4.3 How much change occurs over time in the water and carbon cycles? Flashcards
4.3 How much change occurs over time in the water and carbon cycles?
Key idea ➡ Human factors can disturb and enhance the natural processes and stores in the water and carbon cycles.
Dynamic equilibrium and the water and carbon cycles
-Land-use changes
-Water extraction
-Aquifers and artesian basins
-Fossil fuels and the carbon cycle
-Positive and negative feedback loops in the water and carbon cycles
Dynamic equilibrium and the water and carbon cycles definition
Most natural systems, unaffected by human activity, exist in a state of dynamic equilibrium. They are dynamic in the sense that they have continuous inputs, throughputs, outputs and variable stores of energy and materials. In the short term, inputs, outputs and stores of water or carbon will fluctuate from year to year.
In the long term, however, flows and stores usually maintain a balance, allowing a system to retain its stability.
Negative feedback loops within systems (Dynamic equilibrium and the water and carbon cycles)
In the long term, however, flows and stores usually maintain a balance, allowing a system to retain its stability.
Negative feedback loops within systems restore balance. In a drainage basin unusually heavy rainfall will increase the amount of water stored in aquifers. This in turn will raise the water table, increasing flow from springs until the water table reverts to normal levels.
In the carbon cycle, burning fossil fuels increases atmospheric CO₂ but at the same time stimulates photosynthesis. This negative feedback response should remove excess CO₂ from the atmosphere and restore equilibrium.
Land-use changes (Dynamic equilibrium and the water and carbon cycles)
-Urbanisation
-Farming
-Forestry
Urbanisation (Land-use changes (Dynamic equilibrium and the water and carbon cycles))
Urbanisation is the conversion of land use from rural to urban. Farmland and woodland are replaced by housing, offices, factories and roads; natural surfaces such as vegetation and soil give way to concrete, brick or tarmac.
These artificial surfaces are largely impermeable so they allow little or no infiltration and provide minimal water storage capacity to buffer run-off.
Drainage systems - Urbanisation (Land-use changes (Dynamic equilibrium and the water and carbon cycles))
Urban areas also have drainage systems designed to remove surface water rapidly (e.g. pitched roofs, gutters, sewage systems). As a result a high proportion of water from precipitation flows quickly into streams and rivers, leading to a rapid rise in water level.
Floodplains - Urbanisation (Land-use changes (Dynamic equilibrium and the water and carbon cycles))
In addition to changing land use, urbanisation also encroaches on floodplains. Floodplains are natural storage areas for water. Urban development on floodplains reduces water storage capacity in drainage basins, increasing river flow and flood risks.
Floodplains
The flat area around a river that is covered with sediment as a result of frequent flooding.
Farming (Land-use changes (Dynamic equilibrium and the water and carbon cycles))
Farming brings changes to vegetation and soils which have implications for the carbon and water cycles.
The clearance of forests - Farming (Land-use changes (Dynamic equilibrium and the water and carbon cycles))
The clearance of forest for farming reduces carbon storage in both the above- and below-ground biomass. Soil carbon storage is 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. Soil erosion invariably accompanies arable farming. Erosion by wind and water is most severe when crops have been lifted and soils have little protective cover.
Below-ground biomass
One of seven key agriculture, forestry, and land-use carbon pools. It includes all living biomass of live roots. Fine roots of less than 2 mm diameter are often excluded because these often cannot be distinguished empirically from soil organic matter or litter.
Above-ground biomass
One of seven key agriculture, forestry, and land-use carbon pools. It includes trees defined as generally 5 cm or greater in diameter (at breast height (1.31 m above ground)).
Arable farming
Growing crops on good land to be eaten directly, or to be fed to animals
Changes to the carbon cycle - Farming (Land-use changes (Dynamic equilibrium and the water and carbon cycles))
Changes to the carbon cycle are less apparent on pasture land or where farming replaces natural grasslands. For instance in North America, the net primary production of annual crops such as wheat on the Great Plains exceeds that of the original Prairie grasslands.
However, carbon exchanges through photosynthesis are generally lower than in natural ecosystems. In part this is explained by a lack of biodiversity in farmed systems, and the growth cycle of crops often compressed into just four or five months.
Interception of rainfall by crops - Farming (Land-use changes (Dynamic equilibrium and the water and carbon cycles))
Interception of rainfall by annual crops is less than in forest and grassland ecosystems. So too is evaporation and transpiration from leaf surfaces. 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 underdrainage increases the rate of water transfer to streams and rivers. Surface run-off increases where heavy machinery compacts soils. Thus peak flows on streams draining farmland are generally higher than in natural ecosystems.
Farming modifies the natural water cycle - Farming (Land-use changes (Dynamic equilibrium and the water and carbon cycles))
Farming also modifies the natural water cycle. Crop irrigation diverts surface water from rivers 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.
Forestry (Land-use changes (Dynamic equilibrium and the water and carbon cycles))
Forest management in plantations modifies the local water and carbon cycles. Changes to the water cycle are shown on pages 120-121.
Changing land use from farmland, moorland and heath to forestry increases carbon stores. In a typical plantation in the UK, mature forest trees contain on average 170-200 tonnes C/ha. This is ten times higher than grassland, and 20 times higher than heathland.
Water extraction (Dynamic equilibrium and the water and carbon cycles)
-Water extraction on the River Kennet catchment
Water extraction definition (Dynamic equilibrium and the water and carbon cycles)
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.
Water extraction on the River Kennet catchment (Water extraction (Dynamic equilibrium and the water and carbon cycles))
The River Kennet in southern England drains an area of around 1,200 km² in Wiltshire and Berkshire. The upper catchment mainly comprises chalk which is highly permeable. This groundwater contributes most of the Kennet’s 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, brown trout, water voles, otters and white-clawed crayfish.
Impacts on the regional water cycle - Water extraction on the River Kennet catchment (Water extraction (Dynamic equilibrium and the water and carbon cycles))
-Rates of groundwater extraction have exceeded rates of recharge, and the falling water table has reduced flows 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.
Aquifers and artesian basins (Dynamic equilibrium and the water and carbon cycles)
-Aquifers
-Artesian basins
Aquifers (Aquifers and artesian basins (Dynamic equilibrium and the water and carbon cycles))
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.