The Freshwater System
Precipitation: the transfer of moisture (rain, snow etc) to the earth's surface from the atmosphere.
Interception: the capture of raindrops by plant cover, which prevents direct contact with the soil.
Runoff: precipitation that does not soak into the ground but flows over it into surface waters.
Groundwater: water held underground in soil or porous rock, often feeding springs and wells.
Evapotranspiration (EVT): the loss of water from vegetation and water surfaces to the atmosphere.
Potential evapotranspiration (pEVT): the rate of water loss from an area if there were no shortages of water
Maximum sustainable yields (MSY): the maximum level of extraction of water that can be maintained indefinitely for a region.
The hydrological cycle, also known as the water cycle, is the continuous movement of water on, above, and below the surface of the Earth.
It's a complex system with various interconnected processes responsible for distributing freshwater globally.
The water cycle involves the exchange of energy, which leads to temperature changes.
For instance, when water evaporates, it takes up energy from its surroundings and cools the environment.
When it condenses, it releases energy and warms the environment.
These heat exchanges influence climate.
The evaporative phase of the cycle purifies water which then replenishes the land with freshwater.
The flow of liquid water and ice transports minerals across the globe.
It is also involved in reshaping the geological features of the Earth, through processes including erosion and sedimentation.
The hydrological cycle is a closed system because water can not be added or lost.
Although water can not be added or lost it can be found in different states and in different locations.
Despite the planet being covered in water, the vast majority is sea water (97.5%). Of the remaining 2.5% the majority is held in glaciers and ice sheets.
Only a very small amount of the world’s water is easily accessible in rivers and lakes (0.00069%)
Closed System:
In a closed system, there is no exchange of matter with the surroundings, only energy.
Hydrological Cycle as a Closed System:
When considering only the movement of water molecules within the Earth's system, it can be simplified as a closed system.
The total amount of water on Earth remains constant, although it continuously changes states and locations through the various processes of the cycle.
Open System:
In an open system, both matter and energy can be exchanged with the surroundings.
Hydrological Cycle as an Open System:
When considering the energy transfers involved in the water cycle, it's clearly an open system.
Solar energy from the sun drives the entire cycle, causing water to evaporate and eventually return as precipitation.
Additionally, a small amount of water vapor escapes Earth's atmosphere, entering the open space, making it a truly open system in that sense.
Precipitation:
This is the initial stage where water vapor in the atmosphere condenses and falls back to Earth as rain, snow, sleet, or hail.
Factors like temperature, pressure, and wind currents influence precipitation patterns, leading to uneven distribution across the globe.
Types of precipitation:
Convectional precipitation: Occurs due to rising warm air, common in tropical regions.
Orographic precipitation: Forced uplift of air over mountains, leading to rain on windward slopes.
Frontal precipitation: Warm and cold air fronts colliding, resulting in widespread precipitation.
Interception:
Before reaching the ground, precipitation encounters various surfaces like leaves, branches, and vegetation.
Some water gets trapped or evaporates directly back into the atmosphere.
Factors affecting interception:
Type and density of vegetation
Leaf surface area
Rainfall intensity and duration
Infiltration:
Water that doesn't evaporate penetrates the ground surface, entering the soil zone. Soil texture, porosity, and moisture content determine the infiltration rate.
Infiltration pathways:
Macropores: Large channels allowing rapid infiltration (e.g., cracks, wormholes)
Micropores: Smaller spaces facilitating slower infiltration
Infiltration impacts:
Replenishes soil moisture crucial for plant growth
Contributes to groundwater recharge
Runoff:
Water that doesn't infiltrate either evaporates or flows over the land surface as runoff.
Types of runoff:
Overland flow: Water sheet flow directly over the land
Channel flow: Water concentrated in streams and rivers
Subsurface flow: Water infiltrating shallow soil layers and moving laterally towards waterways
Runoff factors:
Rainfall intensity and duration
Slope and topography
Soil permeability and saturation
Land cover (e.g., vegetation, urbanization)
Evapotranspiration:
This combined process involves the evaporation of water directly from soil and water bodies and the transpiration of water vapor from plants through their leaves.
Types of evapotranspiration:
Evaporation: Direct change of liquid water to vapor from surfaces
Transpiration: Release of water vapor from plant stomata during photosynthesis
Evapotranspiration impacts:
Returns water vapor to the atmosphere for further precipitation
Regulates surface temperature and humidity
Influences plant growth and ecosystem health
Groundwater Recharge:
A portion of infiltrated water percolates deeper into the soil, eventually reaching the saturated zone below, replenishing groundwater aquifers.
Factors affecting recharge:
Soil permeability and depth
Groundwater table depth
Rainfall patterns and intensity
The water balance basically looks at the balance between inputs and outputs.
You can look at the water balance at a global level (hydrological cycle), at a local level (drainage basin cycle) or even just a field.
At a global level oceans tend to experience greater outputs (evaporation) than inputs (precipitation).
This is because oceans are large areas with no shade that have regular winds blowing saturated air on land, allowing greater evaporation.
In addition oceans don’t tend to suffer from the same amount of relief and convectional rainfall as land does.
On land, inputs (precipitation) tends to be greater than outputs (evaporation).
This is because lands suffers from larger amounts of frontal, relief and convectional rainfall, as well as much of the lands water being protected underground or in shaded areas reducing evaporation.
At a global level there obviously has to be an equilibrium between inputs and outputs.
The excess precipitation on land is returned to the oceans by channel flow, surface run-off and to a lesser extent groundwater flow.
The excess of evaporation is returned to the land from the sea by winds blowing saturated air on land.
Drainage basin: the area drained by a river and its tributaries
Water balance: the relationship between the inputs and outputs of a drainage basin
Soil moisture excess: when soil moisture and groundwater is replenished. The excess may lead to saturation and increased surface run-off
Drainage divide or watershed: the line defining the boundary of a river or stream drainage basin separating it from adjacent basins
Discharge: the volume of water passing a given point over a set time
Peak rainfall: The highest rainfall (usually measured in mm) during a storm.
Inputs:
The main input to the system is precipitation.
The type of precipitation (rain or snow, etc), the intensity, the duration and frequency all have an effect on the amount of water in the system.
Each subsystem of the drainage basin system will also have inputs and ouputs, and the output from one stage of the diagram will form the input for another.
Precipitation: Any moisture that falls from the atmosphere.
The main types of precipitation are rain, snow, sleet, hail, fog and dew.
Inter-basin transfer: Water that either naturally (due to the alignment of the rock) or with human involvement (pumps and pipes) moves from one drainage basin to another.
Transfer:
The sum of all the water flowing over the drainage basin’s surface is called runoff.
It is made up of streamflow, which is flow through permanent river channels and overland flow or surface runoff.
Overland flow transfers water through the basin either as sheetwash, across the surface, or in tiny channels called rills.
Beneath the surface, water is transferred via throughflow, which is the movement of water through the lower soil towards rivers, and groundwater flow.
Groundwater flow is typically very slow.
Water that has been intercepted by foliage may also be transferred, either directly as throughfall, or by running down branches and stems via stemflow.
Stem flow: When intercepted water runs down the trunks and stems of vegetation.
Canopy drip: When intercepted water drips off the leaves of vegetation (drip tip leaves in rainforests are actually designed to allow this to happen).
Throughfall: Precipitation that falls directly through vegetation.
Infiltration: Water that moves from the surface of the earth into the soil below.
Throughflow: Water that travels through unsaturated ground.
Pipeflow: Water that travels through holes left by root systems and animals burrows.
Percolation: Water that travels from unsaturated into saturated ground.
Groundwater flow (baseflow): Water that travels through saturated ground.
Capillary action (or rise): Water that may move upwards towards the surface.
Channel flow: Water that travels in a river.
Surface run-off (overland flow): When water travels across the surface of the earth e.g. down a hill.
Storage
Water is stored in a drainage basin on the surface in lakes and channels or underground in the groundwater store.
Water reaches the groundwater store via the processes of infiltration and percolation.
During these processes, some water will be stored in the soil and rock.
The amount of water stored will vary depending on the porosity of the soil and on the permeability of the rock.
Water can also be temporarily stored via interception.
This refers to the storage of water on leaf and plant stems.
Dense foliage may result in little water reaching the ground, since it often evaporates from the leaves.
Interception: When water is caught and held by vegetation or man-made structures like buildings.
Surface store: When water is held in the surface of the earth.
This may be a puddle, a lake or a garden pond.
Soil moisture store: When water is held in unsaturated soil.
Groundwater store: When water is held in saturated ground.
Outputs
The final release of the water in a drainage basin is known as its output.
Typically, rivers flowing into the sea will be the main output of a drainage basin. Some water will also be lost via evapotranspiration.
This process refers to direct evaporation, and also to the extend that moisture lost from leaves will result in plants withdrawing water from the soil via their roots.
Evaporation: The process of water turning from a liquid into a vapour. Evaporation only takes place from a body of water e.g. a lake, puddle or the sea.
Transpiration: The evaporation of water from vegetation.
Evapotranspiration: The combined action of evaporation and transpiration
Inter-basin transfer: Water that either naturally (due to the alignment of the rock) or with human involvement (pumps and pipes) moves from one drainage basin to another.
River discharge via channel flow: Water entering the sea and leaving a drainage basin.
A very small amount of water also enters the sea via throughflow and groundwater flow (baseflow).
Split into upper course, middle course, and lower course
As river flows, it is shaped by erosion, transportation, and deposition
Erosion
River erosion is the wearing away of the land as the water flows past the bed and banks.
There are four main types of river erosion:
Attrition: occurs as rocks bang against each other gradually breaking each other down (rocks become smaller and less angular as attrition occurs)
Abrasion: this is the scraping away of the bed and banks by material transported by the river
Solution: chemicals in the river dissolve minerals in the rocks in the bed and bank, carrying them away in solution.
Hydraulic Action: this is where the water in the river compresses air in cracks in the bed and banks.
This results in increased pressure caused by the compression of air, mini 'explosions' are caused as the pressure is then released gradually forcing apart parts of the bed and banks.
Transport
Material may be transported by a river in four main ways: solution; suspension; saltation and traction.
The type of transport taking place depends on the size of the sediment and the amount of energy that is available to undertake the transport.
The chemical composition of the parent rock from which sediments originate.
In the upper course of the river there is more traction and saltation going on due to the large size of the bed-load, as a river enters its middle and lower course there is a lot of finer material eroded from further upstream which will be carried in suspension.
Deposition
Where material carried by the river is dropped and occurs when there is no longer sufficient energy to transport material.
May result in the formation of features such as slip off slopes (on the inner bends of meanders); levees (raised banks) alluvial fans; meanders; braided streams and the floodplain.
Eroded material carried in suspension and solution will be dropped last.
Discharge is the volume rate of water flow (velocity), which is transported through a given cross-sectional area. Discharge is normally measured in cumecs (cubic metres a second).
Discharge = cross section of channel (m2) x velocity of water (m/s)
Bed: The bottom of the river channel
Banks: The sides of the river channel.
Channel: The confines of the river, encompassing the bed and two banks.
Wetted Perimeter: The total length of the bed and the banks in contact with the river.
Cross-sectional area: The width of the river multiplied by the depth of the river.
Because the depth of the river will vary across its width, an average depth reading is normally taken. The cross sectional area is normally given in m2.
Velocity: This is the speed that the water in a river is travelling at. The unit of measurement is normally meters a second (m/s). River velocity can be measured using a flowmeter
Channel roughness – as large angular boulders create a rough channel shape and therefore, a large amount of its bed friction.
This creates more resistance to flow than a river with smooth clays and silt forming its banks.
The roughness coefficient is measured using Manning’s ‘n’, which shows the relationship between channel roughness and velocity.
Regimes - variations in a river flow
The regime of a river is expected to have a seasonal pattern of discharge during the year.
This is due to factors such as climate, local geology and human interaction.
Equatorial rivers have regular regimes but in the UK where seasons exist one or two peaks may be recognisable.
Simple regimes: these show times of high water levels followed by lower levels. They exist as a result of a glacier melt, Snowmelt, or seasonal rainfalls such as monsoons.
Complex regimes: if a river has more than one period of high water levels and/or low water levels, this results. It is more common on large rivers that flow through a variety of relief and receive their water supply from large tributaries, for example, the Rhine.
Two types of flow:
Laminar Flow: This rarely occurs, water flows smoothly in a straight channel. It is most common in the lower parts of a river.
Turbulent flow: This is far more common, it occurs where the shape of the rivers channel is varied with pools, meanders, and rapids. A great deal of turbulence results in sediment being disturbed. The greater the velocity the larger the quantity and size of particles that can be transported.
The Bradshaw model, developed by Dr. Anthony D. Bradshaw in the 1980s, provides a framework for understanding the relationships between various factors that influence the morphology (shape and form) of river channels.
This model is especially helpful in analyzing natural, unmodified rivers, although it can also be applied to modified systems with some adjustments.
Key Variables:
Discharge: The volume of water flowing through a channel at a given time. This is the primary driver of channel morphology, with higher discharge leading to wider, deeper channels.
Sediment supply: The amount and size of sediment (sand, gravel, etc.) entering the channel. High sediment supply can lead to braided channels, while low supply can result in meandering channels.
