Water Cycle Flashcards
Global water budget
The global water budget limits water available for human use and water stores have different residence times; some stores are non-renewable (fossil water or cryosphere losses)
The global water budget takes into account all the water that is held in stores and flows of the global hydrological cycle. The most significant feature of the budget is that only 2.5% of it is freshwater; the rest is in oceans. Even more remarkable is that only 1% of all freshwater is ‘easily accessible surface freshwater’. Nearly 70% is locked up in glaciers and ice sheets.
Although water is constantly circling around the hydrological cycle, each store has a 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.
Fossil Water
Ancient, deep groundwater made from pluvial (wetter) periods in the geological past
The Cryosphere
Made up of those areas of the world where water is frozen into snow or ice
Is it non-renewable? This is to be questioned because, come another glacial period, more water will once again be locked in glaciers and ice sheets.
From a human viewpoint, the most critical feature of the global water budget is that accessible surface water is a mere 1% of all the world’s freshwater, and this is the major source of water for human use.
The smallness of this figure emphasises the important point that water is not the abundant resource so many think it is. Indeed, it is a scarce resource needing careful management.
Figures:
All water:
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
The hydrological cycle
Inputs:
The main input is precipitation, which can vary in a number of different ways. All these characteristics can have a significant effect 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.
Flows
There are at least seven different flows that are important in transferring the precipitation that has fallen on the land into the drainage network.
Interception: the retention of water by plants and soils which is subsequently evaporated or absorbed by the vegetation.
Infiltration: the process by which water soaks into, or is absorbed by, the soil.
Percolation: similar to infiltration, but a deeper transfer of water into permeable rocks.
Throughflow: the lateral transfer of water downslope through the soil
Groundwater flow: the very slow transfer of percolated water through pervious (permeable) or porous rocks.
Surface runoff: the movement of water that is unconfined by a channel across the surface of the ground. A.k.a. overland flow.
River or channel flow: takes over as soon as the water enters a river or stream; the flow is confined within a channel.
Outputs
Evaporation: the process by which moisture is lost directly into the atmosphere from water surfaces, soil and rock.
Transpiration: the biological process by which water is lost from plants through minute pores and transferred to the atmosphere.
Discharge (channel flow): into another, larger drainage basin, a lake or the sea.
Basics of drainage basins
Basics of drainage basins
A drainage basin is the area of land drained by a river and its tributaries, sometimes referred to as a river catchment. The boundary of a drainage basin is defined by the watershed.
The drainage basin is a subsystem within the global hydrological cycle. It is an open system with external inputs and outputs. Since those inputs vary over time, so does the amount of water in the drainage basin. Drainage basins vary in size from that of a small local stream up to a huge river such as the Amazon. The drainage basins of tributary streams and small rivers nestle within the drainage basins of larger rivers.
Climate, soils, geology, relief and vegetation all impacting physical factors within drainage basins
Climate
Mainly impacts on the inputs and outputs
Climate has a role in influencing the type and amount of precipitation overall and the amount of evaporation (i.e. the major inputs and outputs)
Climate also has an impact on vegetation type
Soils
Largely affect the relative importance of the different flows within the system (of these flows perhaps the most important is surface runoff)
Soils determine the amount of infiltration and throughflow, and indirectly, the type of vegetation
Geology
Largely affects the relative importance of the different flows within the system (of these flows perhaps the most important is surface runoff)
Geology can impact on subsurface processes such as percolation and groundwater flow (and, therefore, on aquifers)
Indirectly, geology affects soil formation.
Relief
Largely affects the relative importance of the different flows within the system (of these flows perhaps the most important is surface runoff)
Relief can impact on the amount of precipitation.
Slopes can affect the amount of runoff.
Vegetation
Largely affects the relative importance of the different flows within the system (of these flows perhaps the most important is surface runoff)
The presence or absence of vegetation has a major impact on the amount of interception, infiltration and occurrence of overland flow, as well as on transpiration rates.
Human factors disrupting the drainage basin cycle
It is mainly human changes to:
rivers and drainage
the character of the ground surface (its shape, texture and covering)
that disrupt the drainage basin system, often by accelerating its processes.
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.
