Australia State of the Environment Report 2001 (Theme Report)
Prepared by: Ann Hamblin, Bureau of Rural Sciences, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06748 5
Secondary salinity and acidity (continued)
Trends in use of water by vegetation [L Indicator 3.4]
Australia has operated a continental monitoring service for vegetation flushes and declines related to seasonal rainfall, based on the NOAA-NDVHRR satellites, since 1991. The data are received daily and aggregated to fortnightly images that are posted on the website of the monitoring agency, Environment Resource Information Network (ERIN).
The 'greening' flushes are described as the normalised difference in vegetation index (NDVI), referring to differences between the highest 'greening' and the lowest 'background' values that occur in the dry season. Full details are supplied on the ERIN website. As there are now ten years of data for the whole continent in the ERIN archives, possible trends in vegetation condition and extent are just beginning to emerge.
Figure 55 indicates the changes in the amount of rain that have been utilised by green vegetation over the period 1991-1999, using a surrogate measure. The maps show whether any individual 25 km 2 area has experienced positive, negative or no change in the slope of a 'rainfall transfer quotient' over the nine-year period. This quotient compares the difference between each seasonal flush and basal vegetation greenness and the amount of rain falling in the growing season to the long-term trend. The map shows categories of high, medium, low, and no change in the trend, expressed as a statistical probability. Areas with a 95 or 90% probability have undergone a change in the way vegetation is utilising rainfall. When conditions are good (more rainfall, more seasonal growth) there is an increase in the quotient. Parts of northern Australia recovering from droughts in the early 1990s show this trend, while parts of south-eastern and western Australia also show more seasonal growth compared with the basal vegetation that could be ascribed to agricultural productivity.
Figure 55: Water utilisation by vegetation, derived from NDVI and rainfall data 1991-1999.
Source: ERIN (2001)
These findings are unexpected, but should be treated with caution as the method is still being developed and assessed. It should also be noted that over half the continent has not experienced any measurable change in plant water utilisation, despite fluctuating seasonal growing conditions that have occurred from year to year.
Interpretation of these results at this stage is tentative, but may indicate that an increase in permanent perennial vegetation has been taking place in parts of northern Australia. In southern Australia, there is an apparent reduction in water use in areas of lesser proportions of perennial vegetation. While it is tempting to ascribe this to a real change in water use in agricultural lands, there is no confirmation from the more detailed research sites that have been monitored over the decade.
The results are best considered as demonstrating the potential capacity of this technology to provide synoptic continental-scale information rapidly on a range of vegetation and climate related issues. As yet the application of the technology is in its infancy but with succeeding years the increasing size of the data set increases its potential value.