Channel slope: The steepness of the channel bed. Steeper slopes promote faster flow and deeper channels, while flatter slopes encourage meandering and deposition.
Bank material: The strength and cohesion of the material forming the riverbanks. Stronger banks are more resistant to erosion, allowing for steeper channel slopes and narrower channels.
Vegetation: The type and extent of vegetation along the banks and within the channel. Vegetation can stabilize banks, reduce erosion, and influence sediment transport.
Relationships and Interactions:
The Bradshaw model emphasizes that these variables are not independent but interact and influence each other.
High discharge may increase sediment transport capacity, leading to adjustments in channel width and depth.
Channel slope can influence the size of sediment transported, impacting bank erosion and channel form.
Bank material plays a role in how the channel responds to changes in discharge and sediment supply.
Vegetation can modify flow patterns, trap sediment, and influence bank stability.
The drainage basin system is said to be open as both inputs and outputs of energy and material occur.
All rivers receive a water from it.
The boundaries of the basin are known as the watershed and will usually be marked by areas of higher land.
Drainage basins have many different characteristics that influence how quickly or slowly the main river within them responds to a period of intense rainfall, these are outlined in more detail in the section relating to storm hydrographs.
Physical Factors affecting river discharge:
Impermeable Rock (e.g. granite) - Water is unable to infiltrate through, resulting in more surface runoff, increasing volume of the channel and its speed.
Permeable Rock - More infiltration, resulting in less surface runoff and less volume in the river
Size of drainage basin - Small -> Water will enter the river quicker and faster
Relief of drainage basin - If the slope of the basin is more steep, water in the river is likely to move down faster, increasing its speed
Percipitation - heavy rain can cause saturation in the soil and hence cause more water to reach the river (runoff). This also means that the speed of the river increases.
Vegetation - allows more infiltration and interception, causing less surface-runoff and slowing down the speed of the river
Human Factors Affecting River Discharge:
Impermeable man-made surfaces - Concrete and tarmac can cause rivers in urban drainage basins to have a higher discharge due to higher amounts of surface runoff. Speed is also increased due to drainage systems and ground.
Destruction of vegetation (deforestation) - Less infiltration + interception causes more surface run off and increases speed of the water.
River Management - Presence of dams allow river flow to be controlled, which may cause more discharge (before the dam) , or less (below the dam).
Base flow - the normal day to day discharge of a river
The rising limb - the rapid increase of discharge resulting from a rainfall.
Peak flow - when the river reaches the maximum capacity that it can hold.
The recession limb - when the discharge starts to decrease and river levels fall.
Basin lag time - the time difference between the peak of the rain event to the peak flow.
Factors that affect shape of Hydrograph:
Drainage basin
Type of rock (impermeable or permeable) - Impermeable rock will not allow water to seep in, thus causing larger amounts of surface runoff and a shorter lag time.
The gradient of the drainage basin - Steep gradients will cause greater overland flow and a shorter lag time.
Size of drainage basin - larger basins will take longer to reach the river, hence a longer lag time
Present conditions of the drainage basin - soil either saturated, very dry or even frozen
Shifts and Changes to Curve
Type and amount of Precipitation
Rapid rain - soil will saturate at a very rapid rate, excess water quickly transfers by surface runoff thus causing a short lag time
Land Use and Human Impact
Impermeable man-made surfaces - e.g. concrete and tarmac roads, shorter lag times
Vegetation area -infiltrates more and intercepts water, a longer lag time, reducing discharge
Area of deforestation - short lag time, increases discharge
Time/season of the year
Summer - evapotranspiration rates are higher, reducing surface run off, longer lag time
Temperatures
Water Use
Dams and reservoirs near area - slow down the rate of discharge, a much longer lag time, and may also cause a reduced amount of discharge
Located in the south east of Brazil and is the capital of Rio de Janeiro state
flood started on 11 Jan. and continued for days after floods and mudslides killed over 900 people and lost over 3000 homes which caused $1.3 billion of damage
Much of the state is on the drainage basin of the river Paraiba do Sul - total area of 57000km squared
Human Causes:
Deforestation of hillsides - reduced strength of hills by removing root system, decreases interception and transpiration, which means that soil becomes saturated more quickly (also increase in surface run-off causing landslides)
Building on marginal land - increasing rates of rural-urban migration meant more building on marginal land (includes floodplains and steep slopes unsuitable for settlement building)
No building regulations - informal settlements (favelas) on marginal land, vulnerable during times of flood; most will not have any drainage system, which increases saturation of soil and likelihood of floods
Population density - any flood is going to affect a large population
Poor transport and communication - many poor people received no warning because they had no access to media sources; rescue efforts were also made much more difficult
Physical causes:
Steep drainage basin and valley sides - mountainous areas and steep valleys mean rainfall reaches streams and rivers very quickly causing flash floods
High levels of precipitation
Tropical climate - south east of Brazil experiences over 4m of rain a year, meaning that during the summer, the ground remains largely saturated, thus decreases infiltration rates and increases surface run-off
Mudslides - secondary hazard of flood water; flood water saturated the ground, increases stress on slopes, causing mudslides
Much of Bangladesh has been formed by deposition from 3 main rivers - the Brahmaputra, the Ganges, and the Meghna
Sediment from these and over 50 other rivers form a large delta (80% of Bangladesh is located on the delta, thus under the threat from flooding and rising sea level)
Densely populated (900 people per km squared) and rapid growth (2.7% per annum)
High total rainfall and very seasonal - 75% of annual rainfall occurs in the monsoon between June and September
Ganges and Brahmaputra carry snowmelt waters from the Himalayas
Peak discharges are immense (due to snowmelt in the Himalayas combined with heavy monsoonal rain) - up to 10,000 cumecs
Types of flooding - river floods, overland run-off, flash floods, back-flooding and storm surges
Reasons for flooding:
Discharge peaks of big rivers
High runoff from the Meghalaya Hills
Heavy rainfall
High groundwater tables
Spring tides
Causes and Effects
Outside monsoon season, heavy rainfall cases extensive flooding (leads to destruction of agricultural land); however, may be advantageous to agricultural production due to new source of nutrients
Effects of flash floods due to heavy rainfall in northern India have been intensified by destruction of forest, which reduces interception, water retention and increases rate of surface run-off
Human activity exacerbated the problem - attempts to reduce flooding by building embankments and dikes have prevented the back flow of flood water into the river - leads to a ponding of water (drainage congestion) and back-flooding
Embankments have led to a increase in deposition in drainage channels and can cause large-scale deep flooding
Coastal flooding - storm surges caused by intense low-pressure systems are funneled up the Bay of Bengal
4750 people killed, 130,000 cattle killed, 660,000 hectares of crops damaged
66% of land flooded
23m made homeless
400 factories closed, 11000km of roads damaged, 1000 schools damaged or destroyed
Advantages of flooding:
Flood waters replenish groundwater reserves
Provide nutrient-rich sediment (alluvium) for agriculture in dry season
Provide fish (fish supply makes up 75% of dietary protein and over 10% of annual export earnings)
Reduce need for artificial fertilizers
Flush pollutants and pathogens away from domestic areas
Dam: a barrier constructed to hold back water and raise its level, forming a reservoir used to generate electricity or as a water supply.
Reservoir: a large natural or artificial lake used as a source of water supply.
Multipurpose scheme: a scheme or project built for more than one purpose. For example to prevent flooding as well as irrigate the land and also generate HEP
Hydrological changes resulting from the construction of dams and reservoirs:
Changes to the hydrology upstream of dams –
Increased evaporation rates because reservoirs have a larger surface area than rivers.
An increase in the amount of surface store (reservoirs are an artificial store).
A reduction in the velocity of the river upstream.
The river was effectively flowing into a stationary store of water.
Increased sedimentation can lower the depth of the river and the reservoir.
Again this will reduce velocity and may also reduce storage capacity.
Changes to the hydrology downstream of dams –
River discharge will decrease because water is being held behind the dam.
A rivers’ discharge may become more regular (less extremes) because the flow of water is regulated.
Clear water erosion may cause the bed of the river to lower.
There is no sediment (load) to be deposited to replace erosion.
The amount of load transported by the river will reduce because less sediment is reaching downstream.
The salinity of the water and the ground may increase.
The temperature of the water may reduce, as water released from reservoirs is often colder (reservoir deeper than river).
The water may also be less oxygenated than natural free flowing water.
With smaller discharge the velocity of the river may decrease, because the level of the river is further below bank-full discharge so the hydraulic radius is smaller.
The amount of depositional landforms may reduce e.g. alluvial fans, levees, deltas and slip off slopes.
Aswan Dam on the River Nile
Built on the River Nile, south of the city of Aswan in Egypt
2 dams - Aswan Low Dam and Aswan High Dam (completed in 1902 and 1970)
Advantages:
Flood and drought control - dams allow good crops in dry years, e.g. 1972 and 1973 in Egypt (reduces dependency on food imports)
Irrigation - 60% of water from the Aswan Dam is used for irrigation and up to 4000km of the desert are irrigated
HEP - accounts for 7000m kW hours each year (45% of Egypt’s energy needs)
Improved navigation upstream and downstream due to less seasonal variations downstream as the amount of water released is regulated (improved tourism on the river Nile)
Recreation and tourism (dam itself is a tourist attraction)
Amount of fishing behind the dam increased, supporting local fishing industry
Building and maintenance of the dam created many jobs and taught local workers new skills
(Estimated that the value of the Aswan High Dam to the Egyptian economy is about $500m each year)
Costs:
Water losses - dam provides less than half the amount of water expected
Salinization - crop yields have been reduced on up to ⅓ of the area irrigated by water from the dam due to salinization
Groundwater changes - seepage leads to increased groundwater levels and may cause secondary salinization
Displacement of population - up to 100,000 Nubian people have been removed from their ancestral homes
Seismic stress - earthquake of November 1981 is believed to be caused by the dam; as water levels in the dam decrease, so does seismic activity
Channel erosion (clear water erosion) beneath the channel; lowering the channel by 25mm over 18 years
Increased sedimentation may put stress on dam, reduce lake depth, storage levels and preventing the nutrients from reaching farmland downstream
Loss of nutrients - $100m worth of artificial fertilizers used annually to replace nutrients (alluvium) trapped behind the dam
Decreased fish catches - sardine yields are down 90% and 3000 jobs in Egyptian fisheries have been lost
Spread of diseases due to increased stagnant water
Erosion:
Erosion is the wearing away of something. When talking about rivers it normally means the wearing away of the bed, banks and its load. Types of erosion are:
Attrition:
This when load in a rivers flow crash into each other, causing pieces to break off.
Hydraulic Action:
This is when air and water gets trapped in cracks on a rivers beds and banks. The build up of pressure within the cracks causes bits of the bed and banks to break off and the cracks to get bigger.
Corrosion (solution):
When the slight acidity of water cause bits of load and the bed and the banks to dissolve.
Corrasion (abrasion):
When bits of load crash into the bed and banks. This process causes the load, bed and banks to wear away.
Transportation
When a river has surplus energy it may carry some of the material that it has eroded. The different types of erosion are:
Traction: Load that is rolled along the bed of the river.
Saltation: Load that is bounced along the bed of the river.
Suspension: Load that is transported in a rivers’ flow (current).
Solution: Load that is dissolved by a river and then transported by it.
Flotation: Material transported on the surface of a river.
Deposition
When the velocity of a river falls causing its energy to fall.
Because the energy of the river is falling so does its capacity and competence, causing to put down its load. This process of putting down load is deposition.
Hjulstrom Curve: A graph that shows the relationship between river velocity and particle size when looking at a rivers’ ability to erode. transport and deposit.
The Hjulström Curve is a graph used to determine whether a river will erode, transport, or deposit sediment depending upon the flow velocity.
The x-axis shows the size of the particles in mm.
The y-axis shows the velocity of the river in cm/s.
Competence: The maximum diameter of a piece of load that a river can transport.
Capacity: The maximum amount of load that a river can transport.
Critical Erosion Velocity: The minimum velocity that a river needs to be traveling for it to start eroding and then transporting material.
Settling (or fall) Velocity: The velocity that a river needs to fall below to start depositing its load.
What apparent anomaly with the Hjulstrom curve is that it can erode sand at a much lower velocity than it can erode clay and silt.
This is because that clay and silt are very cohesive (they stick together).
This means that even though the particles sizes are small they have a very strong bond between them.
Upper Course
The upper course is nearest the source.
This is where load is biggest and most erosion is vertical.
Most landforms are made by erosion and include; waterfalls, gorges, rapids, v-shaped valleys and interlocking spurs.
Alluvial River: any river that carries load.
Nearly all rivers (except some rivers flowing over ice shelves and glaciers) carry load.
Fluvial: Anything found on or made by a river. This includes all landforms.
Characteristics:
Lowest volume of water
A narrow channel with a steep gradient;
The river erodes downwards.
This vertical erosion results in a number of distinctive landforms including:
V shaped valley cross section
narrow valley floor
interlocking spurs
river's load is of various sizes and angular.
V-Shaped Valley Formation:
Vertical erosion in the river channel
Weathering of the sides of the valley sides
Mass movement of materials down the valley sides,
Material is gradually transported away by the river.
As the river flows through the valley it is forced to swing from side to side around more resistant rock outcrops (spurs).