The components of the drainage basin most affected by humans are:
evaporation and evapotranspiration
interception
infiltration
groundwater
surface runoff
Amazonia
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:
A lowering of humidities
Less precipitation
More surface run off and infiltration
More evaporation, less transpiration
More soil erosion and silt being fed into rivers.
Storm hydrography
Whereas river regimes are usually graphed over the period of a year, storm hydrographs show discharge changes over a short period of time, often no more than a few days. The storm hydrograph plots two things: the occurrence of a short period of rain (maybe a heavy shower or storm) over a drainage basin and the subsequent discharge of a river.
Main features of a hydrograph:`
Once the rainfall starts, the discharge begins to rise - rising limb
Peak discharge is reached some time after the peak rainfall because the water takes time to move over and through the ground to reach the river.
The time interval between peak rainfall and peak discharge is known as lag time.
Once the input of rainwater into the river starts to decrease, so does the discharge; this is shown by the falling or recessional limb
Eventually the river’s discharge returns to its normal level, or base flow.
The shape of a storm hydrograph of the same river may vary from one rain event to the next. This variation is closely linked to the nature of the rainfall event. The shape of the hydrograph also varies from one river to another. This is a result of the particular physical characteristics of individual drainage basins.
Some hydrographs have very steep limbs, especially rising limbs, a high peak discharge and a short lag time. These are often referred to as ‘flashy’ hydrographs. In contrast, there are some hydrographs with gently inclined limbs, a low peak discharge and a long lag time. These are called ‘delayed’, or ‘flat’ or ‘subdued’ hydrographs.
Urbanisation
When it comes to evaluating the factors affecting the character of storm hydrographs, particularly their ‘flashiness’, none is more important than urbanisation. Not least of its impacts is that it changes the characteristics of the land surface. Its effects on hydrological processes include the following:
Construction work leads to the removal of the vegetation cover. This exposes the soil and increases vegetation cover
Bare soil is eventually replaced by a covering of concrete and tarmac, both of which are impermeable and increase surface runoff.
The high density of buildings means that rain falls on roofs and is then swiftly fed into drains by gutters and pipes.
Drains and sewers reduce the distance and time rainwater travels before reaching a stream or river channel.
Urban rivers are often channelised with embankments to guard against flooding. When floods occur, they can be more devastating.
Bridges can restrain the discharge of floodwaters and act as local dams, thus prompting upstream floods.
In short, the overall impact of urbanisation is the increase the flood risk. The problem is made worse by the fact that so many towns and cities are located close to rivers. Historically, this was for reasons of water supply and sewage disposal. Often the historic nucleus was located at a point where a river could be easily crossed.
Synoptic themes
Planners have become important players in managing the impacts of urbanisation on flood risk. This is because:
many towns and cities are naturally prone to flooding because of their locations
of the number of people who live in urban places and who therefore need protection
of the huge amount of money invested in urban property.
Flood risk management involves such actions as:
strengthening the embankments of streams and rivers
putting in place flood emergency procedures
steering urban development away from high-risk areas such as floodplains
Causes of drought
The Four Types of Drought (Meteorological and Hydrological most important)
Drought is defined in meteorological terms as a shortfall or deficiency of water over an extended period, usually at least a season.
This differs from hydrological drought, where there is reduced stream flow, lowered groundwater levels and reduced water stores.
Agricultural drought is when agricultural activity is greatly impacted by drought.
This can lead to food shortages, famine and starvation (socio-economic drought)
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Causes of Meteorological Drought
The physical causes of drought are only partially understood. They lie somewhere in the complex interactions between atmosphere, oceans, cryosphere, biosphere and the land, which produces the climates of the globe.
Droughts can range from short-term and localised precipitation deficits to longer-term trends that are part of climate change.
Research suggests that sea surface temperature anomalies are an important causal factor in short-term precipitation deficits.
El Niño Southern Oscillation (ENSO)
Temperature anomalies provide the key to ENSO, which, in turn, is thought to trigger the occurrence of droughts.
Normal conditions in the Pacific Basin
In a 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
Conditions during an 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
When this happens, 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.
El Niño events usually occur every 3-7 years, and usually last for 18 months. El Niño events seem to trigger very dry conditions throughout the world, usually in the second year. For example, the monsoon rains in India and South East Asia often fail.
