The Hydrological Cycle
- Evaporation: water from lakes & oceans turning into vapour
- Transpiration: water vapour given off by plants/trees.
- Evapotranspiration: the sum of evaporation & transpiration
- Condensation: water vapour cooling & turning into water droplets.
- Precipitation: water falling back to earth (snow, hail, rain, sleet).
- Interception: trees etc stop some rain reaching the ground.
- Throughfall: water dropping from leaves.
- Stemflow: water running down plant stems/tree trunks.
- Surface run-off: water flowing through streams & rivers.
- Infiltration: water soaking into the surface soils.
- Through-flow: water flowing downhill through the soil
- Percolation: water seeping through rock & deeper soils.
- Groundwater: water deep in saturated rocks underground.
- Groundwater flow:(baseflow) slowly moving to river channel or the sea.
The Drainage Basin
- Source: the start of a river
- Tributary: a smaller river that runs into a larger one
- Confluence: the meeting point of two rivers/tributaries
- Mouth: the end of the river where it flows into the sea
- Watershed: Also known as drainage divide, it is the line defining the boundary of a river drainage basin.
Changing Balance of Water Storage
Reduction in ice storage
Global warming is melting mountain glaciers and Arctic/Antarctic ice sheets.
- Increased amounts of surface runoff (higher levels of meltwater) leading to increased flood risk.
- Rising sea levels due to additional water.
- Possible lower infiltration rates due to increased surface runoff.
- Impact on groundwater levels if infiltration is reduced.
Maximum Sustainable Yield
The maximum level of extraction of water that can be maintained indefinitely for a given area.
This concept relates to a balance between the inputs and outputs of the water system. If we increase water use for irrigation then there will be an increase in the amount of water lost through evapotranspiration. Similarly – rising populations are increasing water consumption and then sending the waste water quickly to the seas where it is lost to the system.
Storm hydrographs show how specific river systems respond to rainfall events. They are very useful for identifying the flood risk posed by heavy or prolonged rainfall. Knowing the lag-time allows effective emergency response procedures to be put in place.
Storm hydrographs also allow the identification of longer-term patterns and trends in flooding and river behaviour.
- Discharge: the volume of water that passes a certain point over a certain period of time, usually measured in cumecs (cubic meters per second). Discharge tends to increase as the river travels downstream due to tributaries joining but also increased velocity (less friction)
- Peak Discharge: the time at which the river is flowing at its fullest after the peak rainfall.
- Peak Rainfall: represents the time at which there was the heaviest rainfall during the storm.
- Lag Time: the time between peak rainfall and peak discharge. It is significant since it represents how quickly the water from the rain reached the river.
- Rising Limb: Linked to lag time – a steep rising limb (quick increase in discharge levels in a river) – flash flood risk.
- Falling Limb: This shows how quickly the discharge returns to pre-storm levels.
- Bankfull Discharge: the level at which the river is flowing at capacity. Above this line represents flooding.
- Surface Runoff: increases rapidly as heavy rainfall exceeds infiltration capacity.
- Throughflow: lower and slower than surface runoff due to the time taken to infiltrate and pass through the soil.
- Base Flow: changes in base flow are small and more gradual. It takes more time to infiltrate and percolate to base flow.
Factors Affecting Flood Hydrographs
- Infiltration rates: impermeable soils and surfaces (urban areas) reduce the amount of water that can soak in. This increases the amount of surface runoff & will cause a steeper rising limb.
- Drainage density: a higher density of streams and tributaries speeds up the rate at which the rainfall enters the main channel – steeper rising limb.
- Vegetation cover: areas with vegetation are likely to have higher interception rates which reduces the amount of water reaching the river channel & slows down the speed at which it reaches the channel.
- Urban areas: these not only increase the amount of runoff, they deliberately increase the speed at which it reaches the river.
