?
- At the ITCZ the air rises as a result of intense heating by the Sun's energy.
- There is also high evaporation, especially from oceans.
- It therefore causes an alternating wet season (when it arrives) and dry season (when it moves away) in some world regions.
111
How do sub-tropical high pressure zones impact the ITCZ?
- Sometimes the subtropical high-pressure zones, associated with the descending part of the convection cell (Hadley cell) block the high humidity, rain-bearing air masses associated with the ITCZ, so that the pattern is modified.
112
How can the modified ITCZ pattern caused by sub-tropical high pressures impact continental areas?
- Over continental areas such as Africa, there may be lower humidity levels because less water evaporates.
- And if high pressure blocks the arrival of the wet season, a severe drought can occur in the Sahel.
113
How is frontal precipitation created in mid latitudes?
- In the mid-latitudes, frontal precipitation is created in low-pressure systems that form along the polar front, where warm tropical air rises over cold polar air.
114
How do depressions move in mid latitudes?
- Depressions move from west to east in the mid-latitudes as a result of the Coriolis force (caused by the rotation of the Earth).
- Their track is directed by the polar front jet stream, which is a very fast-moving, meandering belt of air in the upper troposphere.
115
How are anticyclones able to move from the subtropics northwards when the jet stream usually prevents this?
- The loops of the jet stream occasionally stabilise, or even break up, and this allows high-pressure areas (anticyclones) from
the subtropics to move northwards.
116
How do northwards moving anticyclones form subtropics impact mid latitude countries?
- The anticyclones move northwards when the jest stream stabilises.
- They bring stable weather conditions with very little precipitation.
- The rain-bearing depressions are forced around them, usually to the north but occasionally to the south, causing drought in mid-latitude countries such as the UK
117
How are mid latitude anticyclones able to block weather systems from the west?
- The stability of anticyclones, with their sinking air and calm conditions, means that they can persist and block weather systems from the west for up to two weeks.
- If this situation is repeated over the space of a few months, normal precipitation levels are greatly reduced and this may cause a drought.
118
What did the Met Office report about mid latitude blocking anticyclones in the UK?
- The Met Office reported that from 2010 to early 2012 much of central, eastern and southern England and Wales experienced a prolonged period of below-average rainfall due to blocking anticyclones.
119
What are the main physical causes of drought?
- ITCZ
- Mid latitude blocking anticyclones
- ENSO cycles
120
What is the El Nino-Southern Oscillation (ENSO)?
- The El Nino-Southern Oscillation (ENSO) is a naturally occurring large mass of very warm seawater in the equatorial Pacific Ocean.
121
What happens during an ENSO cycle?
- Normally, the large mass of warm water in the equatorial pacific ocean is normally located in the western Pacific.
- It reaches the western pacific being pushed by ocean currents, trade winds and the Walker circulation cell in the atmosphere.
- However, on average every seven years these pushing forces weaken.
- This allows the mass of warm water to move eastwards towards the west coasts of Central and South America.
122
What impact does an El-Nino event have on countries around the Pacific Ocean?
- Wherever the mass of warm water is located, evaporation rates are higher and precipitation greater.
- This means that Central and South America will experience higher precipitation during an El-nino.
- Areas of cooler water, such as the cold current (the Humboldt Current) that flows along the Peru- Chile coastline will bring drier weather to the western pacific and the east coast of Australia as it moves west.
123
What are the normal conditions in the Pacific Ocean when it is not an El-nino year?
- There are large masses of warm water in the western pacific bringing high levels of precipitation to the east coast of Australia.
- Cold masses of water for example the Humboldt current which flows along Peru-Chile coastline bring dry weather to the wets coast of South America.
124
What is an example of a drought which occurred due to an El-Nino event?
- The 1997 to 2009 Millennium Drought in Australia.
125
What is a La Nina?
- La Nina occurs when the warm mass of water is pushed even further west than normal, which causes drought in Peru and California, for example.
126
How can ENSO cause global impacts?
- While it originates in the Pacific, ENSO is now known to cause global variations in rainfall patterns, by changing the global atmospheric circulation, creating both drought and floods in different areas of the world.
127
What is an example of a drought caused by a La Nina?
- The severe East African drought of 2011 was attributed to a strong La Nina
128
What did scientists as the University of Birmingham say about severe droughts in 2016?
- In February 2016, scientists at the University of Birmingham published research arguing that
severe droughts should no longer be seen as purely natural hazards.
- Dr Anne Van Loon concluded that 'society is not a passive victim of drought'; human responses to water shortages influence water levels in reservoirs, aquifers and rivers.
129
What does research identify about the direct impact of humans on drought?
- Severe droughts in settled environments, such as those experienced by China, Brazil and the USA in the 21st century, are not just natural hazards, because human activities play a role.
- Research has identified that people have directly affected
the development of droughts by abstracting water from rivers and groundwater, and by reducing the downstream supply of water by building reservoirs and water transfers.
130
What does research identify about the indirect impact of humans on drought?
- Humans have
indirectly affected the development of droughts by changing land uses and altering hydrological processes.