As there is little energy for lateral erosion, between spurs of higher land creating interlocking spurs
Middle Course and Lower Course:
The middle course when the river leaves the mountains and enters are more hilly environment.
The valley floors starts to widen as you get more horizontal erosion.
The landforms found in the middle course include alluvial fans and meanders.
The lower course is closest to the mouth.
Here the river is travelling over much flatter land and the load is much smaller and smoother.
This is more horizontal erosion here as the river nears its base level.
The landforms found in the lower course include meanders, oxbow lakes, braided rivers, levees and deltas
Meander:
A meander is when water flows in a curvy, bendy path, like a snake.
As a river makes its way through an area that is relatively flat, it often develops bends as it erodes its way through the path of least resistance.
Forms as a watercourse erodes the sediments of an outer, concave bank and deposits sediments on an inner, convex bank (point bar), leading to a meandering channel
Oxbow Lake:
An oxbow lake is a meander that has become cut off from the main river channel.
If you have the outside of two meanders near each other they will eventually connect.
They connect because erosion is at its maximum on the outside of the meander.
When they eventually connect the thalweg (fastest flow) will no longer go around the old meander, but actually go in a straight line.
This means that the outside of the river channel now has a slower flow so deposition takes place cutting off the old meander.
Braided River:
A braided river is a river with a number of smaller channels, separated by small and often temporary islands called eyots.
Braided rivers usually form on rivers with the variable flow (wet and dry season or snow melt season) and high quantities of load.
When a river is at maximum discharge it is able to transport most of its load.
However, when the discharge falls along with the velocity an energy of the river, deposition starts to take place, creating eyots.
Delta:
Form when a river tearing sediment reaches a body of water
Deltas are found at the mouth of a river, where the river meets the sea.
At this point the river is carrying too much load for its velocity and so deposition occurs.
The top of the delta is a fairly flat surface.
This is where the coarsest river load is dropped.
The finer particles are carried into deeper water.
The silt is dropped to form a steep slope on the edge of the delta while the clay stays in suspension until it reaches the deeper water.
Levees:
Levees are embankments found on the sides of a river channel. Levees can be made by or enlarged by humans, but we are only interested in levees that are made naturally.
Levees are made when a river exceeds bankfull discharge i.e. it is in flood.
Floodplain:
The floor of the valley floor that gets flooded when a river exceeds bankfull discharge.
Floodplains tend to be much wider in a rivers’ lower course where horizontal erosion has had a greater effect.
Bluff line:
The outer limits of the floodplain.
The bluff line is basically the edge of the valley floor.
Strand line:
A line of load (usually sticks and litter) that is deposited at the limit of a flood.
Alluvial deposits (alluvium):
Load that is deposited by a river in time of flood.
Floodplains and leveés are formed by deposition in times of river flood.
The river’s load is composed of different sized particles.
When a river floods it deposits the heaviest of these particles first. The larger particles, often pebble-sized, form the leveés.
The sands, silts and clays are similarly sorted with the sands being deposited next, then the silts and finally the lightest clays.
Every time the river floods deposition builds up the floodplain.
Meanders & Oxbow Lakes
deposition and erosion
Floodplains, Levees & Deltas
deposition
The river is now flowing over flatter land and so the dominant direction of erosion is lateral (from side to side).
The river has a greater discharge and so has more energy to transport material. Material that is transported by a river is called its load.
Deposition is also an important process and occurs when the velocity of the river decreases or if the discharge falls due to a dry spell of weather.
Materials Transported Downhill:
Traction: boulders and pebbles are rolled along the river bed at times of high discharge
Saltation: sand sied particles are bounced along the river bed by the flow of the water
Suspension: Find clay and sans particles are carried along within the water even at low discharges
Solution: some minerals dissolve in the water (Ex. Calcium carbonate). THey require little energy
Case Study: Floodplain Management
River Conwy, North Wales - Floodplain Management
Source in Snowdonia and mouth in the Irish Sea
Only 27 miles long but has regular floods
Steep gradient and sits on impermeable slates (little infiltration, high rates of surface run-off)
Weather near source is very wet, receives up to 4m of rainfall a year
During spring, the river is also fed by snowmelt
Deforestation and tidal rivers make it very prone to flash floods
Flood in 2005 damaged railways, roads, farmland, parkland, houses and businesses
Management techniques used:
River wall - a 3m concrete wall built to protect the village of Llanrwst
River training - rocks placed in river channel to slow river near village and cause deposition (redirected away from village)
Channelization - little tributaries that flow through Llanrwst have been lined with concrete; aim is to get water through the villages quicker by reducing friction
Embankments levees - raised banks built along river sections to increase river’s cross-sectional area and reduce flood risk
Raised buildings and pathways - built on stilts so they don’t get damaged if river bursts its banks
Controlled flooding - low value farmland allowed to flood to protect high value settlements
Flood proofing houses - designed with no carpets and removable furniture on lower floors
Urbanization: Urbanisation tends to cause deforestation reducing interception and transpiration. Sewers also reduce surface stores and therefore evaporation. Urban areas usually create large impermeable surfaces which can lead to greater surface run-off.
Sewer Systems: Generally sewer systems create artificial channels, which often reduces a rivers’ lag time and can lead to increased flooding downstream.
Pollution: Transport, industry and housing all create pollution which works its way into the water system. Areas that don’t have proper sewers and water treatment tend to be effected more. Metals and chemicals are particularly polluting.
Water table (groundwater depletion): Unsustainable use of groundwater can cause subsidence.
Mexico City has experienced subsidence because of aquifer depletion underneath the city. On the scale, London has actually seen its water table rise since deindustrialisation has meant the demand for water has fallen.
Deforestation: Deforestation reduces interception and transpiration. Removal of trees can also increase the risk of mudslide by reducing slope stability and stops root uptake. Less interception speeds up the rate the ground become saturated and therefore increases the risk of flooding
Micro-climate: Urban areas create heat islands which can increase convectional rainfall. Particulates released by industry and transport also make excellent condensation nuclei.
Channelization: Artificially smoothing channels may remove river discharge from one area, but areas down stream that haven’t been smoothed are likely to experience an increase risk of flooding.
Channel Enlargement (widening/deepening): Making the width and depth of the river wider and deeper to increase its cross- sectional area.
Advantages: By enlarging the cross-sectional area you are increasing the bankfull discharge of the river along with its hydraulic radius.
This will increase the velocity of the river and reduce the chances of it flooding in the immediate area by moving the floodwater further on downstream.
Disadvantages: If buildings are built up to the river bank it might not be possible to enlarge the channel.
Also the process can be expensive and can cause problems to areas downstream who are receiving more flood water quicker, but with an un- enlarged channel.
Channel Straightening: Removing meanders from a river to make the river straighter.
Advantages: By removing meanders the velocity of the water through a settlement will increase.
This will stop a backlog of water and should reduce the risk of flooding. It also improves navigation.
Disadvantages: By changing the course of the river, you might remove flowing water from industries that depend on it.
There might also be building that have to be demolished to allow straightening.
Again it is expensive and may cause flooding problems downstream.
Flood Relief Channels: Building new artificial channels that are used when a river nears bankfull discharge.
Advantages: They take the pressure off the main channels when floods are likely therefore reduce flood risk.
Disadvantages: It can be hard find land to build relief channels, they are expensive and when empty can become areas to dump rubbish, etc.
If river levels rise significantly it is also possible for relief channels to flood as well
Artificial Stores: Creating reservoirs or lakes that can store excess water in times of flood.
Advantages: They can remove pressure of the main channel and can become new habitats and serve other purposes e.g. leisure, drinking water.
Disadvantages: Building dams, sluices, diversion channels are all expensive.
They also involve flooding areas of land which may be hard to find near large vulnerable urban populations
Flood Embankments (levees): Like levees these increase the channel depth of a river, raising its bankfull discharge and reducing the risk of flood.
Advantages: They increase the cross-sectional area of the river and therefore its hydraulic radius.
This should reduce the risk of flooding.
Disadvantages: Like in New Orleans under extreme conditions, embankments may fail causing even bigger problems.
They are expensive to build and again may cause problems downstream.
Controlled Flooding: Allowing low value land e.g. farmland to flood, therefore protecting higher value areas.
Advantages: By allowing the river to flood naturally you are taking the pressure of high value areas, you are letting the river behave more naturally and it adds alluvium to the floodplain.
Disadvantages: You have to make the decision what is worth protecting which is always going to upset someone.
You also have to protect areas that you don't want to flood which costs money (cost benefit analysis)
Afforestation / Reforestation: Simply planting more trees in a drainage basin.
Advantages: This is a natural process, increasing the amount of interception, transpiration and root uptake.
People would not normally protest against trees being planted.
Disadvantages: It is not possible to cover the whole drainage basin in trees, so if it rains in an area with no trees, then there is no reduction in flooding.
Also, most trees lose there leaves in autumn and winter reducing interception in those months.
Flood Proofing: This is making property less vulnerable to flooding or flood damage. This might be temporary like using sandbags or design by removing carpets downstairs.
Advantages: This can be done on an individual level and can be relatively cheap.
Temporary protection can be removed under normal circumstances so it does not change the aesthetics of properties.
Disadvantages: Temporary defences can usually only protect against minor floods.
Not everyone will be happy with having to redesign their houses.
Insurance: Although it doesn't prevent flooding, it can help individuals and industries to recover and protect against future flooding.
Advantages: It helps individuals and settlements to recover after flood events and may help them protect property and be less vulnerable in the future.
Disadvantages: They do not actually prevent flooding.
Not everyone can afford insurance and insurance companies may not insure high risk areas.
Land Use Planning (zoning): Mapping areas by looking at there likelihood to flood and then only building low value uses on areas with high flood risk.
Advantages: Very good at removing high value areas and high density populations from hazardous areas.
Disadvantages: It is not always possible to change land uses that already exist in an area.
You have to decide what size flood to map for e.g. a once in ten year flood or once in one hundred year flood.
Often poor will still choose to live on marginal land.
Contour Ploughing and Strip Cultivation: Either creating temporary surface stores or leaving vegetation to increase interception and transpiration
Advantages: Contour ploughing is simply a cheap and easy change in existing farming methods.
Keeping vegetation is natural and relatively cheap.
Disadvantages: Won't protect against big floods and farmers may not be happy giving up farmland, simply to grow trees.
Interception Channels: These are channels that divert a rivers' discharge around settlements.
The old channel remains but with a smaller discharge.
Advantages: They remove pressure of the main river and areas of high land value.
They may also develop into new habitats for plants and animals.
Disadvantages: They are expensive, may flood themselves in times of heavy floods and may restrict future urban
Settlement Removal: Moving settlements from high risk flood areas to less vulnerable locations often on higher land.
Advantages: Is probably the most effective because you remove high value property and humans from vulnerable areas.
Disadvantages: It is usually not practical to move whole settlements, because of the cost and the problems of finding alternative locations.
Also many settlements depend on water for their survival.
Dams: Often built as part of a multipurpose scheme, they create artificial stores which can hold water in times of increased precipitation.
Advantages: They can store large amounts of water and can be used for other purposes.
Disadvantages: If rain is downstream of the dam then they have no effect.
In large flood events they are vulnerable to breaking and are expensive to build.
Wing Dykes: Barriers placed out into a river, these can be used to divert the cause of rivers by shifting the thalweg of rivers.
This may move the channel away from high value areas.
Advantages: They can move the main channel from vulnerable areas to protect high value areas.
Disadvantages: They are expensive to build and during big flood events the flood water may go over the wing dykes.
Also if there is property on both sides of a river, which side do you protect.
Electronically Controlled Sewers: Advanced sewers which can control the flow of rain water tostop increased discharge into rivers and therefore flooding.
Advantages: They can be very effective at controlling smaller floods.
They are underground so do not cause any visual pollution.
Disadvantages: This involves a complete redesign of sewers.
Sewers usually have to be increased in size and electronic sluices have to be added.
They also have to be operated from a central command centre and with all electronically operated equipment can break.
Also they might not be able to cope with large scale floods, so water has to be released into rivers anyway.
Channelization: The concreting of beds and banks.
Advantages: Reduces friction and increases velocity of river, removing water from the channelised area quicker.
Bank erosion is also reduced.
Disadvantages: It is expensive and is not natural so vegetation and animal life will find it harder to grow and live.
Flooding maybe caused downstream of the channelised area.
Dredging: The removal of material from the bed of the river deepening it.
Advantages: Channel cross-section is increased so the river can hold greater discharge.
It can look more natural because no structures are built.
Disadvantages: Deposition can mean that dredging needs to happen regularly.
River bank conservation: Protecting the banks and sides of the river to reduce erosion.
This can be done through planting vegetation.
Advantages: It looks natural, promoted wildlife and is relatively cheap compared to hard-engineering.
Disadvantages: During large flash floods vegetation can be easily removed.
River restoration: Returning a river to its natural state before it had been managed. This might involve removing channelization.
Advantages: This looks natural, is attractive and can attract wildlife. Can allow the floodplain to become more fertile.
Disadvantages: Can't protect against big floods and may have to coincide with zoning
Key Terms
Artesian basin: An artesian basin or aquifer is a confined aquifer containing groundwater under positive pressure.
This causes the water level in the well to rise to a point where hydrostatic equilibrium has been reached (balance between pressure on the aquifer and pressure from the aquifer).