- Drainage Density: the relationship between the total length of all the rivers and streams in a drainage basin (km) and the total area of a drainage basin (km2). If a drainage basin has a high density, it means that precipitation gets into streams quicker, If a drainage basin has a low drainage density, it means that more precipitation has to travel further by surface run-off, throughflow and baseflow.
Flooding – Case Study: Bangladesh 1998
- Most of Bangladesh has been formed by the deposition of alluvial soils over millions of years by the rivers running through it.
- Due to the nature of its formation as a floodplain it is very flat, low lying and naturally prone to flooding.
- It has 3 major rivers (Ganges, Brahmaputra & Meghna) converging and at times of peak flow the confluences are likely to experience flooding.
- Continued deposition of material by the rivers slowing reduces their capacity and increases the flood risk.
- This geographical region experiences the monsoon rainy season from May to September with high levels of rain falling in the 3 main river basins that flow through Bangladesh.
- Deforestation in the Himalayas has reduced interception rates, increased surface runoff and led to higher peak discharge levels.
- Deforestation is leading to increased soil erosion in mountain areas which is increasing silting of the river channel downstream.
- Increasing levels of population in Bangladesh & India are causing urban expansions and larger impermeable areas.
- Lack of available finance is leading to lack of investment in flood defences (maintenance & new defences).
- Almost $1 billion of damage. Extensive damage to rice plantations and huge loss of livestock – impact on food supply & price.
- 4750 people killed.
- Drinking water supplies contaminated which lead to spread of cholera & dysentery.
- 7 million homes destroyed and almost 25 million people made homeless.
Dams & reservoirs
Hydrological Changes Resulting from Dams & Reservoirs
Increased channel storage in the form of the lake created.
- Less extreme variations in discharge downstream of the dam as water is regulated (held back in flood risk times, released in dry periods).
- Decrease in sediment deposition downstream of the dam due to it being deposited in the dam itself (reducing the dams capacity).
- Example: shrinking Nile delta & loss of fertile soil in the lower reaches of the river Nile.
Costs & Benefits of Dams as Multi-Purpose Schemes
- HEP generation: Arenal Dam (Costa Rica) provides an important energy source for a country with very limited domestic reserves of fossil fuels.
- Leisure & recreation opportunities: Water sport opportunities and lakeside housing developments on many dam-created lakes.
- Navigation: dams with lock can make the river more navigable by regulating the levels downstream and creating deepwater passages upstream. Example: 3 Gorges Dam, China
- Large scale dams flood huge areas of vegetated land, as this vegetation starts to rot once submerged it releases methane – a powerful greenhouse gas. Dams contribute to the enhanced greenhouse effect in this way.
- Hydraulic Action: pressure/force of the water loosening material & widening cracks in the river bank..
- Abrasion: the scraping action of material being transported.
- Attrition: the breaking of stones when they collide with other material.
- Corrosion: the dissolving of rock (limestone etc) by acid in the water.
Traction: the rolling of stones & rocks. Usually larger rocks.
- Saltation: stones/rocks bouncing. Usually smaller rocks due to the energy required.
- Suspension: very small particles being carried along in the river current.
- Solution: dissolved material (invisible).
- Braiding: formation of interweaving channels in the river due to small temporary islands being created by deposition.
- Levees: natural embankments formed due to repeated flooding of a river. The flood waters deposit material as soon as the water bursts its banks since the increased friction slows it down. This forms natural levees.
- Floodplains: areas of flat land created by repeated deposition of alluvial soil in times of flood.
- Delta: near the mouth the river splits into many channels (distributaries) due to high levels of deposition which block the channel and force the water to find new routes. Example: Nile delta.
The Bradshaw Model
This diagram shows the change in river characteristics from upstream to downstream.
Particles are eroded as they are transported downstream so the particle size is larger closer to the source. Smaller particles are moved further downstream.
The gradient is steep near the source (mountains) & gentle by the mouth.
Discharge increases as tributaries join the main channel and increase the volume of water. The reduced friction leads to faster flowing water.
- Created when rivers flow over layers of soft and hard rock.
