Australian Natural Resources Atlas

Natural Resource Topics

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:

Figure 8: A systems model of catchment function.
Figure 9: Model of catchment condition used to develop the assessment framework.
The city of Hobart at the bottom of the Derwent River catchment.

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:

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.

Table 4: Catchment condition indicator selection criteria.
Rationale
  1. Relevant to landscape function at the scale of intended use (e.g. large catchment) based on current knowledge/expert opinion
  2. Relevant to action planning, management, policies and regulations
  3. Sensitive to system change
Data availability
  1. Are the data available at the scale of intended use? (compilation scale)
  2. Cost limitations in acquiring and processing data
Data quality
  1. Are the methods of data collection and sources of error well documented?
  2. Is the variability in the data large enough to affect the interpretation of the attribute at the scale of intended use?
Meaning
  1. Has the attribute been validated to have meaning relative to the assessment question being asked and the scale of intended use?
  2. Does the attribute indicate a response in condition relative to management action?
  3. Understood widely by users?

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:

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.

Table 5: Indicators used to define the water, land and biota subindicies and the catchment condition index.
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).

Figure 10: Process for selecting catchment condition assessment indicators.
Table 6: Example of an application of the indicator selection process.
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
  • Changes to watertable depth
  • Salt stores (hazard)
  • Mobilisation risk
  • Area of cleared land in salt affected areas
Criteria Technical selection criteria (Table 4) test to reduce or remove poor measures
Indicator Area of cleared land in salt affected areas
Figure 11: Classification approaches used for catchment condition indicator and index values.
Figure 12: Number of subcatchments with predicted extent high salinity risk/hazard.

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