Aquifer: Rocks that can hold water.
Saturated: When all pore space is full and rocks or soil can hold no more water.
Groundwater: Water held under the surface of the earth.
Depletion: When something is reducing, aquifers can become depleted in dry periods or when they are managed unsustainably.
Aquiclude: Rock that will not hold water or allow its movement. i.e. they are non-porous and impermeable
Causes of Groundwater Usage:
Evapotranspiration from shallow stores, capillary action will draw moisture up to near the surface
Natural discharge by springs and into lakes, rivers and oceans
Artificial abstraction (removal) for domestic, industrial and agricultural use
Leakage into nearby aquifers
Interbasin transfers
Causes of Groundwater Recharge:
Artificial recharge. Either leakage from irrigation channels and reservoirs or the pumping of water into aquifers.
Infiltration and percolation after precipitation or snow melt
Seepage from river channels, lakes and oceans
Leakage from nearby aquifers
Interbasin transfers
Groundwater Pollution in Bangladesh - Groundwater Management
Increase in incidence of cancers in Bangladesh
Caused by naturally occurring arsenic in groundwater pumped up through tube wells
As many as 85m of the country’s 125m population will be affected by arsenic-contaminated drinking water
UNICEF has sunk millions of tube wells in Bangladesh, providing a convenient supply of drinking water free from bacterial contamination of surface water
But the water from wells was never tested for arsenic contamination, which occurs naturally in the groundwater
1 in 10 who diners water containing arsenic will ultimately die of lung, bladder or skin cancer
Arsenic poisoning is a slow disease - skin cancer occurs 20 years after people start ingesting the poison, then internal cancers
One solution is a concrete butt, collecting water by pipe from gutters
Another is a filter system
Wetland is an area of land where soil is saturated with moisture either permanently or seasonally.
Such areas may also be covered partially or completely by shallow pools of water. Wetlands include swamps, marshes and bogs.
The water found in wetlands can be saltwater, freshwater, or brackish (a mixture of fresh and salt water).
The world’s largest wetland is the Pantanal which straddles Brazil, Bolivia and Paraguay in South America.
Brackish water: Water that has a higher salinity content than freshwater, but not as high as saltwater.
Importance of Wetlands
Flood control:
Many wetlands are covered in vegetation which can intercept precipitation, absorb rainwater and transpire water.
Wetland vegetation can also reduce the velocity of rivers flowing into them or from them and act as natural stores of water.
If you remove or drain areas of wetland more pressure is placed upon the main river channel.
Coastal and marine wetland areas can also absorb the energy of tropical storms, tsunamis etc.
Groundwater recharge:
Wetlands can collect large areas of precipitation and river discharge.
As this water is held in storage it will infiltrate and percolate into the ground to recharge groundwater.
Transport Network:
Wetland provide many natural waterways that people can move around on easily.
Tourism and Leisure:
Some wetlands, like the everglades in Florida or the fens in East England become tourist attractions.
They also become popular locations to bird watch, fish and hunt.
Flora and Fauna:
Many wetlands are unique habitats that support indigenous aquatic plants and animals.
Many wetlands support rare reptilian and amphibian species.
Many migratory birds also rest in wetlands flying to and from nesting and breeding grounds.
Fisheries:
Wetlands can support large numbers of fish which can support local populations.
Wetlands are not normally viable commercial fisheries.
Water purification:
The soils, geology and vegetation of wetlands can help clean and purify water.
Storage of organic matter:
Wetlands support large areas of organic matter that can hold large stores of methane (greenhouse gas).
Coastal stabalisation:
Wetlands that occur along the coastline and on river banks have prevent erosion from the sea or by rivers.
Factors Causing Loss and Degradation of Wetlands
Increased demand for agricultural land:
As the world population grows there is an increasing demand for food.
With the amount of viable agricultural land decreasing, increasingly areas of wetland are being artificially drained to make ways for agricultural land e.g. the draining of the fens in East England.
Population growth:
As the world’s population grows, it demands more water, more food and more land.
The increasing demand for water can mean wetlands are drained of their water or their source of water.
This problems is made worse as the world’s population develops and uses more water e.g. showers and toilets.
Urbanization:
With the world population growing, there is a greater demand for housing. Increasingly this demand for housing is in urban areas.
With urban areas growing more and more wetland areas are being drained or inhabited.
Urbanisation on or near wetlands can cause pollution, changes in river flow and river channels and disturbance of wildlife.
Land reclamation is the process of reclaiming land from the water.
Sea level rises:
Global warming is causing glaciers and ice sheets to melt causing sea levels to rise.
These rising sea levels can flood coastal and marine wetland areas.
Even if the whole wetland is not flooded, water conditions can be changed from fresh to brackish.
River flow changes:
Many rivers have been channelised and straightened, reducing the amount of wetlands.
Others have been drained or dams have altered flow.
Some have been polluted or redirected.
All these natural changes are removing or changing the ecosystems of many wetland areas.
Pollution:
Any form of pollution, but particular chemicals and metals can change the delicate ecosystems of wetlands.
Process like eutrophication, caused by fertiliser run-off can completely kill whole wetland areas by preventing the wetland oxygenating properly and receiving sunlight.
Infrastructure projects:
As populations grow and we become more mobile, there is an increasing demand for new roads, airports, railways. etc.
Unfortunately wetlands are often drained or disrupted (bridges, dykes and causeways) to make way for these projects.
Alien species invasion:
Many alien species like the cane toad in Australia or the American mink in the UK have been introduced to wetlands and devastated indigenous species.
The introduction of any alien, however small can disrupt food webs and ecosystems.
Tropical storms:
Although wetlands can be a natural defence against tsunamis and tropical storms, they can also been damaged by them.
Freshwater wetlands in particular can be flooded by storms surges associated with tropical storms, changing the salinity of water and damaging vegetation.
Kissimmee River - Wetland Management
In south central Florida, drainage basin of 7800km squared and approximately 200km long was home to wetland plants, fish and wading birds.
However, the 5km wide floodplain with populated settlements nearby were regularly flooded.
Thus, the river was channelized and transformed into a 90km, 10m deep drainage canal - to provide an outlet canal for draining floodwaters from upper Kissimmee lake basin and to provide flood protection for land adjacent to the river
Impacts of channelization:
Loss of 12000-14000 hectares of wetlands
Floodplain dried up after channelization - no longer exceeds bankfull discharge
Egret, heron, and wood stork populations decreased by ⅔
Catches of largemouth bass decreased
Fishing, bird watching and hunting tourism declined
Concerns about the sustainability of existing ecosystems led to the Kissimmee River Restoration Project (large scale, supported by the state and federal)
Aim: restore over 100km squared of river and wetland floodplain by 2015
Started in 1999
River is being de-channelized by refilling the flood canal and re-establishing the old natural course of the river
Restored sections now flood naturally - restored floodplains could benefit avian species e.g. wading birds and waterfowl, by providing increased feeding and breeding habitats
Dissolved oxygen levels have doubled in restored sections
Increase in revenue from tourism potential could significantly enhance local and regional economies
Possible negative impacts from restoration:
Greater evaporation due to more surface stores
Increase risk of flooding
River will be less navigable in dry periods
Restoration will cost $578m
Agriculture:
Agriculture the artificial cultivation (growing or rearing) of plants or animals.
Agriculture that grows crops is known as arable agriculture, agriculture that involves rearing animals is known as pastoral agriculture.
Irrigation:
This means artificially watering the land.
There are three main types of irrigation; gravity flow, sprinklers and drip systems.
Eutrophication:
This is the processing of artificially adding nitrates and phosphates (through fertilsers and sewage) to wetland areas e.g. rivers and lakes.
The added nitrates and phosphates causing excessive growth of algaes.
The algae growth can reduce the oxygen content of the water as well as reducing the amount of sunlight that it receives.
The nitrates and phosphates often come from agro-chemical run-off, but can also come from domestic sewage and industrial waste.
Salinisation:
This is the increase in the salt content of water.
Salinisation can happen because of evaporation or unsustainable water extraction.
If the water become to salinated it becomes less fertile.
Growing Demand for Agricultural Products:
The world’s population is growing.
The current population is about 7 billion, but it is expected to peak at nearer to 9 billion.
Because fossil fuels are finite, alternative forms of energy are being looked at.
One form of renewable energy being used are biofuels.
Biofuels are made out of biological matter and therefore are increasing the demand for agricultural products.
Economic development.
As more of the world’s population is removed from poverty, their calorific intake increases.
This increase in food consumption, is increasing the demand for agricultural products.
Pastoral farming.
As the world population increase, the demand for meat also increases.
Most farm animals are omnivores or herbivores so need agricultural products like corn to eat.
Decreasing Supply of Agricultural Products or Land
Urbanisation.
As the world develops, urbanisation increases tends to happen increasing the size of urban areas.
As urban areas grow they eat into greenfield sites in rural areas, reducing the amount of agricultural land.
Land degradation and desertification.
Land that is overcultivated or overgrazed can become degraded (less fertile).
As farmers try to react to demand by growing more intensively, more land is being degraded.
In extreme circumstances, the land may turn to desert (desertification).
Rising sea levels.
Some of the earth’s most fertile agricultural areas are floodplains and deltas.
As world sea levels (eustatic changes) increase much of this fertile land is lost.
Conversion to biofuels.
Although not strictly reducing the amount of agricultural products (biofuels are agricultural products), this does decrease the supply of agricultural products available for human consumption.
Biofuels are often favoured by farmers, because they demand a higher price.
Hazards.
Natural hazards like tropical storms, volcanoes and tsunamis can reduce the amount of agricultural land available for cultivation.
Disease.
There is an increasing amount of intensive monoculture (growing of one crop) taking place.
Monoculture always runs the risk of been impacted by the outbreak of diseases or pests that attack the particular crop e.g. wheat leaf rust fungus.
Case Study: Local Scale Competition for Demand for Water
Israel’s Aquifers - Demand for Water: Local/national Scale
Water is one of the most sensitive and unsolvable problems in the Middle East
Created friction between the Arabs and Jews (Israeli-Palestinian tensions)
For decades, Israel has obtained up to 80% of the 670m cubed of water provided by mountain aquifer mostly located under the West Bank
Israelis have occupied the West Bank since 1967 and have prevented the Palestinians from obtaining better access to the resource
Mountain aquifer is important for Israel as it provides:
⅓ of its water consumption
4% of its drinking water
50% of its agricultural water
120,000 Jewish settlers in the West Bank use 60m cubed annually compared to 137million m cubed used by 1.5m Arabs
The WB and Gaza are served by Israel’s water carrier and groundwater in aquifers
WB’s aquifers, replenished by rainfall, flow west, north, and east from the watershed
Palestinians were forbidden to dig new wells or deepen old ones (Israel claims that they have the right to use the aquifer because some of the water flows into its territory) - thus kept very short for their crops
The Gazans, like West Bankers, get little domestic water from Israel’s national carrier - most of their supplies come from an aquifer that has been exploited - Gazans pump twice as much as can be safely withdrawn, leading to salt water intrusion (kills citrus trees)
Gaza Strip is part of the Palestinian territories - coastal aquifers becoming exhausted and at threat of salt water intrusion and domestics and industrial pollution
Case Study: International Scale
The Mekong - Demand for Water: International Scale
South-east Asia’s largest river flat, well-watered and fertile land in the basin lies around Tonle Sap Lake, but annual flood makes intensive agriculture difficult there surface area of lake can increase up to ten times during the monsoon remained untouched until 1990s
First dam non the river, at Man Wan, in China was completed in 1993
Population growth and economic growth - place strain on the Mekong
HEP potential of the Mekong and its tributaries is considerable - so far, only 5% of the lower basin’s HEP have been developed
Dams generate electricity, aid irrigation and regulate flooding
However, caused damage to fisheries - annual harvest amounts to 2m tonnes
Case Study: International Scale
The River Nile - Conflict at International Scale
Importance of the River Nile:
Tourism - rapids in Uganda
agriculture and irrigation - Egypt depends on the Aswan Dam to irrigate the desert
Transport - promote trade
Wildlife
Drinking water
HEP - drought control
Longest river in the world, 6650km long
2 main tributaries - White Nile and Blue Nile
Confluence is in the Sudanese capital, Khartoum
Source of WN: Burundi
Source of BN: Ethiopia
WN, BN and the Nile flow through 11 countries
Conflicts:
Have arisen since Ethiopia began dam building
In 2010, 6 of 9 upstream countries signed a Cooperative Framework Agreement seeking more water shares from the Nile
Sudan and Egypt rejected agreement because it challenged their historic water allocations
A major dam on the BN, called the Grand Renaissance Dam is under construction by Ethiopians
Egypt, draws much of its drinking water, natural resources, and energy from the Nile, has protested the dam’s construction (will siphon resources away)
Dispute between Egypt and Sudan over the dam construction has reignited a 60-year old dispute (Sudan, downriver of the Nile, has supported Ethiopia’s attempts to build the dam)
Ethiopia denies that the dam would damage Egypt’s water supplies
Egypt now hope to pull the Europeans to its side ad to pressure Ethiopia before protesting before the security council
Egypt has threatened to defend its historical claims over the Nile in numerous occasions - it will even use air power against other countries to protect its flow of the Nile
Precipitation: the transfer of moisture (rain, snow etc) to the earth's surface from the atmosphere.
