Friday, March 29, 2024

Selecting an appropriate baseline: Frank Batini

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Water Corporation advertisement
Water Corporation advertisement, March 2022.

BaselineA set of data that are used as a reference to gauge the direction and extent of change.

Frank Batini

If we wish to compare current observations with past performance, we need to establish a set of relevant data, or baseline. The baseline selected by the Water Corporation of Western Australia for rainfall and streamflow comparisons in WA is the period 1911-1975.

Since the 1970’s “Climate Change”, attributed to rising levels of carbon dioxide, has been promoted as the cause of lower rainfall, of falling water-tables, reduced stream-flow and some deaths of vegetation in the forested water catchments of the South–West. World-wide reduction in carbon dioxide levels in the atmosphere is usually offered as the main solution.

 Water Corporation’s advertisements claim that “Perth’s rainfall is declining due to climate change” showing a graph of reduced stream-flows into the reservoirs as convincing proof that “Climate change is real”.

Such claims deserve scrutiny.

Rainfall  

 Water Corporation data show a rainfall reduction of about 16 percent since 1975.  However, a thorough analysis of rainfall must use all of the available data sets, not only those since 1911.

I have reviewed BOM rainfall data for Jarrahdale, located close to the catchments of the Wungong and Serpentine Dams, from 1882.  Recalculating the averages from 1882-1910, 1911-1975 and post 1975, the means are: 1100mm, 1251mm and 1054mm.The data show a natural, long-term cycle in rainfall.

The several very dry years recorded before 1911 at Jarrahdale cannot be attributed to “Climate Change” as the phrase is used these days.

Catchments

There have been many changes to water catchments in the jarrah forest during the past 150 years. These include extensive timber harvesting and regeneration which altered the structure and age-class of the forest; large-scale bauxite mining which converted thousands of hectares of forest into dense rehabilitation, wildfire, prescribed burning and eucalypt decline associated with Phytophthora disease.  

Research by various agencies has shown that younger, smaller trees transpire at twice the rate of older-growth and that about half of the reduction in stream-flow in recent decades is the result of changes in vegetation, not lower rainfall or “climate change”.

Jarrah forest comparison
Un-thinned (L) and thinned (R) jarrah forest.
Photos: Dr Colin Terry, Water Corporation.
Regeneration comparison
Regeneration 2005 wildfire at 15 months (DBCA) and five years.
Photos: Dr Colin Terry, Water Corporation.

Moreover, when overstocked stands of trees are thinned, there is a significant increase in streamflow for several years. For various reasons (mostly environmental activism) wide-scale commercial thinning of regrowth forests does not occur, despite the water supply and environmental benefit that would follow.

 On-going, professional management of forested water catchments with a view to optimising forest health and water yield is required. In the current political climate, where the Government has decided to close all commercial timber harvesting by end of 2023, this outcome is unlikely.

Streamflow

The Water Corporation’s streamflow into dams are shown to commence in 1911. However none of the major dams feeding the city of Perth were operational before 1940. All pre-1940 inflows are inferred by modelling and may be incorrect. A more extensive system of continuous stream monitoring was not established until the mid-1960s.

Comment

Domestic water consumption has increased markedly with population growth since 1911. Efforts by the Water Corporation to make Perth’s water supply less dependent on annual rainfall are sound. They have encouraged water conservation, developed two sea-water desalination plants, and have injected treated water into aquifers.

 However, their assertions of a direct link between rainfall, streamflow and “climate change” are unsubstantiated and misleading. I have contacted Corporation and its Board on several occasions and offered to meet, but my offer has not been taken up.

Most of the research and monitoring of ecosystems and catchments in the south-west forests of WA has been done since the 1960s and has only measured a “receding tide”: falling water-tables, reduced streamflow, changes in aquatic biodiversity and tree health.

That these changes have occurred against a baseline of a long and unusually wet period in the rainfall cycle is mostly ignored by land managers and academics.

While the data are sound, the conclusions drawn may be wrong.

Long-term monitoring of vegetation (1973-2021) in the 31 mile brook catchment has shown only a slight shift in species composition, despite major changes in hydrology.  Species that expanded their range during the wetter years are now simply retreating back.

In the 2019/2020 wildfire area there should now be evidence of regeneration from seed, coppice and tubers. If regeneration is extensive and successful, it will have negative effects on water supply in about 10 years time. There are sound data from NSW and Victorian forests to show that these changes will then last for decades.

Due to the extent of the area burnt, any attempt at mitigation will be costly and difficult to implement, especially if there is no longer a viable timber industry in either NSW or Victoria.

Because of the time lapse, current State Governments and Departments will likely treat this as “someone else’s problem” or attribute the consequences to “climate change”, the all-purpose excuse and scapegoat.

Frank Batini is an experienced forester, environmental scientist and consultant in the management of natural resources. From 2002 to 2012 Frank was employed as a consultant to the Water Corporation.

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