3.a. Human factors can disturb and enhance the natural processes and stores in the water and carbon cycles Flashcards
How do most natural systems exist?
- Most natural systems unaffected by human activity exist in a state of dynamic equilibrium
- Dynamic in the sense that they have continuous inputs, throughputs, outputs and stores of energy and materials
What does dynamic equilibrium mean?
- Short term - inputs, outputs and stores of water or carbon fluctuate from year to year
- Long term - flows and stores usually maintain balance, allows a system to retain stability - negative feedback loops within systems restore balance
Examples of negative feedback loops?
- In drainage basin, unusually heavy rainfall will increase amount of water stores in aquifers, this then raises the water table, increasing the flow from springs until the water table reverts to normal levels
- Carbon - burning fossil fuels increases atmospheric CO2, but also stimulates photosynthesis, negative feedback response should remove excess CO2 from the atmosphere and restore equilibrium
What is urbanisation?
Conversion of land use from rural to urban, farmland and woodland replaced by housing, offices, factories and roads
How does urbanisation affect the water cycle?
Natural surfaces such as vegetation and soil give way to concrete, brick, tarmac, etc - these artificial surfaces are largely impermeable and provide minimal water storage capacity to buffer run-off
Urban areas also have drainage systems designed to remove surface water rapidly (eg. pitched roofs, sewerage systems), which means that streams and rivers which drain urban areas have characteristically short lag times and high, but short-lived, peak flows
In addition to changing land use, urbanisation also encroaches on floodplains, which are natural storage areas for water - urban development on floodplains reduces water storage capacity in drainage basins, increasing river flow and flood risks
How does farming impact stores in the water cycle?
Soil erosion by wind and water is most severe when crops have been lifted and soils have little protective cover
How does farming impact flows in the water cycle?
Crop irrigation diverts surface water from rives and groundwater to cultivated land, some of this water is extracted by crops from soil storage and released by transpiration, but most is lost to evaporation and in soil drainage
Interception of rainfall by annual crops, evaporation and transpiration from leaf surfaces are all less in farmland compared to forest and grassland ecosystems
Ploughing increases evaporation and soil moisture loss, and furrows ploughed downslope act as drainage channels, accelerating run-off and soil erosion
Infiltration due to ploughing is usually greater in farming systems, while artificial under drainage increases the rate of water transfer to streams and rivers, surface runoff increases where heavy machinery compacts soil, therefore peak flows on streams draining farmland are generally higher than in natural ecosystems
How does farming impact stores in the carbon cycle?
Clearance of forest for farming reduces carbon storage in both the above and below ground biomass
Soil carbon storage also reduced by ploughing and the exposure of soil organic matter to oxidation
Further losses occur through the harvesting of crops with only small amounts of organic matter returned to soils
How does farming impact flows in the carbon cycle?
Where farming replaces natural grassland, carbon exchanges are generally lower than in natural ecosystems - this is in part explained by a lack of biodiversity in farmed systems and the growth cycle of crops are often compressed into just four or five months
How does forestry impact the water cycle?
- Higher rates of rainfall interception in plantations than in natural forest east, in eastern England, interception rates for Sitka spruce are as high as 60%, in upland Britain where temperatures and evaporation are lower, interception is about 30% - in the UK preferred plantation species are conifers, the needle-like structure of consider leaves, their evergreen habit and high density of planting all contribute to high rates of interception
- Increased evaporation, a large proportion of intercepted rainfall is stored on leaf surfaces and is evaporated directly to the atmosphere
- Reduced run-off and stream discharge, with high interception and evaporation rates and the absorption of water by tree roots, drainage basin hydrology is altered, streams draining plantations typically have relatively long lag times, low peak flows and low total discharge, the effect of conifer plantations in upland catchments is often to reduce water yield for public supply
- Compared to farmland and moorland, transpiration rates are increased, typical transpiration rates for Sitka spruce in the Pennines are around 350mm/yr-1 of rainfall equivalent
- Clear felling to harvest timber creates sudden but temporary changes to the local water cycle, increasing run-off, reducing evapotranspiration and increasing stream discharge
How does forestry impact the carbon cycle?