- The softer rock erodes more quickly and undercuts the hard rock (hydraulic action).
- Eventually the hard rock ledge collapses into the plunge pool and the waterfall moves upstream (attrition and abrasion will occur).
- This process repeats – often leaving a gorge downstream as the waterfall cuts a deep channel through the rock.
Human Modifications of Floodplains
- The fertility of deposited alluvial soils has always attracted farmers to cultivate floodplains. They also benefit from a plentiful supply of fresh water for irrigation.
- In recent decades many floodplains have become desirable places to live due to the river view and access. Riverside developments often make use of brownfield land that was once industry or docks and command high prices.
- Urbanisation of floodplains removes vegetaion and creates impermeable surfaces. This reduces infiltration, speeds up runoff into the river channel and is likely to increase the risk and scale of flooding.
- Straightening river channels speeds the flow of water & reduces the risk of flooding. Expensive.
- Man-made levees (New Orleans) & man made embankments can be made to reduce flood risk.
Flood Management Strategies
- Channel Enlargement:Making the width and depth of the river wider and deeper to increase its cross-sectional area and therefor the bankful discharge that it can accommodate.
- Advantages: 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 riverbank it might not be possible to enlarge the channel. Also the process can be expensive and can cause problems to areas downstream that are receiving more floodwater quicker, but with an un-enlarged channel.
- Channel Straightening:Removing meanders from a river to make the river straighter.
- Advantages: This increases the velocity of the water through a settlement and should stop the 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 buildings 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 pressure off the main channels when floods are likely therefore reduces 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.
- Flood Embankments: these increase the channel depth of a river, raising its bankfull discharge and reducing the risk of flood.
- Advantages: relatively easy to construct and maintain.
- Disadvantages: embankments may fail causing even bigger problems. They are expensive to build and again may cause problems downstream.
- Groundwater refers to stores of water in saturated rock layers. Humans exploit groundwater aquifers as a source of fresh water for drinking and agriculture. Wells are often relatively cheap and easy to drill.
- Groundwater takes thousands of years to be recycled naturally.
- Aquifers are recharged naturally through infiltration and percolation from precipitation and lakes/rivers.
* Rising populations and increased irrigation in many places has led to unsustainable extraction of ground water. This results in falling water table levels which causes environmental and human impacts:
- Tanzania: Open cast gold mining has resulted in the mine pumping ground water out to allow the mine to go deeper. This has resulted in local villages’ wells no longer reaching groundwater levels.
- India: Extensive arable agriculture & irrigation in Punjab has resulted in wells needing to be sunk deeper and deeper. Simultaneously the ground water quality has decreased as it becomes more saline.
- Spain: Over-extraction of water for tourism has seen Benidorms water table fall. This has resulted in sea water seeping in & making the groundwater saline.
* Humans can artificially recharge groundwater stores through piping surplus water from dams & reservoirs into the aquifers.
- Agriculture (fertilisers and pesticides)
- Industry (chemicals and metals)
- Landfill sites (decomposing rubbish: plastics, batteries and electronic items)
- Household waste (non-biodegradable products e.g. shampoo, detergent)
- Sewage (open or broken sewers, lack of water treatment facilities)
- Natural (arsenic, calcium, magnesium and chloride can all occur naturally)
Problems of Depletion
- Saltwater intrusion: If aquifers near coastal areas are depleted enough, saltwater may seep in to fill them. Once an aquifer has become filled with saltwater, then it is useless for human consumption.
- Drought: Periods of drought can be worsened if groundwater has been depleted. Under normal circumstances groundwater can be relied upon in times of drought. However, if groundwater has been managed unsustainably, then there might be no groundwater to rely on. This also increases the risk of crop failure, famine and ultimately death of livestock and humans.
- Ground subsidence: groundwater depletion may cause the land above to subside (sink). Mexico City is a good example, where its growing population has exploited groundwater unsustainably. Buildings are starting to lean as the ground subsides.
