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In this section you will find detailed information, including national overviews and state and regional level assessments, from the 2000-2002 National Land and Water Resources Audit theme assessments.
Australian Catchment, River and Estuary Assessment 2002
National Land and Water Resources Audit, 2002
ISBN 0 642 37125 3
Assessing catchments
Catchment condition is a value judgement that depends on biophysical attributes interacting with social values and economic factors. A systems model of catchment function was developed to capture the main elements needed to define catchment condition (Figure 8).
This comparative assessment was limited to a biophysical definition of catchment condition with catchment function defined in terms of land, water and biota components (Figure 9). These components can be viewed through:
- individual indicators;
- combined indicators for each component (subindices); or
- combined across the three components into an overall catchment condition index.
The city of Hobart at the bottom of the Derwent River catchment.
Photo: Department of Primary Industries, Water and Environment, Tasmania
Selection criteria (Table 4) were developed and applied to 110 biophysical attributes to screen for suitable indicators. Criteria incorporated:
- data quality;
- ease of interpretation;
- compilation scale;
- relevance to catchment function; and
- relevance to management.
Fourteen indicators were selected (Table 5) and used to calculate the composite catchment condition index and subindices. Calculations were done using a data compilation system (CatCon) based on the ASSESS decision support system (Veitch 1997). This system allows spatially referenced indicators to be viewed, weighted, reclassified and combined to form composite indices within a geographic information system.
Rationale
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A five-point condition scale from better to poorer was used to rank and map relative catchment condition. The result is colour-coded maps for:
- individual indicators;
- composite subindices for water, land and biota condition; and
- an overall composite catchment condition index.
The CatCon system allows patterns for classes of relative catchment condition to be defined. These patterns provide the basis for decision support on priorities and opportunities for protective catchment management or remedial action.
Comparisons of catchment condition were made across Australia. The method also has application to a smaller number of catchments to determine relative ranking within a State/Territory or a drainage division (e.g. catchments draining to the Great Barrier Reef or the Murray-Darling Basin).
Regional differences in catchment function: building a framework to set targets
Catchment management issues and biophysical responses to similar land uses vary regionally due to differences in climate, land forms, soil types and land use patterns. Social and economic aspirations will also affect regional priorities for catchment management.
Weightings for indicators can be assigned from zero upwards to facilitate this type of comparative assessment within a region. As more data sets become available, additional indicators can be included, enhancing the query functionality of the system and its regional applicability as a tool for catchment management target setting.
| Indicators | Related catchment management issue |
|---|---|
| Water | |
| Suspended sediment load | Modelled post-settlement change in suspended sediment loads |
| Pesticide hazard | Pesticide use is a surrogate for pesticide pollution risk |
| Industrial point source hazard | Industrial pollution contamination risk |
| Nutrient point source hazard | Nutrient point source loading of waterways |
| Impoundment density | Ecosystem changes associated with altered flows |
| Land | |
| 2050 high dryland salinity risk/hazard | Modelled risk assessment of salinity impacts |
| Soil degradation hazard | Soil and land use assessment of soil degradation risk |
| Hill slope erosion ratio | Modelled assessment of changes in hill slope erosion potential from natural conditions |
| Biota | |
| Native vegetation fragmentation | Deterioration in native habitat |
| Native vegetation extent | Habitat quantity and distribution |
| Protected areas | How much habitat is protected |
| Road density | Human population and land use intensity pressures |
| Feral animal density | Extent feral animals have impacted on native biota |
| Weed density | Extent of disturbance to native vegetation |
Indicators for assessment
The indicator approach (Figure 10, Table 6) for assessing catchment condition selects indicators either as specific or aggregated measures.
The approach recognises that broad-scale data sets are often more readily available and better depict regional pattern than fine-scale data. Broad-scale coverages are usually generalised from detailed data and so tend to highlight the predominant biophysical processes and characteristics that determine catchment condition. The major benefit of broad-scale data in decision-support systems is a clearer identification of key and dominant patterns than can be provided through aggregating a collection of discontinuous and inconsistent fine-scale data sets.
Indicator values were ranked into five classes reflecting values from poorer to better condition. Indicators or composite indices can either be ranked on equal intervals or equal areas under a frequency distribution curve (Figure 11). Indicators had extremely skewed distributions and an equal area ranking was found to be the most appropriate. Composite indices had distributions that approached a normal distribution and equal interval rankings were used.
Maps based on rankings are useful for comparing relative conditions, but do not convey actual values. The probability that an interpretation in relative terms will be meaningful increases at a national scale. Histograms showing the number of catchments versus indicator value are a useful tool for estimating the range of values associated with a particular group of catchments (Figure 12).
| Societal value | Good land condition/sustainable landscapes |
|---|---|
| Issue | Loss of crop production due to salinisation |
| Assessment | What areas are at risk to future salinisation |
| Attribute |
|
| Criteria | Technical selection criteria (Table 4) test to reduce or remove poor measures |
| Indicator | Area of cleared land in salt affected areas |
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