- In a typical plantation in the UK, mature forest trees contain on average 170-200 tC/ha^-1, which is 10x higher than grassland and 20x higher than heathland
- The soil represents an even larger carbon pool, in England measurements of forest soil carbon are around 500 tC/ha^-1
- Forest trees extract CO2 from the atmosphere and sequester it for hundreds of years, most of the carbon is stored in the wood of the tree stem, however forest trees only become an active carbon sink (i.e. absorbing more carbon than they release) for the first 100 years after planting
- After this, the amount of carbon captured levels off and is balanced by inputs of litter to the soil, the release of CO2 in respiration and by the activities of soil decomposers
- In consequence, forestry plantations usually have a rotation period of 80-100 years, after this time the trees are felled and reforestation begins afresh
What is water extraction?
Water is extracted from surface and groundwater to meet public, industrial and agricultural demand, direct human intervention in the water cycle changes the dynamics of river flow and groundwater storage
What is the significance of the River Kennet?
- River Kennet in southern England drains an area of around 1200km^2 in Wiltshire and Berkshire
- Upper catchment mainly comprises chalk which is highly permeable, so groundwater contributes most of the Kennet’ flow
- As a chalk stream, the river supports a diverse range of habitats and wildlife, its water filtered through the chalk has exceptional clarity, high oxygen levels and is fast flowing - among the native fauna are Atlantic salmon, water voles and white-clawed crayfish
- Within and close to the catchment, several urban areas rely on water from the Kennet basin to meet public supply - Swindon, the largest, has a population of over 200,000
- The Kennet also supplies water for local industries, agriculture and public use, Thames Water abstracts groundwater from the upper catchment from boreholes - none of this water is returned to the river as waste water
What effects has water extraction had on the river and regional water cycle?
- Rates of groundwater extraction have exceeded rates of recharge, and the falling water table has reduced lows in the River Kennet by 10-14%
- During the 2003 drought flows fell by 20% and in the dry conditions of the early 1990s, by up to 40%
- Lower flows have reduced flooding and temporary areas of standing water and wetlands on the Kennet’s floodplain
- Lower groundwater levels have caused springs and seepages to dry up and reduced the incidence of saturated overland flow on the chalk
What are aquifers?
- Aquifers are permeable or porous water-bearing rocks such as chalk and New Red Sandstone
- Groundwater is abstracted for public supply from aquifers by wells and boreholes, emerging in springs and seepages, groundwater feeds rivers and makes a major contribution to their base flow
- Within an aquifer the upper surface of saturated is known as the water table, its height fluctuates seasonally and is also affected by periods of exceptional rainfall, drought and abstraction
- In normal years in southern England, the water table falls between March and September as rising temperatures increase evapotranspiration losses - recharge resumes in the late autumn
What are artesian basins?
When sedimentary rocks form a syncline or basin-like structure, an aquifer confined between impermeable rock layers may contain groundwater which is under artesian pressure - if this groundwater is tapped by a well or borehole, water will flow to the surface under its own pressure, this is known as an artesian aquifer
The level to which the water will rise, the potentiometric surface, is determined by the height of the water table in areas of recharge on the edges of the basin
London is located at the centre of a synclinal structure which forms an artesian basin, groundwater in the chalk aquifer is trapped between impermeable London Clay and Gault Clay
Rainwater enters the chalk aquifer where it outcrops on the edge of the basin in the North Downs and Chilterns, groundwater then flows by gravity through the chalk towards the centre of the basin, thus under natural conditions the wells and boreholes in the London area are under artesian pressure
Groundwater from the chalk is an important source of water for the capital, however overexploitation in the 19th century and in the first half of the 20th century caused a drastic fall in the water table - in central london it well by nearly 90m
In the past 50 years declining demand for water by industry in London and reduced rates of abstraction have allowed the water table to recover - by the early 1990s it was rising at a rate of 3m/yr^-1 and began to threaten buildings and underground tunnels
Since 1992, Thames Water has been granted abstraction licenses to slow the rise of the water table which is now stable