Interception: the capture of raindrops by plant cover, which prevents direct contact with the soil.
Runoff: precipitation that does not soak into the ground but flows over it into surface waters.
Groundwater: water held underground in soil or porous rock, often feeding springs and wells.
Evapotranspiration (EVT): the loss of water from vegetation and water surfaces to the atmosphere.
Potential evapotranspiration (pEVT): the rate of water loss from an area if there were no shortages of water
Maximum sustainable yields (MSY): the maximum level of extraction of water that can be maintained indefinitely for a region.
The hydrological cycle, also known as the water cycle, is the continuous movement of water on, above, and below the surface of the Earth.
It's a complex system with various interconnected processes responsible for distributing freshwater globally.
The water cycle involves the exchange of energy, which leads to temperature changes.
For instance, when water evaporates, it takes up energy from its surroundings and cools the environment.
When it condenses, it releases energy and warms the environment.
These heat exchanges influence climate.
The evaporative phase of the cycle purifies water which then replenishes the land with freshwater.
The flow of liquid water and ice transports minerals across the globe.
It is also involved in reshaping the geological features of the Earth, through processes including erosion and sedimentation.
The hydrological cycle is a closed system because water can not be added or lost.
Although water can not be added or lost it can be found in different states and in different locations.
Despite the planet being covered in water, the vast majority is sea water (97.5%). Of the remaining 2.5% the majority is held in glaciers and ice sheets.
Only a very small amount of the world’s water is easily accessible in rivers and lakes (0.00069%)
Closed System:
In a closed system, there is no exchange of matter with the surroundings, only energy.
Hydrological Cycle as a Closed System:
When considering only the movement of water molecules within the Earth's system, it can be simplified as a closed system.
The total amount of water on Earth remains constant, although it continuously changes states and locations through the various processes of the cycle.
Open System:
In an open system, both matter and energy can be exchanged with the surroundings.
Hydrological Cycle as an Open System:
When considering the energy transfers involved in the water cycle, it's clearly an open system.
Solar energy from the sun drives the entire cycle, causing water to evaporate and eventually return as precipitation.
Additionally, a small amount of water vapor escapes Earth's atmosphere, entering the open space, making it a truly open system in that sense.
Precipitation:
This is the initial stage where water vapor in the atmosphere condenses and falls back to Earth as rain, snow, sleet, or hail.
Factors like temperature, pressure, and wind currents influence precipitation patterns, leading to uneven distribution across the globe.
Types of precipitation:
Convectional precipitation: Occurs due to rising warm air, common in tropical regions.
Orographic precipitation: Forced uplift of air over mountains, leading to rain on windward slopes.
Frontal precipitation: Warm and cold air fronts colliding, resulting in widespread precipitation.
Interception:
Before reaching the ground, precipitation encounters various surfaces like leaves, branches, and vegetation.
Some water gets trapped or evaporates directly back into the atmosphere.
Factors affecting interception:
Type and density of vegetation
Leaf surface area
Rainfall intensity and duration
Infiltration:
Water that doesn't evaporate penetrates the ground surface, entering the soil zone. Soil texture, porosity, and moisture content determine the infiltration rate.
Infiltration pathways:
Macropores: Large channels allowing rapid infiltration (e.g., cracks, wormholes)
Micropores: Smaller spaces facilitating slower infiltration
Infiltration impacts:
Replenishes soil moisture crucial for plant growth
Contributes to groundwater recharge
Runoff:
Water that doesn't infiltrate either evaporates or flows over the land surface as runoff.
Types of runoff:
Overland flow: Water sheet flow directly over the land
Channel flow: Water concentrated in streams and rivers
Subsurface flow: Water infiltrating shallow soil layers and moving laterally towards waterways
Runoff factors:
Rainfall intensity and duration
Slope and topography
Soil permeability and saturation
Land cover (e.g., vegetation, urbanization)
Evapotranspiration:
This combined process involves the evaporation of water directly from soil and water bodies and the transpiration of water vapor from plants through their leaves.
Types of evapotranspiration:
Evaporation: Direct change of liquid water to vapor from surfaces
Transpiration: Release of water vapor from plant stomata during photosynthesis
Evapotranspiration impacts:
Returns water vapor to the atmosphere for further precipitation
Regulates surface temperature and humidity
Influences plant growth and ecosystem health
Groundwater Recharge:
A portion of infiltrated water percolates deeper into the soil, eventually reaching the saturated zone below, replenishing groundwater aquifers.
Factors affecting recharge:
Soil permeability and depth
Groundwater table depth
Rainfall patterns and intensity
The water balance basically looks at the balance between inputs and outputs.
You can look at the water balance at a global level (hydrological cycle), at a local level (drainage basin cycle) or even just a field.
At a global level oceans tend to experience greater outputs (evaporation) than inputs (precipitation).
This is because oceans are large areas with no shade that have regular winds blowing saturated air on land, allowing greater evaporation.
In addition oceans don’t tend to suffer from the same amount of relief and convectional rainfall as land does.
On land, inputs (precipitation) tends to be greater than outputs (evaporation).
This is because lands suffers from larger amounts of frontal, relief and convectional rainfall, as well as much of the lands water being protected underground or in shaded areas reducing evaporation.
At a global level there obviously has to be an equilibrium between inputs and outputs.
The excess precipitation on land is returned to the oceans by channel flow, surface run-off and to a lesser extent groundwater flow.
The excess of evaporation is returned to the land from the sea by winds blowing saturated air on land.
Drainage basin: the area drained by a river and its tributaries
Water balance: the relationship between the inputs and outputs of a drainage basin
Soil moisture excess: when soil moisture and groundwater is replenished. The excess may lead to saturation and increased surface run-off
Drainage divide or watershed: the line defining the boundary of a river or stream drainage basin separating it from adjacent basins
Discharge: the volume of water passing a given point over a set time
Peak rainfall: The highest rainfall (usually measured in mm) during a storm.
Inputs:
The main input to the system is precipitation.
The type of precipitation (rain or snow, etc), the intensity, the duration and frequency all have an effect on the amount of water in the system.
Each subsystem of the drainage basin system will also have inputs and ouputs, and the output from one stage of the diagram will form the input for another.
Precipitation: Any moisture that falls from the atmosphere.
The main types of precipitation are rain, snow, sleet, hail, fog and dew.
Inter-basin transfer: Water that either naturally (due to the alignment of the rock) or with human involvement (pumps and pipes) moves from one drainage basin to another.
Transfer:
The sum of all the water flowing over the drainage basin’s surface is called runoff.
It is made up of streamflow, which is flow through permanent river channels and overland flow or surface runoff.
Overland flow transfers water through the basin either as sheetwash, across the surface, or in tiny channels called rills.
Beneath the surface, water is transferred via throughflow, which is the movement of water through the lower soil towards rivers, and groundwater flow.
Groundwater flow is typically very slow.
Water that has been intercepted by foliage may also be transferred, either directly as throughfall, or by running down branches and stems via stemflow.
Stem flow: When intercepted water runs down the trunks and stems of vegetation.
Canopy drip: When intercepted water drips off the leaves of vegetation (drip tip leaves in rainforests are actually designed to allow this to happen).
Throughfall: Precipitation that falls directly through vegetation.
Infiltration: Water that moves from the surface of the earth into the soil below.
Throughflow: Water that travels through unsaturated ground.
Pipeflow: Water that travels through holes left by root systems and animals burrows.
Percolation: Water that travels from unsaturated into saturated ground.
Groundwater flow (baseflow): Water that travels through saturated ground.
Capillary action (or rise): Water that may move upwards towards the surface.
Channel flow: Water that travels in a river.
Surface run-off (overland flow): When water travels across the surface of the earth e.g. down a hill.
Storage
Water is stored in a drainage basin on the surface in lakes and channels or underground in the groundwater store.
Water reaches the groundwater store via the processes of infiltration and percolation.
During these processes, some water will be stored in the soil and rock.
The amount of water stored will vary depending on the porosity of the soil and on the permeability of the rock.
Water can also be temporarily stored via interception.
This refers to the storage of water on leaf and plant stems.
Dense foliage may result in little water reaching the ground, since it often evaporates from the leaves.
Interception: When water is caught and held by vegetation or man-made structures like buildings.
Surface store: When water is held in the surface of the earth.
This may be a puddle, a lake or a garden pond.
Soil moisture store: When water is held in unsaturated soil.
Groundwater store: When water is held in saturated ground.
Outputs
The final release of the water in a drainage basin is known as its output.
Typically, rivers flowing into the sea will be the main output of a drainage basin. Some water will also be lost via evapotranspiration.
This process refers to direct evaporation, and also to the extend that moisture lost from leaves will result in plants withdrawing water from the soil via their roots.
Evaporation: The process of water turning from a liquid into a vapour. Evaporation only takes place from a body of water e.g. a lake, puddle or the sea.
Transpiration: The evaporation of water from vegetation.
Evapotranspiration: The combined action of evaporation and transpiration
Inter-basin transfer: Water that either naturally (due to the alignment of the rock) or with human involvement (pumps and pipes) moves from one drainage basin to another.
River discharge via channel flow: Water entering the sea and leaving a drainage basin.
A very small amount of water also enters the sea via throughflow and groundwater flow (baseflow).
Split into upper course, middle course, and lower course
As river flows, it is shaped by erosion, transportation, and deposition
Erosion
River erosion is the wearing away of the land as the water flows past the bed and banks.
There are four main types of river erosion:
Attrition: occurs as rocks bang against each other gradually breaking each other down (rocks become smaller and less angular as attrition occurs)
Abrasion: this is the scraping away of the bed and banks by material transported by the river
Solution: chemicals in the river dissolve minerals in the rocks in the bed and bank, carrying them away in solution.
Hydraulic Action: this is where the water in the river compresses air in cracks in the bed and banks.
This results in increased pressure caused by the compression of air, mini 'explosions' are caused as the pressure is then released gradually forcing apart parts of the bed and banks.
Transport
Material may be transported by a river in four main ways: solution; suspension; saltation and traction.
The type of transport taking place depends on the size of the sediment and the amount of energy that is available to undertake the transport.
The chemical composition of the parent rock from which sediments originate.
In the upper course of the river there is more traction and saltation going on due to the large size of the bed-load, as a river enters its middle and lower course there is a lot of finer material eroded from further upstream which will be carried in suspension.
Deposition
Where material carried by the river is dropped and occurs when there is no longer sufficient energy to transport material.
May result in the formation of features such as slip off slopes (on the inner bends of meanders); levees (raised banks) alluvial fans; meanders; braided streams and the floodplain.
Eroded material carried in suspension and solution will be dropped last.
Discharge is the volume rate of water flow (velocity), which is transported through a given cross-sectional area. Discharge is normally measured in cumecs (cubic metres a second).
Discharge = cross section of channel (m2) x velocity of water (m/s)
Bed: The bottom of the river channel
Banks: The sides of the river channel.
Channel: The confines of the river, encompassing the bed and two banks.
Wetted Perimeter: The total length of the bed and the banks in contact with the river.
Cross-sectional area: The width of the river multiplied by the depth of the river.
Because the depth of the river will vary across its width, an average depth reading is normally taken. The cross sectional area is normally given in m2.
Velocity: This is the speed that the water in a river is travelling at. The unit of measurement is normally meters a second (m/s). River velocity can be measured using a flowmeter
Channel roughness – as large angular boulders create a rough channel shape and therefore, a large amount of its bed friction.
This creates more resistance to flow than a river with smooth clays and silt forming its banks.
The roughness coefficient is measured using Manning’s ‘n’, which shows the relationship between channel roughness and velocity.
Regimes - variations in a river flow
The regime of a river is expected to have a seasonal pattern of discharge during the year.
This is due to factors such as climate, local geology and human interaction.
Equatorial rivers have regular regimes but in the UK where seasons exist one or two peaks may be recognisable.
Simple regimes: these show times of high water levels followed by lower levels. They exist as a result of a glacier melt, Snowmelt, or seasonal rainfalls such as monsoons.
Complex regimes: if a river has more than one period of high water levels and/or low water levels, this results. It is more common on large rivers that flow through a variety of relief and receive their water supply from large tributaries, for example, the Rhine.
Two types of flow:
Laminar Flow: This rarely occurs, water flows smoothly in a straight channel. It is most common in the lower parts of a river.
Turbulent flow: This is far more common, it occurs where the shape of the rivers channel is varied with pools, meanders, and rapids. A great deal of turbulence results in sediment being disturbed. The greater the velocity the larger the quantity and size of particles that can be transported.
The Bradshaw model, developed by Dr. Anthony D. Bradshaw in the 1980s, provides a framework for understanding the relationships between various factors that influence the morphology (shape and form) of river channels.
This model is especially helpful in analyzing natural, unmodified rivers, although it can also be applied to modified systems with some adjustments.
Key Variables:
Discharge: The volume of water flowing through a channel at a given time. This is the primary driver of channel morphology, with higher discharge leading to wider, deeper channels.
Sediment supply: The amount and size of sediment (sand, gravel, etc.) entering the channel. High sediment supply can lead to braided channels, while low supply can result in meandering channels.
