Technical report No.2 was finally released by the Murray-Darling Authority last Friday, at 4pm but only as an electronic version. It is a bloated document with large print — with dozens of  almost full-page colour photos and drafts — making it virtually impossible to print out this 453-page report from a home computer. It is also surprisingly light on research.

So what kinds of research is missing?  The lack of analysis of the impact of native vegetation management within the Basin’s catchment stands out. On pages 14 and 34 there are maps of the historical runoff and rainfall within the catchment, which likely shows the forested areas — yet there is no discussion of the importance of the management of these wetter forests to streamflow in the Basin or maps of their vegetation, its age and management.

After the 1939 fires, the managers of the Melbourne Water supply catchments established research on a large area of forest that was salvaged logged and replanted.  Many Victorians have seen this forest driving up the Black Spur to Marysville.

It was established that the streamflow from these catchments diminished relative to rainfall, peaking when these forests reached 30 years of age. Young eucalypt forests grow very fast and use massive amounts of groundwater water leaving little to flow into streams.  This affect is most likely in the spring/summer when trees grow fastest — and when we need the water the most.  As these planted forests age their use of water ceased when they reached 120 years of age.  To maintain forests of this average age the rotation period for logging forest would have to be 240 years. In Victoria, for instance, the logging rotation is at 80 years (and less) apparently maintaining forest of an average age of 40 years that use large amounts of water.

The streamflow data and the history of forested catchments is available for almost the entire basin. This can be used to evaluate the impact of the regrowth of native vegetation on streamflow. There there appears to be no consideration of the impact on streamflows into the Murray-Darling Basin from forest management in this technical report.

If forests are burned, as is planned for most currently, what will be the impact on streamflow as they regrow?  When forests are salvaged logged after fires, what is the impact of that management on future streamflow compared to allowing these forest to regrow?  Where forests are simply clearfalled as part of normal forestry operations, what amount of water is lost from streamflow as they regrow?

The rapid growth in plantation forests of fast-growing eucalypts within the Murray-Darling Basin too has had limited evaluation (pp 39).  The area under plantation is given as 290,000 hectares.  A “figure” for water use is given as 341 gigalitres per annum. However, this figure appears to exclude “other plantations” in areas where native forest that have been cleared to establish them and plantations that have been replanted!  This estimate  is not related to their likely increased water use with age, the total area of “other” plantations or the expansion of that area.  Melbourne Water research clearly implies groundwater high in a catchment can become streamflow lower down as springs emerge and add to the streamflow. What will be the rotation time for these forests and how will that affect their impact of streamflow?

Pre-European farming, the entire Murray-Darling catchment was vegetated and nearly all of its forests were intact, old and likely did not use water — especially its wet forests along the Dividing Range that runs from Victoria to Queensland prescribing the eastern boundary of this vast catchment.

It would seem reasonable to expect the Murray-Darling Basin Authority to undertake extensive research into the impact of forest and plantation management on streamflow — specially given the further increase in plantation forests for fast growing eucalypt species for carbon sequestration within the catchment flagged by the Bureau of Research Science  (pp 39). Research and monitoring of water use of these plantations is surely essential to water management in the basin.

The impact of native forest management on basin streamflow is needed too. This is especially so with the proposed increases in fuel reduction burning in forests that likely currently yield the most reliable streamflow to the basin and the greatest volume relative to their area.

Why just cut allocations to irrigators without also addressing these critical forest and plantation management issues properly?

Peter Fray

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