water paper 1 Flashcards
5.4a the course of drought (meteorological & hydrological), short term precipitation deficit, longer term trends and ENSO cycles
meteorological: shortfalls in precipitation as a result of short term variability within longer term average (when amount of precipitation in a specific area is less than the average e.g Sahel)
hydrological: when reduced precipitation impacts on water supply, eg there is reduced stream flow, lowered groundwater levels and reduced water stores, soil moisture, reservoir and lake levels.
agricultural: decrease in soil moisture and soil water availability has a knock on effect on plant growth and reduces biomass
famine drought: humanitarian crisis where widespread famine of agriculture leads to food shortages and famine
physical factors
higher temp leads to high evaporation reducing amount of water available
natural variability in climate causes temporary declines in rain causing stores to not be replenished
climate effects precipitation
ENSO - El Niño South Oscillation
can trigger dry conditions (in South East Asia, India, Australia, South East USA, brazil)
Temperature anomalies provide the key to ENSO, which, in turn, is thought to trigger the occurrence of droughts.
Normal conditions in the Pacific Basin
In a non-El niño year, the trade winds blow from east to west along the equator
The air pushes the warm water westerwards
Thermocline, upwelling,
Warm, moist air rises, cools and condenses, forming rain clouds
Conditions during an El Niño event
The trade wind pattern is disrupted - it may slacken or even reverse and this has a knock-on effect on the ocean currents
Air circulation loop reversed
When this happens, cool water normally found along the coast of Peru is replaced by warmer water.
At the same time, the area of warmer water further west, near Australia and Indonesia, is replaced by cooler water.
El Niño events usually occur every 3-7 years, and usually last for 18 months. El Niño events seem to trigger very dry conditions throughout the world, usually in the second year. For example, the monsoon rains in India and South East Asia often fail.
La Niña events may sometimes, but not always, follow an El Niño event. They involve the build-up of cooler-than-usual subsurface water in the tropical part of the Pacific. This situation can lead to severe drought conditions, particularly on the western coast of South America.
Very strong air circulation and very warm water moving east-west.
5.4b contribution of human activity makes risk of drought : over abstraction of surface water resources & ground water aquifers
Desertification = once-productive land changes into a desert-like landscape. It takes place in semi-arid land on the edges of existing deserts. e.g Sahel region drought: Mauritania to Ethiopia.
Causes: Changing rainfall patterns with less reliable rainfall seasonally and annually. The occasional drought year sometimes extends to several years.
The vegetation cover becomes stressed and dies, leaving bare soil. The bare soil is eroded by wind and the occasional intense shower.
Only short, intense periods of rainfall = difficult for the remaining soil to capture and store it.
The U.N. defines water security as: The capacity of a population to safeguard sustainable access to adequate quantities of and acceptable quality water for sustaining livelihoods, human well-being, and socio-economic development, for ensuring protection against water-borne pollution and water-related disasters
factors increasing vulnerability to desertification
over-abstraction of surface water from rivers and ponds, and of groundwater from aquifers.
overpopulation & increase of wealth = increase of demand of water
overgrazing = increased need for livestock creates soil depletion land is stripped of protective grass cover
farming = reduces soil fertility and soil is eroded
deforestation = removed vegetation increasing soil erosion
Australian drought – linked to El Nino events. Rainfall deficiency lots of drought occurring use of almond farms are unsustainable and a waste of water. A careful management of scarce water resources and sorting out the competing demands of irrigation and urban dwellers has stopped desertification. Other actions e.g. large-scale recycling of grey water (waste bath, shower, sink and washing water), constructing desalination plants and planning water conservation strategies.
5.4c the impacts of drought on ecosystem functioning (wetlands, forest stress) and the resilience of these ecosystems.
Wetland: area of marsh, peatland or water where water flows. Cover 10% of earths surface and perform key functions
Act as temporary water stores, reduce flood risk and recharge aquifers.
Water filters: trap and recycle nutrients and pollutants to maintain water quality.
High biological productivity: have diverse food web good for fish & birds
Store carbon: peat acts as carbon storage.
Drought impact on wetlands: less transpiration = they become less functional, impacts due to exploiting them for resources e.g., peat. Dams harm wetlands.
Forest: do interception which means reduced infiltration and overland flow. Forests are characterised by high levels of transpiration.
Drought threatens forest by increasing the susceptibility of pines and firs to fungal diseases. Tree mortality increases = less carbon stored= global warming people & deforestation more threatening
5.5a meteorological causes of flooding include: intense storms leading to flash flooding, unusually heavy or prolonged rainfall, extreme monsoonal rainfall and snowmelt.
meteorological causes of flooding are:
intense storms, lead to flash flooding (short lag time), as in semi-arid areas but more common in mountainous areas
prolonged, heavy rain e.g during the Asian monsoon and with passage of deep depressions across UK
rapid snowmelt during a warm spring e.g on plains of Siberia.
likelihood of flooding is also increased by:
in low-lying areas with impermeable surfaces e.g in cities
where the ground surface is underlain by impermeable rocks
when ice dams melt and the waters in glacial lakes are released
where volcanic activity generates meltwater beneath ice sheets that is suddenly released (jökulhlaups)
where earthquakes cause the failure of dams / landslides that block rivers
tidal flooding, a result of storm surges or when high river flows meet particularly high spring tides in estuaries. a storm surge is caused by very low air pressure which raises the height of the high-tide sea. Strong onshore winds then drive the sea towards coast.