Channel slope: The steepness of the channel bed. Steeper slopes promote faster flow and deeper channels, while flatter slopes encourage meandering and deposition.
Bank material: The strength and cohesion of the material forming the riverbanks. Stronger banks are more resistant to erosion, allowing for steeper channel slopes and narrower channels.
Vegetation: The type and extent of vegetation along the banks and within the channel. Vegetation can stabilize banks, reduce erosion, and influence sediment transport.
Relationships and Interactions:
The Bradshaw model emphasizes that these variables are not independent but interact and influence each other.
High discharge may increase sediment transport capacity, leading to adjustments in channel width and depth.
Channel slope can influence the size of sediment transported, impacting bank erosion and channel form.
Bank material plays a role in how the channel responds to changes in discharge and sediment supply.
Vegetation can modify flow patterns, trap sediment, and influence bank stability.
The drainage basin system is said to be open as both inputs and outputs of energy and material occur.
All rivers receive a water from it.
The boundaries of the basin are known as the watershed and will usually be marked by areas of higher land.
Drainage basins have many different characteristics that influence how quickly or slowly the main river within them responds to a period of intense rainfall, these are outlined in more detail in the section relating to storm hydrographs.
Physical Factors affecting river discharge:
Impermeable Rock (e.g. granite) - Water is unable to infiltrate through, resulting in more surface runoff, increasing volume of the channel and its speed.
Permeable Rock - More infiltration, resulting in less surface runoff and less volume in the river
Size of drainage basin - Small -> Water will enter the river quicker and faster
Relief of drainage basin - If the slope of the basin is more steep, water in the river is likely to move down faster, increasing its speed
Percipitation - heavy rain can cause saturation in the soil and hence cause more water to reach the river (runoff). This also means that the speed of the river increases.
Vegetation - allows more infiltration and interception, causing less surface-runoff and slowing down the speed of the river
Human Factors Affecting River Discharge:
Impermeable man-made surfaces - Concrete and tarmac can cause rivers in urban drainage basins to have a higher discharge due to higher amounts of surface runoff. Speed is also increased due to drainage systems and ground.
Destruction of vegetation (deforestation) - Less infiltration + interception causes more surface run off and increases speed of the water.
River Management - Presence of dams allow river flow to be controlled, which may cause more discharge (before the dam) , or less (below the dam).
Base flow - the normal day to day discharge of a river
The rising limb - the rapid increase of discharge resulting from a rainfall.
Peak flow - when the river reaches the maximum capacity that it can hold.
The recession limb - when the discharge starts to decrease and river levels fall.
Basin lag time - the time difference between the peak of the rain event to the peak flow.
Factors that affect shape of Hydrograph:
Drainage basin
Type of rock (impermeable or permeable) - Impermeable rock will not allow water to seep in, thus causing larger amounts of surface runoff and a shorter lag time.
The gradient of the drainage basin - Steep gradients will cause greater overland flow and a shorter lag time.
Size of drainage basin - larger basins will take longer to reach the river, hence a longer lag time
Present conditions of the drainage basin - soil either saturated, very dry or even frozen
Shifts and Changes to Curve
Type and amount of Precipitation
Rapid rain - soil will saturate at a very rapid rate, excess water quickly transfers by surface runoff thus causing a short lag time
Land Use and Human Impact
Impermeable man-made surfaces - e.g. concrete and tarmac roads, shorter lag times
Vegetation area -infiltrates more and intercepts water, a longer lag time, reducing discharge
Area of deforestation - short lag time, increases discharge
Time/season of the year
Summer - evapotranspiration rates are higher, reducing surface run off, longer lag time
Temperatures
Water Use
Dams and reservoirs near area - slow down the rate of discharge, a much longer lag time, and may also cause a reduced amount of discharge
Located in the south east of Brazil and is the capital of Rio de Janeiro state
flood started on 11 Jan. and continued for days after floods and mudslides killed over 900 people and lost over 3000 homes which caused $1.3 billion of damage
Much of the state is on the drainage basin of the river Paraiba do Sul - total area of 57000km squared
Human Causes:
Deforestation of hillsides - reduced strength of hills by removing root system, decreases interception and transpiration, which means that soil becomes saturated more quickly (also increase in surface run-off causing landslides)
Building on marginal land - increasing rates of rural-urban migration meant more building on marginal land (includes floodplains and steep slopes unsuitable for settlement building)
No building regulations - informal settlements (favelas) on marginal land, vulnerable during times of flood; most will not have any drainage system, which increases saturation of soil and likelihood of floods
Population density - any flood is going to affect a large population
Poor transport and communication - many poor people received no warning because they had no access to media sources; rescue efforts were also made much more difficult
Physical causes:
Steep drainage basin and valley sides - mountainous areas and steep valleys mean rainfall reaches streams and rivers very quickly causing flash floods
High levels of precipitation
Tropical climate - south east of Brazil experiences over 4m of rain a year, meaning that during the summer, the ground remains largely saturated, thus decreases infiltration rates and increases surface run-off
Mudslides - secondary hazard of flood water; flood water saturated the ground, increases stress on slopes, causing mudslides
Much of Bangladesh has been formed by deposition from 3 main rivers - the Brahmaputra, the Ganges, and the Meghna
Sediment from these and over 50 other rivers form a large delta (80% of Bangladesh is located on the delta, thus under the threat from flooding and rising sea level)
Densely populated (900 people per km squared) and rapid growth (2.7% per annum)
High total rainfall and very seasonal - 75% of annual rainfall occurs in the monsoon between June and September
Ganges and Brahmaputra carry snowmelt waters from the Himalayas
Peak discharges are immense (due to snowmelt in the Himalayas combined with heavy monsoonal rain) - up to 10,000 cumecs
Types of flooding - river floods, overland run-off, flash floods, back-flooding and storm surges
Reasons for flooding:
Discharge peaks of big rivers
High runoff from the Meghalaya Hills
Heavy rainfall
High groundwater tables
Spring tides
Causes and Effects
Outside monsoon season, heavy rainfall cases extensive flooding (leads to destruction of agricultural land); however, may be advantageous to agricultural production due to new source of nutrients
Effects of flash floods due to heavy rainfall in northern India have been intensified by destruction of forest, which reduces interception, water retention and increases rate of surface run-off
Human activity exacerbated the problem - attempts to reduce flooding by building embankments and dikes have prevented the back flow of flood water into the river - leads to a ponding of water (drainage congestion) and back-flooding
Embankments have led to a increase in deposition in drainage channels and can cause large-scale deep flooding
Coastal flooding - storm surges caused by intense low-pressure systems are funneled up the Bay of Bengal
4750 people killed, 130,000 cattle killed, 660,000 hectares of crops damaged
66% of land flooded
23m made homeless
400 factories closed, 11000km of roads damaged, 1000 schools damaged or destroyed
Advantages of flooding:
Flood waters replenish groundwater reserves
Provide nutrient-rich sediment (alluvium) for agriculture in dry season
Provide fish (fish supply makes up 75% of dietary protein and over 10% of annual export earnings)
Reduce need for artificial fertilizers
Flush pollutants and pathogens away from domestic areas
Dam: a barrier constructed to hold back water and raise its level, forming a reservoir used to generate electricity or as a water supply.
Reservoir: a large natural or artificial lake used as a source of water supply.
Multipurpose scheme: a scheme or project built for more than one purpose. For example to prevent flooding as well as irrigate the land and also generate HEP
Hydrological changes resulting from the construction of dams and reservoirs:
Changes to the hydrology upstream of dams –
Increased evaporation rates because reservoirs have a larger surface area than rivers.
An increase in the amount of surface store (reservoirs are an artificial store).
A reduction in the velocity of the river upstream.
The river was effectively flowing into a stationary store of water.
Increased sedimentation can lower the depth of the river and the reservoir.
Again this will reduce velocity and may also reduce storage capacity.
Changes to the hydrology downstream of dams –
River discharge will decrease because water is being held behind the dam.
A rivers’ discharge may become more regular (less extremes) because the flow of water is regulated.
Clear water erosion may cause the bed of the river to lower.
There is no sediment (load) to be deposited to replace erosion.
The amount of load transported by the river will reduce because less sediment is reaching downstream.
The salinity of the water and the ground may increase.
The temperature of the water may reduce, as water released from reservoirs is often colder (reservoir deeper than river).
The water may also be less oxygenated than natural free flowing water.
With smaller discharge the velocity of the river may decrease, because the level of the river is further below bank-full discharge so the hydraulic radius is smaller.
The amount of depositional landforms may reduce e.g. alluvial fans, levees, deltas and slip off slopes.
Aswan Dam on the River Nile
Built on the River Nile, south of the city of Aswan in Egypt
2 dams - Aswan Low Dam and Aswan High Dam (completed in 1902 and 1970)
Advantages:
Flood and drought control - dams allow good crops in dry years, e.g. 1972 and 1973 in Egypt (reduces dependency on food imports)
Irrigation - 60% of water from the Aswan Dam is used for irrigation and up to 4000km of the desert are irrigated
HEP - accounts for 7000m kW hours each year (45% of Egypt’s energy needs)
Improved navigation upstream and downstream due to less seasonal variations downstream as the amount of water released is regulated (improved tourism on the river Nile)
Recreation and tourism (dam itself is a tourist attraction)
Amount of fishing behind the dam increased, supporting local fishing industry
Building and maintenance of the dam created many jobs and taught local workers new skills
(Estimated that the value of the Aswan High Dam to the Egyptian economy is about $500m each year)
Costs:
Water losses - dam provides less than half the amount of water expected
Salinization - crop yields have been reduced on up to ⅓ of the area irrigated by water from the dam due to salinization
Groundwater changes - seepage leads to increased groundwater levels and may cause secondary salinization
Displacement of population - up to 100,000 Nubian people have been removed from their ancestral homes
Seismic stress - earthquake of November 1981 is believed to be caused by the dam; as water levels in the dam decrease, so does seismic activity
Channel erosion (clear water erosion) beneath the channel; lowering the channel by 25mm over 18 years
Increased sedimentation may put stress on dam, reduce lake depth, storage levels and preventing the nutrients from reaching farmland downstream
Loss of nutrients - $100m worth of artificial fertilizers used annually to replace nutrients (alluvium) trapped behind the dam
Decreased fish catches - sardine yields are down 90% and 3000 jobs in Egyptian fisheries have been lost
Spread of diseases due to increased stagnant water
Erosion:
Erosion is the wearing away of something. When talking about rivers it normally means the wearing away of the bed, banks and its load. Types of erosion are:
Attrition:
This when load in a rivers flow crash into each other, causing pieces to break off.
Hydraulic Action:
This is when air and water gets trapped in cracks on a rivers beds and banks. The build up of pressure within the cracks causes bits of the bed and banks to break off and the cracks to get bigger.
Corrosion (solution):
When the slight acidity of water cause bits of load and the bed and the banks to dissolve.
Corrasion (abrasion):
When bits of load crash into the bed and banks. This process causes the load, bed and banks to wear away.
Transportation
When a river has surplus energy it may carry some of the material that it has eroded. The different types of erosion are:
Traction: Load that is rolled along the bed of the river.
Saltation: Load that is bounced along the bed of the river.
Suspension: Load that is transported in a rivers’ flow (current).
Solution: Load that is dissolved by a river and then transported by it.
Flotation: Material transported on the surface of a river.
Deposition
When the velocity of a river falls causing its energy to fall.
Because the energy of the river is falling so does its capacity and competence, causing to put down its load. This process of putting down load is deposition.
Hjulstrom Curve: A graph that shows the relationship between river velocity and particle size when looking at a rivers’ ability to erode. transport and deposit.
The Hjulström Curve is a graph used to determine whether a river will erode, transport, or deposit sediment depending upon the flow velocity.
The x-axis shows the size of the particles in mm.
The y-axis shows the velocity of the river in cm/s.
Competence: The maximum diameter of a piece of load that a river can transport.
Capacity: The maximum amount of load that a river can transport.
Critical Erosion Velocity: The minimum velocity that a river needs to be traveling for it to start eroding and then transporting material.
Settling (or fall) Velocity: The velocity that a river needs to fall below to start depositing its load.
What apparent anomaly with the Hjulstrom curve is that it can erode sand at a much lower velocity than it can erode clay and silt.
This is because that clay and silt are very cohesive (they stick together).
This means that even though the particles sizes are small they have a very strong bond between them.
Upper Course
The upper course is nearest the source.
This is where load is biggest and most erosion is vertical.
Most landforms are made by erosion and include; waterfalls, gorges, rapids, v-shaped valleys and interlocking spurs.
Alluvial River: any river that carries load.
Nearly all rivers (except some rivers flowing over ice shelves and glaciers) carry load.
Fluvial: Anything found on or made by a river. This includes all landforms.
Characteristics:
Lowest volume of water
A narrow channel with a steep gradient;
The river erodes downwards.
This vertical erosion results in a number of distinctive landforms including:
V shaped valley cross section
narrow valley floor
interlocking spurs
river's load is of various sizes and angular.
V-Shaped Valley Formation:
Vertical erosion in the river channel
Weathering of the sides of the valley sides
Mass movement of materials down the valley sides,
Material is gradually transported away by the river.
As the river flows through the valley it is forced to swing from side to side around more resistant rock outcrops (spurs).
