Scoring river condition
A suite of condition indicators have been developed for some of the key elements of our river health assessments. A single score can be calculated for each of these indicators, which allows results at a single site to be easily compared to other sites across the south-west of Western Australia, and through time.
Scores are primarily designed for use as a first-pass assessment, identifying potential relationships between response-variables and variables indicating stress or pressure on the system.
Examining SWIRC indicator scores can help in determining where more detailed assessments may be required (such as to investigate possible threats to reduced condition). Condition indicators based on desktop data can be scored for large areas in relatively short time-frames and, as such, can be particularly effective in identifying priority areas for the costlier field assessments.
Note: We currently do not have condition indicators for our aquatic habitat theme, but we are working on it.
The scoring system used for the SWIRC indicators complies with national assessment programs such as the National framework for the assessment of river and wetland health and the Aquatic Ecosystems Toolkit's Integrated Ecological Condition Assessment Framework. As a result, the scoring system can be used to generate data for national comparison and reporting purposes.
Creating a condition score
SWIRC scoring protocols are based on a reference condition approach, with each score providing a measure of the departure of the observed values from expected values. The expected values are those typically anticipated under minimal disturbance conditions, and can be derived from historical data, data from minimally disturbed sites or expert opinion. How expectations were derived for each indicator of the SWIRC is described Storer et al 2011a and Storer et al 2011b.
The SWIRC scores are calculated on a scale between 0 and 1 (lower scores representing poorer condition or greater departure from the reference condition) and can be divided into several condition bands – such as those shown in the diagram.
The SWIRC indicators follow a nested structure (see the SWIRC scoring-disk diagram on the top of this page) – with each of the six themes represented by a single ‘index’ score. This score is comprised of scores from one or more sub-indices which, for some, are derived from scores of several component indicators.
Use in management
Management targets, triggers or thresholds are not included in the SWIRC standard methodology as these need to be determined on a case-by-case basis—relative to the specific management question of the condition assessment. However, incorporating these levels on the SWIRC condition scale can be an effective management and communication tool. For example, these can be used to reflect a state we want to achieve or the point where intervention needs to occur. See example for a hypothetical assessment of water quality.
Comparing condition across areas
The following examples illustrate a few options for using the SWIRC scores to compare conditions between sites and reaches.
The example on the right shows SWIRC scores for fringing zone width for all south-west rivers. This is a reach-based indicator that reflects the percentage cover of vegetation within a 50 m corridor on both banks of the river.
This level of assessment could help in understanding the effect of broad-scale pressures such as land use type or climate change, or to identify areas requiring more detailed investigation. When combined with information on values (such as biodiversity), this can be an effective tool in prioritising management effort.
Integrated condition scores
The example on the left shows the results of a full SWIRC assessment—all indicators—of four sites and their respective reaches.
As with the previous example, this level of assessment can be used to identify management concerns whilst enabling some degree of interrogation. For example, a loss in condition in water quality or aquatic biota could be linked with stressors associated with changes in fringing zone, hydrological connectivity or land use.