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  3. SALINISATION Salinisation (the accumulation of salts i

SALINISATION Salinisation (the accumulation of salts i

游客2024-01-09  22
问题                       SALINISATION
    Salinisation (the accumulation of salts in soil) is one of today’s worst environmental disasters and yet it does not share the global spotlight with other ecological issues. Considering the threat that salinisation poses to nearly all irrigated drylands and the consequential impact on traditional agriculture, such lagging public awareness is, at the very least, disconcerting. Reports indicate that between 2.5 and 6 million hectares of land are affected by salinisation and unless precautionary and remedial measures are implemented, economic and environmental repercussions will be inevitable.
    Contributing ecological factors such as soil-type, climate, rainfall and topography make some lands more vulnerable to the salinisation process. However, the vast majority of land degraded through salinisation can be directly linked to human activity, whether it be destroying natural vegetation and bushlands to clear the land for farms, over-irrigation of cultivated land and/or poor farming practices. Countries like Australia are beginning to see the devastation of increasing salinity levels after a relatively short period of introduced European farming methods.
Salts such as sodium chloride and calcium sulphates, occur naturally in many soils and waterways. When the concentration of salt levels (salinity) in soil becomes too high, plant growth is adversely affected and the soil structure can be damaged.
Nature dealt reasonably successfully with salinity levels in Australia prior to European settlement. Natural vegetation, including perennial grasses and deep-rooted trees, ensured that salts were dissolved as rainwater slowly filtered down through the soil. Salt was moved downward and remained in the groundwater below the root zone of the plants. The fibrous root structure of those native plants, acted as an effective filter with their high water-holding capacity. Some rainwater was held by the roots and some rainwater leached downwards through the soil carrying the dissolved salts. Groundwater transpired back into the air through the foliage of the native plants.
    As Europeans introduced traditional forms of agriculture and cleared large areas of land for grazing in the 1800s, much of the native vegetation in Australia was replaced by generally shallow-rooted annual crops and pastures, substantially changing the natural process that had been working well. These crops and pastures use less water than original native vegetation and the roots do not have the same water-holding capacity. With the introduction of these crops, more rainwater and irrigated water makes its way downwards through the soil to below the root-zone. The water still carries the dissolved salts downwards past the root-zone but the amount and level of groundwater is increased. In addition, because the roots of these crops do not hold as much water, the water is drawn back up from the groundwater later, as the plant needs moisture.
    We see then that the amount and level of groundwater increases because of an increase in the amount of water being put into the soil and not being held in fibrous root structures like that of native vegetation. This causes the water table to rise, bringing dissolved salts with it that eventually reach the surface of the soil. Water is then evaporated from the surface leaving high concentrations of salts behind. This is the process of salinisation.
    Soils with high salinity levels occur naturally in Australia but these were mostly confined to particular coastal and sub-coastal areas. Over the past 200 years, the total area affected by salinisation has expanded and has now spread to inland areas. Groundwater salts that have been accumulating over thousands of years are now rising to the surface. When this groundwater enters the root zone of the cultivated plants which are naturally not salt tolerant, the plants die. The effects are not limited to the particular cleared site where soil becomes unsuitable for plant production. Salty groundwater can travel along the natural contours of the land into other agricultural areas, creating salty discharge sites quite some distance from the recharge zone. Native aquatic and land-based habitats are now at risk, threatening the biological diversity in Australia. It is therefore necessary for groundwater and surface run-off water to be dealt with.
    Land managers or farmers do not have to wait until crops die to recognize that salinity levels are out of control. Declining yields in crop production, sick or dying trees around the property or the appearance of salt-tolerant species all serve as a warning that salinity levels have increased. If these signs are ignored and the lands become degraded, combating salinity will become expensive and time-consuming.
    One current practice is to replant trees in an effort to draw the water table down and slow the salinisation process but this alone will be inadequate. Investigations are also being made into planting salt-tolerant crops and pastures whilst building up and preserving native species and remnant bushland areas. More effective techniques to counteract the drainage problems in the form of drainage canals are also being canvassed. These hope to achieve a balance between the volume of water entering the soil in the recharge zone and the volume of water that leaves as discharge.
    The key to fighting salinity is through long-term management practices on agricultural land that recognize the importance of the role that native vegetation plays in keeping water balance in the soil. These practices will need to consider agricultural requirements along with land and water care i.e. balancing economic development with environmental protection. Moreover, a public awareness campaign with government-funded incentives must make it clear that current farming practices in at-Ask landscapes are not sustainable and cannot be tolerated.
    GLOSSARY
    salinisation   - the build-up of concentrations of salt levels within the soil
    groundwater    - water that is held in the soil
    water table    - the level of groundwater
    transpiration  - the loss of water through the leaves of plants
    Complete the summary below. Choose words and phrases from the box below the summary and write your answers in boxes 28-35 on your Answer Sheet.
    Note: use each word or phrase ONCE only. [br] *
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答案 surface
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