5.5b human actions that can exacerbate (increase) flood risk (changing land use within the river catchment, mismanagement of rivers using hard engineering systems.)
economic and population growth during 20th century has caused many floodplains to be built upon and many natural landscapes to be modified for agricultural, industrial and urban purposes.
human actions:
Impermeable areas of tarmac
Wells sunk to supply settlements
Sprinkling of groundwater onto arable crops
Dams built to supply towns with water
Streams channelled into culverts to aid drainage of farmland
natural streams meander & have marshy areas; but channelisation doesnt
Ploughing compacts soil
Grazing animals trample soil
Woodlands intercept rain and transpire moisture; roots give good soil structure. Deforestation destroys this.
Natural grasslands allow water to sink in, replaced by improved pasture
Bridge supports built in rivers, ramps on floodplain
Sewers feed water into channel
River mismanagement: (hard engineering)
channelisation: an effective way of improving river discharge and reducing the flood risk. The trouble is that it simply displaces the river downstream. Some other location may well be overwhelmed by the increased discharge
dams: block the flow of sediment down a river, so the reservoir gradually fills up with silt; downstream there is increased river bed erosion
river embankments: designed to protect from floods of a given magnitude. They can fail when a flood exceeds their capacity. Inevitably, when this happens, the scale of flooding is that much greater.
soft-engineering methods of reducing the flood risk are preferable e.g. making greater use of floodplains as temporary stores of flood water & using them for nature conservation, agriculture and recreation.
5.5c damage from flooding has both environmental impacts (soils and ecosystems) and socio-economic impacts (economic activity, infrastructure and settlement) (UK flood events, 2007 or 2012)
Socio-economic impacts death, injury, trauma spread of water-borne diseases damage to property & housing disruption of transport & communications interruption of water & energy supplies destruction of crops & loss of supplies disturbance of everyday life & work
Environmental impacts (there are some positives):
recharged groundwater stores
increased connectivity between aquatic habitats
soil replenishment
for many species flood events trigger breeding, migration and dispersal
Most ecosystems have a degree of ecological resilience that can cope with the effects of moderate flooding.
It is where the environment has been degraded human activities that negative impacts are more evident e.g. the removal of soil and sediment by floodwaters can lead to the eutrophication of water bodies (environment becomes enriched with nutrients). This can be a problem in marine habitats such as lakes as it can cause algal blooms. . That same floodwater can also leach pollutants into water courses with disastrous effects for wildlife, while diseases carried by floodwater can weaken or kill trees.
UK severe floods, wettest summer of 2007 (double the average)
Over 3 months in 2007 summer -> 397mm of rain fell
Rainfall along River Severn hit record levels, a storm sent the river into flood
Unpredictable flash floods occurred
economic cost 3.2billion
80% properties affected, 420 roads damaged.
cause: prolonged heavy rainfall, but at different times of the year.
no flood defences in place so 34 defences built protecting 30k homes.
There were recriminations after the apparent inadequacy of flood protection measures. The following were singled out for blame:
budget cuts in the amount of money being spent on flood defences
an EU Directive that puts environmental conservation ahead of the regular dredging of rivers
poor land management, resulting in blocked ditches
global warming
What tends to be forgotten in post-flood enquiries is that flood protection measures are designed to cope with flood events of a given magnitude. When an event of a very rare order of magnitude occurs, no amount of money or engineering is going to prevent the hoped for degree of protection.
5.6a Climate change affects inputs and outputs within the hydrological cycle: trends in precipitation and evaporation
Increasing greenhouse gases increase temp of sea & land = ENSO cycles happen more, evaporation increases = more moisture in atmosphere = more precipitation= more storms
Both evaporation and precipitation are increasing with climate change.
Evaporation over large areas of Asia and North America appears to be increasing. Transpiration is linked to vegetation changes, which in turn are linked to changes in soil moisture and precipitation. The length and frequency of heatwaves is increasing in some locations = increased occurrence of drought. With climate warming, more precipitation in northern regions is falling as rain rather than snow
5.6b Climate change affects both stores and flows, size of snow and glacier mass, reservoirs, lakes, amount of permafrost, soil moisture levels as well as rates and runoff and stream flow.
Stores
Surface runoff and stream flow = more low flows (droughts) and high flows (floods), increased runoff and reduced infiltration
Groundwater flow = uncertain, because of human abstraction
Flows
Reservoir, lake and wetland storage = changes in wetland storage cannot be linked to climate change, storage is decreasing as temperatures increase
Soil moisture = little change, higher precipitation and evaporation cancelling each other out, uncertain as soil moisture depends on many factors of which climate is only one where precipitation is increasing, soil moisture will also increase.
Permafrost = deepening of the active layer is releasing more groundwater, Methane released may be accelerating change
Snow = decreasing length of snow-cover, spring melt starts earlier, decreasing temporary store
Glacier ice = evidence of glacier retreat and thinning since the 1970s, decreasing because more precipitation is falling as rain
Oceans = more data on surface temperatures needed where ocean warming means more evaporation = more storms. storage capacity being increased by meltwater
rising sea level