La Niña
La Niña events may sometimes, but not always, follow an El Niño event. They involve the build-up of cooler-than-usual subsurface water in the tropical part of the Pacific. This situation can lead to severe drought conditions, particularly on the western coast of South America.
Very strong air circulation and very warm water moving east-west.
Human activity and drought q
People are not the cause of drought, but their actions can make droughts more severe.
Desertification in the Sahel
The Sahel region of Africa stretches from Mauritania eastwards to Ethiopia.
Desertification is the process by which once-productive land gradually changes into a desert-like landscape. It usually takes place in semi-arid land on the edges of existing deserts. It’s not necessarily irreversible.
The causes of desertification are essentially natural. They set in motion a downward spiral:
Changing rainfall patterns with rainfall becoming less reliable, seasonally and annually. The occasional drought year sometimes extends to several years.
The vegetation cover becomes stressed and begins to die, leaving bare soil.
The bare soil is eroded by wind and the occasional intense shower.
When rain does fall, it is often only for short, intense periods. This makes it difficult for the remaining soil to capture and store it.
The northern Sahel region is experiencing a 30-40% annual departure from normal rainfall. 25-30% below that, and 20-25% in the southern Sahel.
Human factors act like a feedback loop. Humans enhance the impacts of drought by the over-abstraction of surface water from rivers and ponds, and of groundwater from aquifers. Key human factors encouraging this are:
Population growth: rapid population growth puts pressure on the land to grow more food. Migrants fleeing from one disaster area help to make another.
Overgrazing: too many goats, sheep and cattle destroy the vegetation cover
Overcultivation: intense use of marginal land exhausts the soil and crops will not grow
Deforestation: trees are cut down for fuel, fencing and housing. The roots no longer bind the soil, and erosion ensues.
In the case of the Sahel, the situation has been made worse by frequent civil wars. Crops, livestock and homes have been deliberately destroyed.
Drought in Australia
Drought is a recurrent annual feature in Australia, with up to 30% of the country affected by serious of severe rainfall deficiency. The link with El Niño events is well established. However, droughts are becoming more frequent and more severe.
The worst event so far has been the ‘Big Dry’ of 2006. This was assessed as a 1-in-1000 year event and is thought to have been associated with a longer-term climate change.
Unlike the Sahel, Australia has not followed the same downward spiral of desertification. A careful management of scarce water resources, and sorting out the competing demands of irrigation and urban dwellers, has stopped this from happening. Other actions include the large-scale recycling of grey water (waste bath, shower, sink and washing water), constructing desalination plants and devising new water conservation strategies.
Drought and ecosystems
The impacts of drought on ecosystem functioning (wetlands, forest stress) and the resilience of these ecosystems.
Ecological resilience is the capacity of an ecosystem to withstand and recover from a natural event (e.g. drought or flooding) or some form of human disturbance.
Wetlands
Wetlands currently cover about 10% of the Earth’s land surface and until 50 years ago they were considered as wastelands, only good for draining and infilling to provide building land. However, it is now understood that wetlands perform a number of important functions: from acting as temporary water stores to the recharging of aquifers, from giant filters trapping pollutants to providing nurseries for fish and feeding sites for migrating birds.
Drought can have a major impact on wetlands. With less precipitation there will be less interception (as vegetation becomes stressed), as well as less infiltration and percolation. Water tables will fall. Evaporation will also increase. This, together with the decrease in transpiration, will reduce the valuable functions performed by wetlands.
While droughts pose a threat to wetlands, the major challenge to their survival still remains artificial damage.
Resilience???
Forests
Forests have significant impacts on the hydrological cycle. They are responsible for much interception which, in turn, means reduced infiltration and overland flow. Forests are characterised by high levels of transpiration.
Like wetlands, drought threatens forests, but it is people and deforestation that most threaten their survival. In coniferous forests, drought is not only causing direct physiological damage but it is also increasing the susceptibility of pines and firs to fungal diseases. Tree mortality is on the increase. The same applies to the tropical rainforest, expect that the increased mortality attributed to drought appears to be having a greater impact on large trees. Here there is the added concern of what this increased tree mortality will eventually do to this incredibly important carbon store.
As ecosystem play such a vital role within the hydrological cycle, it is important to ensure that their ecological resilience is not overstretched by either the destructive activities of people or natural events such as droughts and floods.