As there is little energy for lateral erosion, between spurs of higher land creating interlocking spurs
Middle Course and Lower Course:
The middle course when the river leaves the mountains and enters are more hilly environment.
The valley floors starts to widen as you get more horizontal erosion.
The landforms found in the middle course include alluvial fans and meanders.
The lower course is closest to the mouth.
Here the river is travelling over much flatter land and the load is much smaller and smoother.
This is more horizontal erosion here as the river nears its base level.
The landforms found in the lower course include meanders, oxbow lakes, braided rivers, levees and deltas
Meander:
A meander is when water flows in a curvy, bendy path, like a snake.
As a river makes its way through an area that is relatively flat, it often develops bends as it erodes its way through the path of least resistance.
Forms as a watercourse erodes the sediments of an outer, concave bank and deposits sediments on an inner, convex bank (point bar), leading to a meandering channel
Oxbow Lake:
An oxbow lake is a meander that has become cut off from the main river channel.
If you have the outside of two meanders near each other they will eventually connect.
They connect because erosion is at its maximum on the outside of the meander.
When they eventually connect the thalweg (fastest flow) will no longer go around the old meander, but actually go in a straight line.
This means that the outside of the river channel now has a slower flow so deposition takes place cutting off the old meander.
Braided River:
A braided river is a river with a number of smaller channels, separated by small and often temporary islands called eyots.
Braided rivers usually form on rivers with the variable flow (wet and dry season or snow melt season) and high quantities of load.
When a river is at maximum discharge it is able to transport most of its load.
However, when the discharge falls along with the velocity an energy of the river, deposition starts to take place, creating eyots.
Delta:
Form when a river tearing sediment reaches a body of water
Deltas are found at the mouth of a river, where the river meets the sea.
At this point the river is carrying too much load for its velocity and so deposition occurs.
The top of the delta is a fairly flat surface.
This is where the coarsest river load is dropped.
The finer particles are carried into deeper water.
The silt is dropped to form a steep slope on the edge of the delta while the clay stays in suspension until it reaches the deeper water.
Levees:
Levees are embankments found on the sides of a river channel. Levees can be made by or enlarged by humans, but we are only interested in levees that are made naturally.
Levees are made when a river exceeds bankfull discharge i.e. it is in flood.
Floodplain:
The floor of the valley floor that gets flooded when a river exceeds bankfull discharge.
Floodplains tend to be much wider in a rivers’ lower course where horizontal erosion has had a greater effect.
Bluff line:
The outer limits of the floodplain.
The bluff line is basically the edge of the valley floor.
Strand line:
A line of load (usually sticks and litter) that is deposited at the limit of a flood.
Alluvial deposits (alluvium):
Load that is deposited by a river in time of flood.
Floodplains and leveés are formed by deposition in times of river flood.
The river’s load is composed of different sized particles.
When a river floods it deposits the heaviest of these particles first. The larger particles, often pebble-sized, form the leveés.
The sands, silts and clays are similarly sorted with the sands being deposited next, then the silts and finally the lightest clays.
Every time the river floods deposition builds up the floodplain.
Meanders & Oxbow Lakes
deposition and erosion
Floodplains, Levees & Deltas
deposition
The river is now flowing over flatter land and so the dominant direction of erosion is lateral (from side to side).
The river has a greater discharge and so has more energy to transport material. Material that is transported by a river is called its load.
Deposition is also an important process and occurs when the velocity of the river decreases or if the discharge falls due to a dry spell of weather.
Materials Transported Downhill:
Traction: boulders and pebbles are rolled along the river bed at times of high discharge
Saltation: sand sied particles are bounced along the river bed by the flow of the water
Suspension: Find clay and sans particles are carried along within the water even at low discharges
Solution: some minerals dissolve in the water (Ex. Calcium carbonate). THey require little energy
Case Study: Floodplain Management
River Conwy, North Wales - Floodplain Management
Source in Snowdonia and mouth in the Irish Sea
Only 27 miles long but has regular floods
Steep gradient and sits on impermeable slates (little infiltration, high rates of surface run-off)
Weather near source is very wet, receives up to 4m of rainfall a year
During spring, the river is also fed by snowmelt
Deforestation and tidal rivers make it very prone to flash floods
Flood in 2005 damaged railways, roads, farmland, parkland, houses and businesses
Management techniques used:
River wall - a 3m concrete wall built to protect the village of Llanrwst
River training - rocks placed in river channel to slow river near village and cause deposition (redirected away from village)
Channelization - little tributaries that flow through Llanrwst have been lined with concrete; aim is to get water through the villages quicker by reducing friction
Embankments levees - raised banks built along river sections to increase river’s cross-sectional area and reduce flood risk
Raised buildings and pathways - built on stilts so they don’t get damaged if river bursts its banks
Controlled flooding - low value farmland allowed to flood to protect high value settlements
Flood proofing houses - designed with no carpets and removable furniture on lower floors
Urbanization: Urbanisation tends to cause deforestation reducing interception and transpiration. Sewers also reduce surface stores and therefore evaporation. Urban areas usually create large impermeable surfaces which can lead to greater surface run-off.
Sewer Systems: Generally sewer systems create artificial channels, which often reduces a rivers’ lag time and can lead to increased flooding downstream.
Pollution: Transport, industry and housing all create pollution which works its way into the water system. Areas that don’t have proper sewers and water treatment tend to be effected more. Metals and chemicals are particularly polluting.
Water table (groundwater depletion): Unsustainable use of groundwater can cause subsidence.
Mexico City has experienced subsidence because of aquifer depletion underneath the city. On the scale, London has actually seen its water table rise since deindustrialisation has meant the demand for water has fallen.
Deforestation: Deforestation reduces interception and transpiration. Removal of trees can also increase the risk of mudslide by reducing slope stability and stops root uptake. Less interception speeds up the rate the ground become saturated and therefore increases the risk of flooding
Micro-climate: Urban areas create heat islands which can increase convectional rainfall. Particulates released by industry and transport also make excellent condensation nuclei.
Channelization: Artificially smoothing channels may remove river discharge from one area, but areas down stream that haven’t been smoothed are likely to experience an increase risk of flooding.
Channel Enlargement (widening/deepening): Making the width and depth of the river wider and deeper to increase its cross- sectional area.
Advantages: By enlarging the cross-sectional area you are increasing the bankfull discharge of the river along with its hydraulic radius.
This will increase the velocity of the river and reduce the chances of it flooding in the immediate area by moving the floodwater further on downstream.
Disadvantages: If buildings are built up to the river bank it might not be possible to enlarge the channel.
Also the process can be expensive and can cause problems to areas downstream who are receiving more flood water quicker, but with an un- enlarged channel.
Channel Straightening: Removing meanders from a river to make the river straighter.
Advantages: By removing meanders the velocity of the water through a settlement will increase.
This will stop a backlog of water and should reduce the risk of flooding. It also improves navigation.
Disadvantages: By changing the course of the river, you might remove flowing water from industries that depend on it.
There might also be building that have to be demolished to allow straightening.
Again it is expensive and may cause flooding problems downstream.
Flood Relief Channels: Building new artificial channels that are used when a river nears bankfull discharge.
Advantages: They take the pressure off the main channels when floods are likely therefore reduce flood risk.
Disadvantages: It can be hard find land to build relief channels, they are expensive and when empty can become areas to dump rubbish, etc.
If river levels rise significantly it is also possible for relief channels to flood as well
Artificial Stores: Creating reservoirs or lakes that can store excess water in times of flood.
Advantages: They can remove pressure of the main channel and can become new habitats and serve other purposes e.g. leisure, drinking water.
Disadvantages: Building dams, sluices, diversion channels are all expensive.
They also involve flooding areas of land which may be hard to find near large vulnerable urban populations
Flood Embankments (levees): Like levees these increase the channel depth of a river, raising its bankfull discharge and reducing the risk of flood.
Advantages: They increase the cross-sectional area of the river and therefore its hydraulic radius.
This should reduce the risk of flooding.
Disadvantages: Like in New Orleans under extreme conditions, embankments may fail causing even bigger problems.
They are expensive to build and again may cause problems downstream.
Controlled Flooding: Allowing low value land e.g. farmland to flood, therefore protecting higher value areas.
Advantages: By allowing the river to flood naturally you are taking the pressure of high value areas, you are letting the river behave more naturally and it adds alluvium to the floodplain.
Disadvantages: You have to make the decision what is worth protecting which is always going to upset someone.
You also have to protect areas that you don't want to flood which costs money (cost benefit analysis)
Afforestation / Reforestation: Simply planting more trees in a drainage basin.
Advantages: This is a natural process, increasing the amount of interception, transpiration and root uptake.
People would not normally protest against trees being planted.
Disadvantages: It is not possible to cover the whole drainage basin in trees, so if it rains in an area with no trees, then there is no reduction in flooding.
Also, most trees lose there leaves in autumn and winter reducing interception in those months.
Flood Proofing: This is making property less vulnerable to flooding or flood damage. This might be temporary like using sandbags or design by removing carpets downstairs.
Advantages: This can be done on an individual level and can be relatively cheap.
Temporary protection can be removed under normal circumstances so it does not change the aesthetics of properties.
Disadvantages: Temporary defences can usually only protect against minor floods.
Not everyone will be happy with having to redesign their houses.
Insurance: Although it doesn't prevent flooding, it can help individuals and industries to recover and protect against future flooding.
Advantages: It helps individuals and settlements to recover after flood events and may help them protect property and be less vulnerable in the future.
Disadvantages: They do not actually prevent flooding.
Not everyone can afford insurance and insurance companies may not insure high risk areas.
Land Use Planning (zoning): Mapping areas by looking at there likelihood to flood and then only building low value uses on areas with high flood risk.
Advantages: Very good at removing high value areas and high density populations from hazardous areas.
Disadvantages: It is not always possible to change land uses that already exist in an area.
You have to decide what size flood to map for e.g. a once in ten year flood or once in one hundred year flood.
Often poor will still choose to live on marginal land.
Contour Ploughing and Strip Cultivation: Either creating temporary surface stores or leaving vegetation to increase interception and transpiration
Advantages: Contour ploughing is simply a cheap and easy change in existing farming methods.
Keeping vegetation is natural and relatively cheap.
Disadvantages: Won't protect against big floods and farmers may not be happy giving up farmland, simply to grow trees.
Interception Channels: These are channels that divert a rivers' discharge around settlements.
The old channel remains but with a smaller discharge.
Advantages: They remove pressure of the main river and areas of high land value.
They may also develop into new habitats for plants and animals.
Disadvantages: They are expensive, may flood themselves in times of heavy floods and may restrict future urban
Settlement Removal: Moving settlements from high risk flood areas to less vulnerable locations often on higher land.
Advantages: Is probably the most effective because you remove high value property and humans from vulnerable areas.
Disadvantages: It is usually not practical to move whole settlements, because of the cost and the problems of finding alternative locations.
Also many settlements depend on water for their survival.
Dams: Often built as part of a multipurpose scheme, they create artificial stores which can hold water in times of increased precipitation.
Advantages: They can store large amounts of water and can be used for other purposes.
Disadvantages: If rain is downstream of the dam then they have no effect.
In large flood events they are vulnerable to breaking and are expensive to build.
Wing Dykes: Barriers placed out into a river, these can be used to divert the cause of rivers by shifting the thalweg of rivers.
This may move the channel away from high value areas.
Advantages: They can move the main channel from vulnerable areas to protect high value areas.
Disadvantages: They are expensive to build and during big flood events the flood water may go over the wing dykes.
Also if there is property on both sides of a river, which side do you protect.
Electronically Controlled Sewers: Advanced sewers which can control the flow of rain water tostop increased discharge into rivers and therefore flooding.
Advantages: They can be very effective at controlling smaller floods.
They are underground so do not cause any visual pollution.
Disadvantages: This involves a complete redesign of sewers.
Sewers usually have to be increased in size and electronic sluices have to be added.
They also have to be operated from a central command centre and with all electronically operated equipment can break.
Also they might not be able to cope with large scale floods, so water has to be released into rivers anyway.
Channelization: The concreting of beds and banks.
Advantages: Reduces friction and increases velocity of river, removing water from the channelised area quicker.
Bank erosion is also reduced.
Disadvantages: It is expensive and is not natural so vegetation and animal life will find it harder to grow and live.
Flooding maybe caused downstream of the channelised area.
Dredging: The removal of material from the bed of the river deepening it.
Advantages: Channel cross-section is increased so the river can hold greater discharge.
It can look more natural because no structures are built.
Disadvantages: Deposition can mean that dredging needs to happen regularly.
River bank conservation: Protecting the banks and sides of the river to reduce erosion.
This can be done through planting vegetation.
Advantages: It looks natural, promoted wildlife and is relatively cheap compared to hard-engineering.
Disadvantages: During large flash floods vegetation can be easily removed.
River restoration: Returning a river to its natural state before it had been managed. This might involve removing channelization.
Advantages: This looks natural, is attractive and can attract wildlife. Can allow the floodplain to become more fertile.
Disadvantages: Can't protect against big floods and may have to coincide with zoning
Key Terms
Artesian basin: An artesian basin or aquifer is a confined aquifer containing groundwater under positive pressure.
This causes the water level in the well to rise to a point where hydrostatic equilibrium has been reached (balance between pressure on the aquifer and pressure from the aquifer).
