P3 Flashcards
Human impact on precipitation
Human activity can affect precipitation by cloud seeding: the introduction of silver iodide pellets, or ammonium nitrate, to act as condensation nuclei to attract water droplets. The aim is to increase rainfall in drought-stricken areas. It has variable results.
Human impact on evaporation and evapotranspiration
Changes in global land use, for example deforestation, are a key influence. Also important is the increased evaporation potential resulting from the enormous artificial reservoirs behind mega dams, for example the Aswan Dam and Lake Nasser in southern Egypt. Conversely, the channelisation of rivers in urban areas into conduits cuts down surface storage and, therefore, evaporation.
Human impact on interception
As interception is largely determined by vegetation type and density, deforestation and afforestation both have significant impacts.
Deforestation leads to a reduction in evapotranspiration and an increase in surface run-off. This increases fooding potential, leads to a decline of surface storage and a decrease in the lag time between peak rainfall and peak discharge. In other words, it speeds up the cycle. Research on deforestation in Nepal shows a range of negative impacts that have been linked to deforestation, including increases in the sediment load downstream in northern Nepal. Figure 1.10 summarises possible impacts of deforestation in the Himalayas in Nepal.
In theory, afforestation should have the reverse impact by trapping silt and slowing up the hydrological cycle by lengthening lag times. However, as a recent research project in the Plynlimon area of the catchment of the River Severn in Mid Wales showed, there is a period of time just after the planting of young trees where there is an increase in run-off and sediment loss as a result of compaction of soil by tractors and planting equipment, which only stops after 30 years when the trees are more fully grown.
Deforestation:
The cutting down and removal of all or most of the trees in a forested
Afforestation:
The planting of trees in an area that has not been forested in recent times.
Figure 1.10 The possible impacts of deforestation in the Himalayan foothills of Nepal
Human impact on infiltration and soil water
Human impacts on infiltration largely result from a change in land use. Infiltration is up to five times greater under forests when compared with grassland. With conversion to farmland there is reduced interception, increased soil compaction and more overland flow. This impact is summarised in Figure 1.11. Land-use practices are also important: while grazing cows leads to soil compaction by the trampling of animals, ploughing increases infiltration by loosening and aerating the soil. Waterlogging and salinisation are common if there is poor drainage, so installing drainage mitigates these problems.
Deforestation issues in Amazonia
The environmental impacts are likely to be severe because of the sheer scale of the deforestation in Amazonia. Over twenty per cent of the forest has been destroyed, at an accelerating rate in the last 50 years, by a combination of cattle ranching, large-scale commercial agriculture for biofuels and soya beans, general development of towns and roads, as well as legal and illegal logging.
As the Amazon forests contain 60 per cent of the world’s rainforests, the environmental impact on global life support systems is bound to be highly significant.
The trees act as ‘green lungs’ by removing CO2 as they photosynthesise and act as carbon sinks. Destruction of forests reduces this capacity, so adding to the global greenhouse gas emissions, especially in times of drought.
There is also an enormous impact on water cycling. In a forest environment 75 per cent of intercepted water is returned by EVT to the atmosphere, which reduces to around 25 per cent when the forest is cleared.
Ultimately, the drier climate can lead to desiccation ano further rainforest degradation. The El Niño-Southern Oscillation (ENSO) (see page 25) can lead to significant occurrence of droughts in Amazonia, which can exacerbate forest fires and further destruction.
The sheer scale of Amazonian destruction can have a very significant impact on the water cycle. As more water runs off into the Amazon drainage system, not only does this exacerbate the possibility of severe flooding and mudslides, it also leads to aquifer depletion, as less water infiltrates to recharge them.
Overland flow also increases the amount of soil erosion and degradation as nutrients are ‘washed away’.
Human impact on groundwater
Human use of irrigation for extensive cereal farming has led to declining water table levels in areas such as the Texan aquifers. The Aral Sea, between Kazakhstan and Uzbekistan, is an example of the damaging effects of the overextraction of water. The Aral Sea began shrinking in the 1960s when Soviet irrigation schemes for the growth of cotton took water from the Syr Darya and Amu Darya rivers, which greatly reduced the amount of water reaching the Aral Sea. By 1994, levels had fallen by 16 m, the surface area had declined by 50 per cent, the volume by 75 per cent, and salinity levels had increased by 300 per cent, with major ecological consequences.
