Water Observations from Space FAQs
The Water Observations from Space (WOfS) available via the Australian Flood Risk Information Portal (the portal) provides information to support the understanding of Australia's flood history through time-series analysis of surface water observations.
Satellite surface water mapping provides large scale, regional information on water. Understanding how past floods have behaved improves our ability to predict future behaviour.
What is the Water Observations from Space (WOfS) product?
WOfS is a satellite imagery product depicting observed surface water across the Australian continent.
Geoscience Australia analyses Landsat data to identify areas where surface water may have been present. The Landsat archive incorporates observations from Landsat-5 from 1998 to 2011, and Landsat-7 from 2000 to 2012. The analysis uses an algorithm to determine were water is or is not present on each image. Water presence is calculated for every available Landsat observation in the archive, and displayed as a summary showing how many times water was observed for every point in Australia in a 25m x 25m grid.
In addition every grid cell in WOfS has an associated confidence value. The confidence is a qualitative measure arrived at by examining whether other factors, such as the terrain, are consistent with the presence of water, and how often water has been observed in the cell overall.
How is WOfS produced?
WOfS is generated from Landsat 5 and Landsat 7 imagery that has first been corrected for atmospheric affects, sun and sensor angles and terrain affects. Each Landsat image is analysed using a standard, automated algorithm to ensure each scene is analysed in the same way. The algorithm detects pixels that are indicative of water, and these collectively form the water observations dataset. The final WOfS product combines all water observations from the entire time series into a single dataset for all of Australia.
What time period does the WOfS cover?
WOfS is compiled from Landsat-5 and Landsat-7 images across Australia for the period 1998 to 2012. The composite product depicts observed surface water for the whole of the time period.
Can I view the surface water extents for a particular year?
The composite product only depicts observed surface water for the entire time period, 1998 to 2012. Surface water observations for individual years or shorter time periods are not available at this time.
Does WOfS show maximum flood extents?
No. WOfS shows surface water on the day (and time) that the satellite passed overhead, which might be before, during or after the flood peak. Given the time between satellite passes (approximately once every 16 days) it is unlikely that the satellite will capture the maximum extent of any given flood.
Why are satellites used to observe surface water?
Satellite surface water mapping provides large scale, regional information on water. Satellites capture spectral information which helps to distinguish water from land, and to understand water quality. By comparing water observed during floods in an area with the amount of water that is usually observed for that area, satellite imagery can help to identify areas at risk of flooding. It can also be used to detect changes in the landscape that alter the way water moves, such as levees and dams.
How large does the surface water body need to be to be visible from space?
In general, surface water bodies need to be approximately 50 metres by 50 metres, or half the size of a football field, to be clearly visible to the Landsat satellites. The Landsat satellites have a 25 metre pixel resolution making it difficult to accurately distinguish water in smaller areas, although some smaller water bodies are detected. Smaller water bodies are more likely to be detected if they consistently hold water, and if the water colour contrasts with the surrounding areas.
What are the practical applications of WOfS?
WOfS can be used to identify how frequently surface water has covered a location in the period of observation. It can show where bodies of water half a football field or larger have never been observed; have been occasionally observed; or where surface water is a permanent feature. WOfS is also useful for understanding related processes in the landscape such as groundwater recharge and the way vegetation responds to different water conditions.
I know there was a flood on X date at Y place. Why can’t I see it?
The satellites can only see the earth when there are few clouds and when they are overhead at the right time. If the day of the flood was cloudy or the satellite didn’t pass overhead on that day, an image of the flood won't have been observed.
I saw satellite images of the flood at X. Why isn’t that shown here?
There are many other satellite and aerial photography images of floods that have been shown in the media that we can’t yet display here. This is because the government doesn’t own the rights to all satellite imagery and aerial photography.
Will more types of satellite data become available in future?
GA has no immediate plans to add observations from other satellites to the WOfS product. However, GA will continue to update WOfS with new observations from the Landsat satellites, and may add more data sources in the future.