- For example, deforestation and overgrazing reduce vegetation cover, so reducing evapotranspiration rates, and thereby reducing atmospheric moisture and precipitation.
- The removal of vegetation also changes soil conditions through compaction and reduced organic matter and moisture retention; this reduces infiltration and increases surface runoff, which reduces soil moisture content and water storage .
131
What does research show about the impact of climate change on drought risk?
- The contribution of anthropogenic climate change to drought risk remains uncertain, but recent research in Australia suggests that it is likely to have enhanced the drought hazard in certain world regions.
132
Why are ecosystems vital for human wellbeing?
- Ecosystems are vital for human wellbeing, providing us with innumerable and invaluable services.
- From tangible amenities such as clean air and water, food and fuel, to intangible amenities such as aesthetic and psychological benefits.
133
How can you define an ecosystem?
- Ecosystems are defined as interconnected communities of living organisms within a defined physical environment.
134
What does the term ecosystem functioning mean?
- The term ecosystem functioning refers to the biological, chemical and physical processes that take place within the ecosystem .
135
Why are all ecosystems vulnerable to drought?
- All ecosystem components and processes are dependent on supplies of water so ecosystems are vulnerable to drought.
136
Which ecosystems are most resilient to drought?
- Levels of ecosystem resilience to drought vary, with desert and semi-arid ecosystems being the most resilient.
137
What is a wetland ecosystem?
- A wetland is a land area saturated with water, either permanently or seasonally.
- Wetlands include marshes, swamps, bogs and fens.
138
What are examples of large and small wetland ecosystems?
- The Everglades in Florida are an example of a large wetland.
- While Minsmere in Suffolk is a small example.
139
How do droughts impact habitats in wetland ecosystems?
- The Environment Agency has researched the impact of drought on wetland ecosystems in the
UK.
- The Agency found that as the supply of water is reduced, areas of open water shrink or dry up altogether, resulting in progressive loss of habitat.
140
How do droughts impact future flood risks in wetland ecosystems?
- As soil moisture is reduced, extended drying can lead to soil erosion, for example by the wind, and a reduced ability to store water in times of flood (potentially increasing downstream flood risk) or to release water in times of drought.
141
How do droughts in wetland ecosystems impact the carbon cycle?
- Organic soils may oxidise, releasing carbon into the atmosphere; as water availability diminishes, concentrations of dissolved nutrients or pollutants may increase.
142
Why may wetland ecosystem species show varying levels of resilience to drought?
- Birds, terrestrial wetland vegetation and invertebrates show varying responses to drought.
- Depending on species' resistance and resilience, competitive and predatory interactions, availability of food, and timing and characteristics of a drought.
143
How can droughts in wetland ecosystems eliminate some species?
- Drought can alter communities by eliminating some species and creating gaps in food webs in which other species can establish themselves.
- For example, as open water is lost, aquatic birds such as ducks and moorhen will decline.
- While some non-aquatic birds such as swallows and martins are also likely to be reduced as nesting sites are lost.
- Species such as snipe and thrush will also be affected because the dry soil surface will be less penetrable, limiting their ability to feed on soil invertebrates .
144
How do the communities of river-fed and groundwater-fed wetlands vary?
- In general, river-fed wetlands experience a wider range of water levels naturally and therefore support communities that may be more adapted to such fluctuations.
- By contrast, groundwater-fed wetlands experience a smaller range of water levels, supporting communities that prefer this type of environment but that are therefore more vulnerable to prolonged drought over several seasons.
145
Why are forest ecosystems highly valued?
- Forests are highly valued for their ecological functions and services, which include:
- water storage and regulation of hydrological processes.
- timber production
- wildlife habitat
- carbon sequestration
- recreational opportunities.
146
How long does it take for normal growth rates of living trees to resume after a drought?
- Research published in 2015, from over 1,300 forest sites worldwide, revealed that living trees take an average of two to four years to recover and resume normal growth rates following a period of drought.
147
Which plant species in a forest ecosystem is not resilient to drought?
- Resilience to drought was lower for some species such as pine, which tend to keep using water at a high rate, even during a drought.
148
What long term harm does drought cause to trees in forest ecosystems?
- Researchers suggest that the long-term harm to drought-stressed trees include:
- foliage loss
- impairing growth
- increased accumulation of pests and diseases
- lasting damage to vascular tissues, impairing water transport.
149
What is an example of a drought which caused a significant die-off of pine species in a forest ecosystem?
- Between 2000 and 2003 a combination of severe drought and unusually high temperatures led to a significant die-off of Pinon pines in the Four Corners region of the south-west USA .
150
How did the severe drought in south west USA cause a significant die off of pine species in the forest ecosystem?
- The hot dry conditions made the Pinons more susceptible to pine bark beetle attacks.
- In some areas more than 90 per cent of the Pinons died.
- Warmer winter temperatures have also contributed to bark beetle outbreaks, because more beetles are able to survive through the winter and then reproduce .
151
Why are forest ecosystems important for carbon sequestration?
- The Global Footprint Network (GFN) calculated that forests store an average of 0.73 tonnes of carbon per hectare per year, which is important for regulating climate change.
