The National Land & Water Resources Audit concludes that about 40 per cent of the land in mid to low rainfall (<700 mm) agricultural catchments requires strategic revegetation with woody perennial plants to have a significant impact on the rate of water and land salinisation. Increasing vegetation cover on this scale, using commercial tree crops, would also reduce greenhouse gas emissions (see Section 5.2d), stimulate regional development and provide employment in local processing and manufacturing industries. Hardwood and softwood plantations are being utilised as an important response to the need to increase revegetation and to meet timber supplies into the future (Figure 24). Plantations can provide environmental benefits such as soil erosion protection, improved water quality and reduced dryland salinity, increased carbon storage and increased habitat for some native animals. For example, a study of the biodiversity benefits of hardwood plantations showed they represented a 15–25 per cent improvement for vertebrate species compared to agricultural land, which could be an important addition to the potential conservation value of the overall landscape. The plantation area has expanded by about 600,000 hectares since 1990. Government and industry share a nominal target of 3 million hectares of plantations by 2020 to meet regional industry development and environmental objectives. Figure 24: The ‘Green Triangle’ region of South Australia and Victoria showing larger consolidated blocks of older pine plantations and more recently established, scattered areas of eucalypt plantations.
Source: Keenan RJ, Gerrand A, Nambiar S, Parsons M (2004) ‘Science for Decision Makers: Plantations and Water: Plantation impacts on stream flow.’ Bureau of Rural Sciences, Canberra ACT
In recent times there have been concerns raised that planting large areas of a catchment could lead to reduced water flow, which in turn would affect agricultural industries and town water supplies. The range of different responses has led to some confusion. To help address this, 18 representatives from a range of government and scientific agencies released an agreed statement in late 2003 concerning scientific knowledge of the impact of forest plantations on catchment water yield where there have been conflicting or contrasting views. Box 8 gives an example of one of the issues identified and the agreed response on the state of scientific knowledge. Box 8: An example of one of the issues addressed in the statement on scientific knowledge of the impact of forest plantations on catchment water yield.
Source: Bureau of Rural Sciences (2003) ‘The Impact of Plantations on Water Yield: A statement clarifying key scientific issues.’ Bureau of Rural Sciences,
Canberra ACT
This statement is complementary to a joint statement by CSIRO Divisions of Forests Products and Land and Water covering some of these issues entitled Maximising the Benefits of New Tree Plantations in the Murray-Darling Basin. Both statements refer to a range of recent references that would be useful to regional managers. Plantations and Water Use provides greater detail on the complex interactions that affect the quality and quantity of stream-flow. The report focuses on five regions with potential for development of new plantations: south-west Western Australia, the ‘Green Triangle’ (south-east South Australia and south-west Victoria), south-east Queensland, northern Tasmania and the Murray-Murrumbidgee region. These regions include over one million hectares of plantations, about two-thirds of Australia’s total plantation area. Regions such as northern Tasmania and south-west WA have a significant area of hardwood plantations – both maps and statistics of areas under different vegetation cover are provided. The report synthesises findings from a number of projects and highlights areas for further research. It also identifies issues for regional managers. For example, many results are based on small catchments which means bias may be introduced if the results are extrapolated to larger catchments. As well, plantation management regimes are changing, with lower initial stocking and heavier thinning, so that research based on historical studies may not apply to current circumstances. While it is important to consider the potential for reduced run-off effects in the planning of extensive revegetation, the strengths and limitations of current modelling must also be considered. The Science for Decision Makers: Plantations and Water provides a useful summary of the longer report described above. It concludes that conversion of agricultural land to plantations may reduce run-off and stream flows, especially if this occurs across a large part of a catchment. In most places however, plantations make up only small proportions of catchments, but can contribute positively to regional economies and provide environmental benefits. With appropriate catchment selection, planning and management, plantations can be a viable and positive part of the rural economy with minimal impacts on stream flow. The challenge is to apply scientific knowledge to develop policies and institutional arrangements that provide for plantation development in those parts of selected catchments where they have the most commercial and environmental benefits with little impact on water flow.
| | | | Action: 5.3 |
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| | If plantations are being considered as an option in your region, read the summary and longer reports on plantations and water yield to appreciate the planning and management actions needed. | | | COST |  | | | TIME |  | | | COMPLEXITY |  | | | | |
The degradation of surface water and groundwater supplies by contaminants such as sediment, nutrients, salts or chemicals is a significant issue for regional managers. The clearing of catchments for agricultural land, soil disturbance during forestry operations or urban development, and bare areas such as gravel roads and stock tracks, have led to substantial increases in the amounts of sediment (gravel, sand, silt and clay) entering streams and rivers. The National Land & Water Resources Audit reported that river sediment loads are generally 10 to 50 times greater than pre-European loads in intensively-used river basins, and 90 per cent of the suspended sediment loads reaching estuaries comes from only 20 per cent of catchment areas . The sediment and associated nutrients and chemicals can contaminate human and stock water supplies, smother breeding sites for fish and other in-stream animals and, by filling up stream pools, deprive aquatic animals of the deeper waters that are a vital refuge in dry seasons and prolonged droughts. Whatever the specific impact, the end result is likely to be significantly decreased water quality. Basin Wide Mapping of Sediment and Nutrient Exports in Dryland Regions of the Murray-Darling Basin synthesised information on sediment and nutrient transport, building on earlier investigations undertaken for the Audit. Spatial modelling was used to quantify the patterns and rates of current sediment and nutrient transport, and methods were developed for effectively controlling sediment and nutrient sources, and quantifying the benefits resulting from different investment scenarios. Tools were developed to help inform decision-making on achieving end-of-valley targets, the most effective means of controlling suspended sediment loads, identifying priority management areas, and demonstrating water quality benefits downstream for different levels of investment and different investment strategies. Results show that effective targeting of restoration work can reduce the cost by many times.
| | | | Action: 5.4 |
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| | Read the report on sediment and nutrient exports to see how careful targeting of restoration works can both reduce costs and improve water quality. | | | COST | | | | TIME | | | | COMPLEXITY | | | | | |
Drawing on more detailed information from two riparian management technical reports, the fact sheet on Improving Water Quality indicates that under favourable conditions, both natural vegetation and grassy filter strips can trap around 90 per cent of the sediment moving from upslope (Figure 25). These strips can be just as effective in trapping or absorbing nutrients. Vegetation (for example most grasses) can quickly grow over and through the trapped sediments, thereby protecting them from future storms. It is important to recognise that, although riparian filter strips can be effective in preventing sediment and nutrients from reaching streams, and thereby help to protect and improve water quality, they are not a substitute for good land management elsewhere. Leaving existing native vegetation on riparian land, or replanting with native species, can have multiple benefits over and above the simple measure of using grassy filter strips. Figure 25: How a riparian buffer strip functions to protect streams from contaminants from the surrounding catchment.
Source: Price P, Lovett S (2002) ‘Improving Water Quality.’ Land & Water Australia, Fact Sheet 3, Canberra ACT
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