Monthly Water Update

The Monthly Water Update provides an overview of rainfall, streamflow, stream salinity and storages across Australia.

Methodology

Details of methods used for streamflow, streamflow salinity, rainfall and storages are given in the table below:


Method for Rainfall Streamflow Streamflow salinity Storages
Data The 5 x 5 km monthly decile grid data is obtained from the Bureau's climate records. The data for measured streamflow (ML/day) are obtained for selected gauges for the period from July 1980 up to the month being analysed.

Salinity is expressed as electrical conductivity (EC) in µS/cm at 25°C. EC is a commonly used surrogate for the measurement of water salinity in Australia.

Data are obtained for selected gauges for the period from 2000 up to the month being analysed.

The total storage capacity (GL) and volume (GL) at end of the reporting month data for selected storages are sourced from the Bureau's database.

To calculate the 'aggregated volume' for a system on the end date of the month, the most recent volume are added up for each storage in the collection - provided the information is less than 30 days old. If the information is more than 30 days old it is left out of the calculation. If too many of the storages are left out of the calculation, then the aggregated volume is not considered a valid estimate of the volume held in the system. System will be missing if the accessible capacity of the missing storages is greater than 7% of the capacity of the system.

Analysis The Bureau's standard spatial climate data analysis and presentation methods are used. Monthly deciles grids are based on the long-term record (since 1900). Flow deciles are determined from monthly streamflow for the reporting month ranked against the historical record of data for the same month since July 1980.

Medians of monthly streamflow salinities are calculated and coloured-coded relative to the following classes of EC in µS/cm:

  • <400 (high quality)
  • 401–800 (Fresh)
  • 801–1500 (Brackish)
  • 1501–5000 (Moderately saline)
  • >5000 (saline)

This is done to give an indication of the usability of the water. The first two classes roughly coincide with a usability range for human drinking. The next two ranges extend use to include stock drinking water and irrigation water, respectively. The final category is of limited value without further treatment to remove salts.

The Bureau's standard data analysis methods are used for calculating the storage capacity and the percentage full. See the Bureau's Water Storage website ( www.bom.gov.au/water/dashboards/#/water-storages/summary/state) for further details.

Storage volumes at the end of the reporting month are presented as a circle, with size of the circle representing the capacity and the colour showing how full it was at the end of the month.

The colours are graded to represent differences in the fullness of the storage. Dark orange representing empty or close to empty and dark blue representing storages that are full or almost full.

Gauge selection See Bureau's Climate records website for details.

Gauges were selected for the lower reaches of each of the river regions in the drainage division where suitable data was available. This provides a view of total outflow from the river region.

In the case of larger river regions, additional gauges were selected to represent the major, middle and upper basin tributaries to show the varying status of the major rivers across the river region.

Gauges were selected which:

  • are in ongoing operation
  • possess largely complete records from July 1980
  • are available from the Bureau's data archives.

Gauges were selected in terms of availability and quality of data in major rivers, creeks and tributaries to represent the region. Gauges were selected which:

  • are in ongoing operation and have a history of at least five years to allow some statistical analysis;
  • have sites located where the water in the river is mixed and away from the direct influence of point source pollution such as drains and outlets;
  • have records that where there are limited data gaps and have an appropriate measurement frequency;
  • are available from the Bureau's data archives.

Water storages in the systems were selected based on their representation of the total system storage and system behaviour, and upon suitable data availability from 2005 up to the month being analysed.

See Bureau's Water Storage website for details.

Data quality See Bureau's Climate records website for details.

Provisional streamflow data is made available to the Bureau by various data providers shortly after the data are entered in their databases. Please note that this data is not fully verified by providers.

This data is used in the analysis to provide a timely view of the recent flows. If more than 20% of the daily records in a particular month are missing the decile ranking is not computed and the gauge is marked as having missing data. The 20% missing data rule is also applied to the historic time-series from which the monthly decile rankings are calculated.

Provisional EC data is made available to the Bureau by various data providers shortly after the data are entered in their databases. This data is not fully verified by providers but it is used in the analysis to provide a timely view of recent salinity conditions.

If more than 20% of the daily records in a particular month are missing the median is not computed and the gauge is marked as having missing data.

See Bureau's Water Storage website for details.
Decile ranking

To calculate the proportion of rainfall in each decile group, data is ranked into ten equal parts and deciles are calculated using rainfall from 1900.

  • Lowest on record and very much below average – together represent rainfalls in the lowest 10% of historical totals.
  • Below average - rainfalls in the lowest 30% of historical totals, but not in the lowest 10%.
  • Average - rainfalls in the middle 40% of historical totals.
  • Above average - rainfalls in the highest 30% of historical totals, but not in the highest 10%.
  • Highest on record and very much above average – together represent rainfalls in the highest 10% of historical totals.

The colours are graded to represent the amounts of rainfall from blue (above average) to white (average) and red (below average).

To calculate the proportion of streamflow sites in each in each decile group, data is ranked into ten equal parts and deciles are calculated using flows from 1980.

  • Lowest on record and very much below average – together represent streamflows in the lowest 10% of the historical totals.
  • Below average - streamflows in the lowest 30% of the historical totals, but not in the lowest 10%.
  • Average - streamflows in the middle 40% of the historical totals.
  • Above average – streamflow in the highest 30% of the historical totals, but not in the highest 10%.
  • Highest on record and very much above average – together represent streamflows in the highest 10% of the historical totals.

The colors are graded to represent the streamflow from blue (above average) to white (average) and red (below average).

Below average – EC in the lowest 30% of historical totals including the lowest 10% since 2000.

Average - EC in the middle 40% of historical totals since 2000.

Above average - EC in the highest 30% of historical totals including the highest 10% since 2000.

Below average - storage volume in the lowest 30% of historical totals including the lowest 10% since 2005.

Average - storage volume in the middle 40% of historical totals since 2005.

Above average - storage volume in the highest 30% of historical totals including the highest 10% since 2005.

About the Monthly Water Update

The Monthly Water Update interprets the hydrological status of surface water (quantity and quality) each month using provisional information from data providers. Rainfall is a key driver of surface water and is shown with streamflow, stream salinity and storages systems across drainage divisions in Australia where data is available.

Streamflow gauging stations (from 370 sites) have been selected to best represent the spatial variation of flows across each drainage division. Gauge station name and ID, and the total monthly streamflow volume are presented for each site when hovering over the map. Sites are shown on a map and colour coded to show the relative decile ranking.

Stream salinity is expressed as electrical conductivity (EC) in micro Siemens per cm. Salinity gauging stations (from 163 sites) have been selected to provide an overview of median streamflow salinity across each drainage division. Gauge station name, ID, median, relation to the past and proportion of time in EC classes are presented for each site when hovering over the map. Sites are shown on a map and colour coded to show the median salinity classes.

Sixty fours storage systems have been identified to show the status of storages. Storage system name, capacity, accessible volume, relationship to the past and percentage change from previous month and previous year are presented for each system when hovering over the map. Systems are shown as circles on a map with the size of the circle representing the capacity and colour the fullness of the system.