Aquifer: Rocks that can hold water.
Saturated: When all pore space is full and rocks or soil can hold no more water.
Groundwater: Water held under the surface of the earth.
Depletion: When something is reducing, aquifers can become depleted in dry periods or when they are managed unsustainably.
Aquiclude: Rock that will not hold water or allow its movement. i.e. they are non-porous and impermeable
Causes of Groundwater Usage:
Evapotranspiration from shallow stores, capillary action will draw moisture up to near the surface
Natural discharge by springs and into lakes, rivers and oceans
Artificial abstraction (removal) for domestic, industrial and agricultural use
Leakage into nearby aquifers
Interbasin transfers
Causes of Groundwater Recharge:
Artificial recharge. Either leakage from irrigation channels and reservoirs or the pumping of water into aquifers.
Infiltration and percolation after precipitation or snow melt
Seepage from river channels, lakes and oceans
Leakage from nearby aquifers
Interbasin transfers
Groundwater Pollution in Bangladesh - Groundwater Management
Increase in incidence of cancers in Bangladesh
Caused by naturally occurring arsenic in groundwater pumped up through tube wells
As many as 85m of the country’s 125m population will be affected by arsenic-contaminated drinking water
UNICEF has sunk millions of tube wells in Bangladesh, providing a convenient supply of drinking water free from bacterial contamination of surface water
But the water from wells was never tested for arsenic contamination, which occurs naturally in the groundwater
1 in 10 who diners water containing arsenic will ultimately die of lung, bladder or skin cancer
Arsenic poisoning is a slow disease - skin cancer occurs 20 years after people start ingesting the poison, then internal cancers
One solution is a concrete butt, collecting water by pipe from gutters
Another is a filter system
Wetland is an area of land where soil is saturated with moisture either permanently or seasonally.
Such areas may also be covered partially or completely by shallow pools of water. Wetlands include swamps, marshes and bogs.
The water found in wetlands can be saltwater, freshwater, or brackish (a mixture of fresh and salt water).
The world’s largest wetland is the Pantanal which straddles Brazil, Bolivia and Paraguay in South America.
Brackish water: Water that has a higher salinity content than freshwater, but not as high as saltwater.
Importance of Wetlands
Flood control:
Many wetlands are covered in vegetation which can intercept precipitation, absorb rainwater and transpire water.
Wetland vegetation can also reduce the velocity of rivers flowing into them or from them and act as natural stores of water.
If you remove or drain areas of wetland more pressure is placed upon the main river channel.
Coastal and marine wetland areas can also absorb the energy of tropical storms, tsunamis etc.
Groundwater recharge:
Wetlands can collect large areas of precipitation and river discharge.
As this water is held in storage it will infiltrate and percolate into the ground to recharge groundwater.
Transport Network:
Wetland provide many natural waterways that people can move around on easily.
Tourism and Leisure:
Some wetlands, like the everglades in Florida or the fens in East England become tourist attractions.
They also become popular locations to bird watch, fish and hunt.
Flora and Fauna:
Many wetlands are unique habitats that support indigenous aquatic plants and animals.
Many wetlands support rare reptilian and amphibian species.
Many migratory birds also rest in wetlands flying to and from nesting and breeding grounds.
Fisheries:
Wetlands can support large numbers of fish which can support local populations.
Wetlands are not normally viable commercial fisheries.
Water purification:
The soils, geology and vegetation of wetlands can help clean and purify water.
Storage of organic matter:
Wetlands support large areas of organic matter that can hold large stores of methane (greenhouse gas).
Coastal stabalisation:
Wetlands that occur along the coastline and on river banks have prevent erosion from the sea or by rivers.
Factors Causing Loss and Degradation of Wetlands
Increased demand for agricultural land:
As the world population grows there is an increasing demand for food.
With the amount of viable agricultural land decreasing, increasingly areas of wetland are being artificially drained to make ways for agricultural land e.g. the draining of the fens in East England.
Population growth:
As the world’s population grows, it demands more water, more food and more land.
The increasing demand for water can mean wetlands are drained of their water or their source of water.
This problems is made worse as the world’s population develops and uses more water e.g. showers and toilets.
Urbanization:
With the world population growing, there is a greater demand for housing. Increasingly this demand for housing is in urban areas.
With urban areas growing more and more wetland areas are being drained or inhabited.
Urbanisation on or near wetlands can cause pollution, changes in river flow and river channels and disturbance of wildlife.
Land reclamation is the process of reclaiming land from the water.
Sea level rises:
Global warming is causing glaciers and ice sheets to melt causing sea levels to rise.
These rising sea levels can flood coastal and marine wetland areas.
Even if the whole wetland is not flooded, water conditions can be changed from fresh to brackish.
River flow changes:
Many rivers have been channelised and straightened, reducing the amount of wetlands.
Others have been drained or dams have altered flow.
Some have been polluted or redirected.
All these natural changes are removing or changing the ecosystems of many wetland areas.
Pollution:
Any form of pollution, but particular chemicals and metals can change the delicate ecosystems of wetlands.
Process like eutrophication, caused by fertiliser run-off can completely kill whole wetland areas by preventing the wetland oxygenating properly and receiving sunlight.
Infrastructure projects:
As populations grow and we become more mobile, there is an increasing demand for new roads, airports, railways. etc.
Unfortunately wetlands are often drained or disrupted (bridges, dykes and causeways) to make way for these projects.
Alien species invasion:
Many alien species like the cane toad in Australia or the American mink in the UK have been introduced to wetlands and devastated indigenous species.
The introduction of any alien, however small can disrupt food webs and ecosystems.
Tropical storms:
Although wetlands can be a natural defence against tsunamis and tropical storms, they can also been damaged by them.
Freshwater wetlands in particular can be flooded by storms surges associated with tropical storms, changing the salinity of water and damaging vegetation.
Kissimmee River - Wetland Management
In south central Florida, drainage basin of 7800km squared and approximately 200km long was home to wetland plants, fish and wading birds.
However, the 5km wide floodplain with populated settlements nearby were regularly flooded.
Thus, the river was channelized and transformed into a 90km, 10m deep drainage canal - to provide an outlet canal for draining floodwaters from upper Kissimmee lake basin and to provide flood protection for land adjacent to the river
Impacts of channelization:
Loss of 12000-14000 hectares of wetlands
Floodplain dried up after channelization - no longer exceeds bankfull discharge
Egret, heron, and wood stork populations decreased by ⅔
Catches of largemouth bass decreased
Fishing, bird watching and hunting tourism declined
Concerns about the sustainability of existing ecosystems led to the Kissimmee River Restoration Project (large scale, supported by the state and federal)
Aim: restore over 100km squared of river and wetland floodplain by 2015
Started in 1999
River is being de-channelized by refilling the flood canal and re-establishing the old natural course of the river
Restored sections now flood naturally - restored floodplains could benefit avian species e.g. wading birds and waterfowl, by providing increased feeding and breeding habitats
Dissolved oxygen levels have doubled in restored sections
Increase in revenue from tourism potential could significantly enhance local and regional economies
Possible negative impacts from restoration:
Greater evaporation due to more surface stores
Increase risk of flooding
River will be less navigable in dry periods
Restoration will cost $578m
Agriculture:
Agriculture the artificial cultivation (growing or rearing) of plants or animals.
Agriculture that grows crops is known as arable agriculture, agriculture that involves rearing animals is known as pastoral agriculture.
Irrigation:
This means artificially watering the land.
There are three main types of irrigation; gravity flow, sprinklers and drip systems.
Eutrophication:
This is the processing of artificially adding nitrates and phosphates (through fertilsers and sewage) to wetland areas e.g. rivers and lakes.
The added nitrates and phosphates causing excessive growth of algaes.
The algae growth can reduce the oxygen content of the water as well as reducing the amount of sunlight that it receives.
The nitrates and phosphates often come from agro-chemical run-off, but can also come from domestic sewage and industrial waste.
Salinisation:
This is the increase in the salt content of water.
Salinisation can happen because of evaporation or unsustainable water extraction.
If the water become to salinated it becomes less fertile.
Growing Demand for Agricultural Products:
The world’s population is growing.
The current population is about 7 billion, but it is expected to peak at nearer to 9 billion.
Because fossil fuels are finite, alternative forms of energy are being looked at.
One form of renewable energy being used are biofuels.
Biofuels are made out of biological matter and therefore are increasing the demand for agricultural products.
Economic development.
As more of the world’s population is removed from poverty, their calorific intake increases.
This increase in food consumption, is increasing the demand for agricultural products.
Pastoral farming.
As the world population increase, the demand for meat also increases.
Most farm animals are omnivores or herbivores so need agricultural products like corn to eat.
Decreasing Supply of Agricultural Products or Land
Urbanisation.
As the world develops, urbanisation increases tends to happen increasing the size of urban areas.
As urban areas grow they eat into greenfield sites in rural areas, reducing the amount of agricultural land.
Land degradation and desertification.
Land that is overcultivated or overgrazed can become degraded (less fertile).
As farmers try to react to demand by growing more intensively, more land is being degraded.
In extreme circumstances, the land may turn to desert (desertification).
Rising sea levels.
Some of the earth’s most fertile agricultural areas are floodplains and deltas.
As world sea levels (eustatic changes) increase much of this fertile land is lost.
Conversion to biofuels.
Although not strictly reducing the amount of agricultural products (biofuels are agricultural products), this does decrease the supply of agricultural products available for human consumption.
Biofuels are often favoured by farmers, because they demand a higher price.
Hazards.
Natural hazards like tropical storms, volcanoes and tsunamis can reduce the amount of agricultural land available for cultivation.
Disease.
There is an increasing amount of intensive monoculture (growing of one crop) taking place.
Monoculture always runs the risk of been impacted by the outbreak of diseases or pests that attack the particular crop e.g. wheat leaf rust fungus.
Case Study: Local Scale Competition for Demand for Water
Israel’s Aquifers - Demand for Water: Local/national Scale
Water is one of the most sensitive and unsolvable problems in the Middle East
Created friction between the Arabs and Jews (Israeli-Palestinian tensions)
For decades, Israel has obtained up to 80% of the 670m cubed of water provided by mountain aquifer mostly located under the West Bank
Israelis have occupied the West Bank since 1967 and have prevented the Palestinians from obtaining better access to the resource
Mountain aquifer is important for Israel as it provides:
⅓ of its water consumption
4% of its drinking water
50% of its agricultural water
120,000 Jewish settlers in the West Bank use 60m cubed annually compared to 137million m cubed used by 1.5m Arabs
The WB and Gaza are served by Israel’s water carrier and groundwater in aquifers
WB’s aquifers, replenished by rainfall, flow west, north, and east from the watershed
Palestinians were forbidden to dig new wells or deepen old ones (Israel claims that they have the right to use the aquifer because some of the water flows into its territory) - thus kept very short for their crops
The Gazans, like West Bankers, get little domestic water from Israel’s national carrier - most of their supplies come from an aquifer that has been exploited - Gazans pump twice as much as can be safely withdrawn, leading to salt water intrusion (kills citrus trees)
Gaza Strip is part of the Palestinian territories - coastal aquifers becoming exhausted and at threat of salt water intrusion and domestics and industrial pollution
Case Study: International Scale
The Mekong - Demand for Water: International Scale
South-east Asia’s largest river flat, well-watered and fertile land in the basin lies around Tonle Sap Lake, but annual flood makes intensive agriculture difficult there surface area of lake can increase up to ten times during the monsoon remained untouched until 1990s
First dam non the river, at Man Wan, in China was completed in 1993
Population growth and economic growth - place strain on the Mekong
HEP potential of the Mekong and its tributaries is considerable - so far, only 5% of the lower basin’s HEP have been developed
Dams generate electricity, aid irrigation and regulate flooding
However, caused damage to fisheries - annual harvest amounts to 2m tonnes
Case Study: International Scale
The River Nile - Conflict at International Scale
Importance of the River Nile:
Tourism - rapids in Uganda
agriculture and irrigation - Egypt depends on the Aswan Dam to irrigate the desert
Transport - promote trade
Wildlife
Drinking water
HEP - drought control
Longest river in the world, 6650km long
2 main tributaries - White Nile and Blue Nile
Confluence is in the Sudanese capital, Khartoum
Source of WN: Burundi
Source of BN: Ethiopia
WN, BN and the Nile flow through 11 countries
Conflicts:
Have arisen since Ethiopia began dam building
In 2010, 6 of 9 upstream countries signed a Cooperative Framework Agreement seeking more water shares from the Nile
Sudan and Egypt rejected agreement because it challenged their historic water allocations
A major dam on the BN, called the Grand Renaissance Dam is under construction by Ethiopians
Egypt, draws much of its drinking water, natural resources, and energy from the Nile, has protested the dam’s construction (will siphon resources away)
Dispute between Egypt and Sudan over the dam construction has reignited a 60-year old dispute (Sudan, downriver of the Nile, has supported Ethiopia’s attempts to build the dam)
Ethiopia denies that the dam would damage Egypt’s water supplies
Egypt now hope to pull the Europeans to its side ad to pressure Ethiopia before protesting before the security council
Egypt has threatened to defend its historical claims over the Nile in numerous occasions - it will even use air power against other countries to protect its flow of the Nile