- Salinisation: decreasing the amount of water increases the concentration of salts leading to higher salinity.
Freshwater Wetland Management
Importance of Wetlands
* Wetlands are an important ecosystem that are vulnerable to changes. They provide a habitat for many birds, animals and fish. * They also serve to filter impurities out of water and the slow flow causes deposition of the rivers load. High infiltration rates into the soil layer helps to recharge underground water stores in the area.
* Economic benefits of wetlands can be gained through tourism & recreation.
*The wetlands of the Kissimmee river basin were the habitat for a wide range of birds, animals and plants. The water from the river basin flows down into the Florida Everglades.
Managing the Wetlands
- After prolonged flooding of the area in the 1940s it was decided to straighten & deepen the river and drain the surrounding wetlands.
- A straight channel cut through the meanders and sped up the discharge of water.
- These alterations were successful in achieving flood control but had significant environmental impacts:
- 90% reduction in waterfowl.
- 70% decrease in Bald Eagle nesting areas.
- significant depletion of fish in the river after it became de-oxygenated.
Re-establishing the Wetlands
- 40 miles of the original river channel are being re-established at huge cost.
- 40 square miles of the floodplain ecosystem are being re-established
Irrigation & Agriculture
Increasing pressure of food supplies has seen irrigation increasingly used to allow greater yields. This has had many environmental impacts:
- Groundwater depletion: Many arable areas in India have falling water tables due to irrigation.
- Salinisation: this is increasing levels of salt in the water & occurs in areas where the groundwater level is near the surface. The salts make their way up through capillary action. Irrigation in such areas can keep the salts lower down.
- Chemical runoff: fertilisers, pesticides and fungicides wash off plants in the rain & leach through the soils into the rivers and groundwater aquifers. This contaminates freshwater supplies & has health implications for humans.
- Eutrophication: nitrates from fertilisers leach into the rivers & cause excessive algae growth. The algae uses oxygen from the water & de-oxgenates it which leaves the water unable to support the natural ecosystem. It can lead to ‘dead’ rivers in which fish & other plants cannot survive.
National Water Conflict:Israel
- Water shortages, rising populations and religious tensions have led to conflict within Israel.
- In a country that experiences water scarcity, the main water sources are underground aquifers and the River Jordan.
- Israelis & West Bank Palestinians (under Israeli military rule) share the West Bank area of Israel but conflict has arisen over access and use of water in this region.
- Israel has drilled almost 50 deep wells in the West Bank to supply its cities. They also have water for extensive irrigation and domestic consumption in swimming pools & watered lawns.
- West Bank Palestinians have been restricted from drilling deep wells. They can have shallow wells & use natural springs. When these supplies run dry in the Summer months they have to buy water from the Israelis – that they argue was extracted from under them.
- These restrictions impact on irrigation, food production and development for the Palestinians
- The World Bank (2009) estimated that Israeli use of water was 4x that of the Palestinians in the West Bank.
- With continued population growth this problem looks set to escalate.
International Water Conflict: The Nile River Basin
- Rising populations and old water-use agreements in the Nile river basin are creating conflict.
- The 1929 Nile Waters Agreement was signed by Britain which gave mainly Egypt, but also Sudan, control over the River Nile.
- The agreement banned irrigation or power generation on any of the tributaries.
- In 1959 this agreement was replaced by one giving Egypt 75% of the Nile water & Sudan 25%. It also gave Cairo complete control of the river.
- Recent decades have seen the other countries negotiate to try and increase their control of the river but Egypt has resisted changes.
- Ethiopia has recently built a large hydro-dam & sees river management for irrigation as vital to food supplies. More dams are planned.
- Uganda has two dams on the Nile and plans more to meet the growing demand for energy.
- Tanzania wants to use Lake Victorias waters for irrigation and piping to other drier areas.
These demands by other countries will reduce the flow reaching Egypt and the Aswan dam – with implications for its irrigation & energy generation.