Meteorological causes of flooding
Surplusses within the hydrological cycle more often than not mean flooding. The meteorological causes of flooding are:
intense storms, which lead to flash flooding (short lag time), as in semi-arid areas but more common in mountainous areas
prolonged, heavy rain, such as during the Asian monsoon and with the passage of deep depressions across the UK
rapid snowmelt during a particularly warm spring, as on the plains of Siberia.
Bangladesh is a particularly food-prone country mainly because it is a land of floodplains and deltas built up by mighty rivers such as the Ganges, Padma and Meghna. These rivers are swollen twice a year by meltwater from the Himalayas. and by the summer monsoon. Hilly tracts between the rivers and behind Chittagong are often victims of flash floods.
There is also tidal flooding, often a result of storm surges or when high river flows meet particularly high spring tides in estuaries. A storm surge is caused by very low air pressure which raises the height of the high-tide sea. Strong onshore winds then drive the ‘raised’ sea towards the coast, often breaching coastal defences and flooding large areas.
The likelikhood of flooding is also increased by other physical circumstances:
in low-lying areas with impervious surfaces, as in towns and cities
where the ground surface is underlain by impermeable rocks
when ice dams suddenly melt and the waters in glacial lakes are released
where volcanic activity generates meltwater beneath ice sheets that is suddenly released (jökulhlaups)
where earthquakes cause the failure of dams or landslides that block rivers
Human activity and flooding
A combination of economic and population growth during the 20th century has caused many floodplains to be built upon and many natural landscapes to be modified for agricultural, industrial and urban purposes. The impacts of human activities on the hydrological cycle were examined in 2C. These same activities, all related to changing land use within river catchments, frequently increase the flood risk, none more so than urbanisation.
E.g.:
Impermeable areas of tarmac
Wells sunk to supply settlements
Sprinkling of groundwater onto arable crops
Dams built to supply towns with water
Streams channelled into culverts to aid rapid drainage of farmland
natural streams meander and have marshy areas; but channelisation does not
Ploughing compacts soil
Grazing animals trample soil
Woodlands intercept rain and transpire moisture; roots give good soil structure. Deforestation destroys this.
Natural grasslands allow water to sink in, replaced by improved pasture
Bridge supports built in rivers, ramps on floodplain
Sewers feed water into channel
River mismanagement:
channelisation: an effective way of improving river discharge and reducing the flood risk. The trouble is that it simply displaces the river downstream. Some other location may well be overwhelmed by the increased discharge
dams: block the flow of sediment down a river, so the reservoir gradually fills up with silt; downstream there is increased river bed erosion
river embankments: designed to protect from floods of a given magnitude. They can fail when a flood exceeds their capacity. Inevitably, when this happens, the scale of flooding is that much greater.
These examples of hard-engineering intervention serve as reminders that soft-engineering methods of reducing the flood risk are preferable. These include making greater use of floodplains as nature intended, as temporary stores of flood water, and using them only for nature conservation and perhaps agriculture and recreation.
Impacts of flooding
Socio-economic
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
The environmental impacts of flooding receive much less publicity, perhaps because there are some positives:
recharged groundwater stores
increased connectivity between aquatic habitats
soil replenishment
for many species, flood events trigger breeding, migration and dispersal
Most ecosystems have a degree of ecological resilience that can cope with the effects of moderate flooding, It is where the environment has been degraded human activities that negative impacts are more evident. For example, the removal of soil and sediment by floodwaters can lead to the eutrophication of water bodies. That same floodwater can also leach pollutants into water courses with disastrous effects for wildlife, while diseases carried by floodwater can weaken or kill trees.
UK Floods
The UK has experienced some severe floods in recent years, most notably in the summer of 2007 and the winter of 2015-16.
These unusually severe floods have had the same basic cause, namely prolonged heavy rainfall, but at different times of the year. During the 2016 floods, large areas of the UK received more than twice the average amount of rainfall for that time of year. Carlisle and Cockermouth in Cumbria were along the worst-hit places.
There were recriminations after the apparent inadequacy of flood protection measures. The following were singled out for blame:
budget cuts in the amount of money being spent on flood defences
an EU Directive that puts environmental conservation ahead of the regular dredging of rivers
poor land management, resulting in blocked ditches
global warming
What tends to be forgotten in post-flood enquiries is that flood protection measures are designed to cope with flood events of a given magnitude. When an event of a very rare order of magnitude occurs, no amount of money or engineering is going to prevent the hoped for degree of protection.