In many British cities, including London, recent reductions in water-using manufacturing activity have led to less groundwater being abstracted. As a result groundwater levels have begun to rise, leading to a different set of problems, such as surface water flooding, fooding of cellars and basements in houses, and increased leakage into tunnels such as those used by the London Underground. The water supplies are also more likely to become polluted. circulation and, to an extent, land and sea bodies.
Only two zones - A and B, temperate and tropical - show a positive balance of run-off; they mark zones of convergence and uplift and subsequent precipitation.
Rivers flowing from these zones are vital in supplying zones of deficit (for example the Nile supplies Egypt’s deserts with vital water). Both climate change and human activities such as deforestation have the capacity to modify the situation in the long and short term.
the water balance varies considerably between continents,
with South America the most well-endowed continent and Africa the least. Run-off is divided into surface flow and base flow. This is an important distinction because in some places there are severe seasonal differences in surface flow (for example, monsoonal areas: at certain times of the year there may be a shortage of water but at other times a surplus. The base flow represents the usually available water. Although very generalised,
Table 1.3 does point to water supply problems on some continents, for example, in Africa precipitation and evapotranspiration are very similar, leaving little water to enter rivers as surface run-off. In contrast, South America has a large precipitation/evapotranspiration difference leading to high surface run-off.
Water budgets at a country or regional scale provide a more useful indication of available water supplies. At a more local scale, water budgets show the annual balance between inputs (precipitation) and outputs (EVT), and how this can impact on soil water availability.
The soil moisture budget is a subsystem of the catchment water balance and is of vital importance to agriculturalists.
Drainage basin water budgets are usually called water balances and are usually expressed using the following formula:
P = Q + E ‡ S
Where:
P = precipitation
Q = discharge (stream flow)
E = evapotranspiration
S = changes in storage
Figure 1.13 shows a water budget for southern England. In the UK, the annual precipitation exceeds evaporation in most years and in most places. Therefore, precipitation inputs exceed evaporation losses, so there will be a positive water balance. However, in some years of drought (for example, 1975-6 and 1995-6), and in some summer months, England has a temporary negative water balance.
River regimes
can be defined as the annual variation in discharge or flow of a river at a particular point or gauging station, usually measured in cumecs. Much of this river flow is not from immediate precipitation or run-off, but is supplied from groundwater between periods of rain, which feeds steadily into the river system from base water flow. This masks the Auctuations in stream flow caused by immediate precipitation.
British rivers flowing over chalk, for example the River Kennet, show this feature as well, as they maintain their fow even in very dry conditions, which is a result of base flow from the chalk aquifers.
The character of a regime of the resulting stream or river is influenced by several variable factors: p1
• The size of the river and where measurements are taken in the basin: many large rivers have very complex regimes resulting from varied catchments.
• The amount, pattern and intensity of the precipitation: regimes often reflect rainfall seasonal maxima or when the snow fields or glaciers melt (for snow the peak period is in spring, for glaciers it is early summer).
• The temperatures experienced: evaporation will be marked in summer as the temperatures are warmer.
The character of a regime of the resulting stream or river is influenced by several variable factors: p2
• The geology and overlying soils, especially their permeability and porosity: water is stored as groundwater in permeable rocks and is gradually released into the river as base flow, which tends to regulate the flow during dry periods.
• The amount and type of vegetation cover: wetlands can hold the water and release it very slowly into the system.
• Human activities, such as dam building, which can regulate the flow.
Overall the most important factor determining stream flow is climate. Figure 1.14 (page 16) shows how these factors lead to a variety of regimes.
Figure 1.14 Varied river regimes
Figure 1.15 Features of a storm hydrograph
Flashy river factors
Flat’ river factors
Figure 1.16 The impact of urbanisation on hydrological processes
Figure 1.17 The impact of the spread of urbanisation on hydrological processes
