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Catchment water quality

The Parks and Wildlife Service, in collaboration with the Department of Water and Environmental Regulation (DWER), conducts fortnightly monitoring at 32 sites within 27 of the 31 Swan Canning sub-catchments. Data from these monitoring programs is reported in a number formats including quarterly catchment data reports and sub-catchment nutrient reports.

Quarterly catchment water quality data reports

Data collected fortnightly as part of the catchment water quality monitoring program is summarised each quarter in a report produced by DWER. This quarterly data report presents background and current data representing 27 of the 31 sub-catchments of the Swan Canning river system.

Reports up to 2015 have focused on 15 priority catchments, however data collected from a further 12 sub-catchments will be presented in reports from 2016, depending on data availability.

Data is presented in graphical and tabular format on a catchment-by-catchment basis for each of ten key water quality parameters:

  • Total nitrogen
  • Ammonium nitrogen
  • Total oxidised nitrogen
  • Dissolved organic nitrogen
  • Total phosphorus
  • Soluble reactive phosphorus,
  • Dissolved organic carbon,
  • Total suspended solids,
  • Dissolved oxygen
  • Specific conductivity

Annual reporting against catchment targets

The Parks and Wildlife Service's key performance indicators of water quality for the estuary and its catchment are:

  • total nitrogen and total phosphorus concentrations in 15 priority sub-catchments
  • chlorophyll-a concentration and dissolved oxygen saturation in surface waters of the estuary

Targets for catchments

Median total nitrogen and total phosphorus targets have been developed for catchment tributaries of the Swan Canning river system. In recognition of the long timeframes required for catchment management to affect nutrient levels in tributaries, both short term and long-term targets have been developed.  The long-term targets are based on Australian and New Zealand Environment and Conservation Council (2000) trigger values for near pristine rivers while the short-term targets are double the ANZECC recommendations.

Target Total nitrogen (mgL-1) target Total phosphorus (mgL-1) target
Short-term 2.0 0.2
Long-term 1.0 0.1

Long-term (over 25-40 years) load reduction targets for nitrogen and phosphorus have been established through the Swan Canning Water Quality Improvement Plan.  Although performance against these nutrient load reduction targets is not reported on annually, sub-catchment nutrient reports regularly provide information on nutrient loads from monitored sub-catchments. The nutrient load reduction targets guide long-term investment to improve water quality in those catchments contributing the greatest nutrient loads to the rivers.

Sub-catchment nutrient reports

Data that is collected fortnightly as part of the catchment water quality monitoring program is used to determine the nutrient concentrations and loads entering the Swan and Canning rivers. Long-term trends in this nutrient data are calculated every five years. However, annual calculations of total nitrogen (TN) and total phosphorus (TP) loads are used to report against catchment nutrient targets.

Five-yearly reporting

Nutrient data collected from 15 priority sub-catchments is compiled into 'catchment nutrient reports', which are produced every five years. In these reports, long-term (since 1999) trends in total nitrogen (TN) and total phosphorus (TP) concentrations are calculated and used as a basis for comparison to determine if the nutrient management actions put in place by the Parks and Wildlife Service are having a positive impact. These reports also include:

  • Brief descriptions and maps of each catchment, including size, land use, climate data and annual flows (where available)
  • Analysis of nutrient speciation (e.g. ammonium, oxidised and organic nitrogen as components of total nitrogen)
  • Examination of seasonal variation in nutrient concentrations
  • Exploration of nutrient reduction strategies targeted to each individual catchment
  • Calculation of annual concentrations and loads of TN and TP to assess catchment-by-catchment progress against nutrient targets

The most recent five-yearly nutrient reporting for 15 individual priority sub-catchments was undertaken in 2011, and another 12 in 2014.

The next five-year trend calculation, incorporating all data collected up to (and including) the 2016 calendar year will be available in the second half of 2017. 

Annual reporting

The annual re-calculation of nutrient trends for each catchment is not appropriate because of the high inter-annual variability in nutrient concentrations/loads (which is very dependent on rainfall patterns). However, reporting of annual nutrient concentrations and loads from each catchment is calculated each year to track progress against targets. As a result, the front page of each five-year catchment report is updated annually to provide a catchment-by-catchment measure of annual TN and TP concentrations and loads entering the Swan and Canning rivers.

The most recent annual updates of the nutrient reports use data collected in 2014.