Stores and flows
Stores
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
Flows
Reservoir, lake and wetland storage
Changes in wetland storage cannot be conclusively linked to climate change
It appears that storage is decreasing as temperatures increase
Soil moisture
Possibly little change, with higher precipitation and evaporation cancelling each other out
Uncertain, as soil moisture depends on many factors, of which climate is only one
Where precipitation is increasing, it is likely that soil moisture will also increase.
Permafrost
Deepening of the active layer is releasing more groundwater
Methane released from thawed lakes may be accelerating change
Snow
Decreasing length of snow-cover season
Spring melt starting earlier
A decreasing temporary store
Glacier ice
Strong evidence of glacier retreat and thinning since the 1970s
Less accumulation because more precipitation is falling as rain
A decreasing store
Oceans
More data on surface temperatures needed
Where there is ocean warming, there will be more evaporation
Possibly ocean warming leads to the generation of more cyclones
Storage capacity being increased by meltwater
Rising sea level
Physical and economic scarcity of water
Physical Scarcity
This occurs when more than 75% of a country’s or region’s blue water (liquid, accessible) flows are being used.
Currently applies to about 25% of the world’s population
Qualifying countries are located in the Middle East and North Africa.
Qualifying regions are located in north China, western USA, and southeast Australia
Economic Scarcity
This occurs when the use of blue water sources is limited by lack of capital, technology and good governance. It is estimated that around 1 billion people are restricted from accessing blue water by high levels of poverty. Most of people living in Africa (apart from the north and extreme south). Also parts of continental south-east Asia.
In short, the causes of water scarcity are twofold:
A lack of precipitation, either annually or seasonally
A lack of the wherewithal needed to harness the amount of blue water in demand
Access to safe, potable water is regarded by some as a human right. In the 21st century, however, it is increasingly being seen as a commodity for which a realistic price should be paid. In the developed world, much of the water supply industry there is now in the hands of private companies. People expect to have to pay for water.
In the developing world, however, the situation is very different. Supplying safe water in areas of physical scarcity can be difficult, costly and well beyond the means of very poor people. This is where charities such as WaterAid provide such invaluable help. Their programmes are helping to reduce the extent of economic water scarcity.
Importance of water supply
Agriculture
Agriculture dominates water use; about 3,770 kilometres of water are withdrawn each year, more than twice the total withdrawn for industrial and domestic purposes. Around 20% of the world’s land is under full irrigation. About 30% of this irrigation comes from dams and their network of irrigation canals. But the majority of irrigation water is pumped directly from aquifers and is leading to massive groundwater depletion, especially in China, India, Pakistan and the USA. Clearly, the water situation is unsustainable and hydrological cycles are being seriously disrupted.
Industry and energy
Just over 20% of all freshwater withdrawals worldwide are for industrial and energy production. Industries such as chemicals, electronics, paper, petroleum and steel are major consumers of water. Water pollution is a major problem associated with much of this industrial use of water.
Over half of the water used by energy production is either for generating HEP or as cooling water in thermal and nuclear power stations. So all this water is returned to its source virtually unchanged. However, there is mounting concern about the growth of biofuels for the production of bioethanol and biodiesel, since these crops are very thirsty.
Domestic Use
So With economic development comes rising standards of living and an increasing per capita consumption of water. Safe water is a fundamental human need. However, water does have its risks so far as human well-being is concerned. Water, particularly that polluted by lack of sanitation, is an effective medium for the breeding and transmission of a range of lethal diseases, such as typhoid, cholera and dysentery.
Water is also a productive breeding ground for some disease vectors, such as mosquitoes, snails and parasitic worms. Malaria, dengue and bilharzia are dehabilitating vector diseases. So safe water is vital to human health, particularly in the context of washing and food preparation.
From the above, it can be seen that an inadequate supply of water can easily impede any water-dependent aspects of economic development. Costs may well rise.
An inadequate water supply will also threaten human health. Environmentally, it will encourage people to over-exploit what water resources there are. This could easily prolong periods of drought and possibly be a first step on the downward path to desertification.
