Now it’s definitive. Forestry is not causing significant reductions in catchment water flows.
A new study of the impacts of various rural land uses on water resources has challenged assumptions that plantation forestry is causing reductions in catchment water flows.
IN fact, the study shows that any lessening of water flows in recent years has more to do with lower rainfall, or changed rainfall patterns, than with forestry, or indeed other land use changes such as the switch from grazing to cropping.
This authoritative catchment-based science contradicts many of the fundamentals on which the South-East Natural Resources Management Board (SENRMB) has based its proposed water management plan for the lower south-east region of South Australia.
The study is the latest of a number by Sinclair Knight Mertz (SKM) investigating water and land use planning in South-West Victoria. It was commissioned by the Glenelg Hopkins Catchment Management Authority (GHCMA), equivalent to the SENRMB in the adjacent south-west of Victoria, and supported by a large number of stakeholders, including the SA Department of Land, Water and Biodiversity Conservation (DWLBC) and the SENRMB’s predecessor, the South-East Catchment Water Management Board.
“This new science is a breakthrough in understanding the relevance of new plantation development, and provides some much needed context for the ongoing water debate in the south-east of South Australia,” said Dr John Kellas, executive officer of the Green Triangle Regional Plantation Committee and member of the Water and Land Use change Steering Committee of the GHCMA.
The SKM study looked at five sub-catchments, each with different degrees of forest cover. The study found that all forms of land use change lead to reductions in surface and groundwater recharge. However, the relationship between declining rainfall and streamflow is clearly stronger than the influence of the various forms of land use change that has occurred since the 1990s. In other words, climate variation or change has altered the rainfall-runoff relationship.
“The timing of the change is coincidental and . . . the change in rainfall-runoff relationship is due to climatic influences,” the report said.
The SKM approach of using whole-of-catchment modelling over a number of catchments is in stark contrast to the SENRMB approach that uses artificial catchments and attempts to estimate the whole-of-catchment impact of forestry by extrapolating from individual tree water use data.
“Estimating regional water use by measuring an individual plant’s water use is an approach fraught with danger. Sample errors, which appear minor at the individual plant water use level, are magnified significantly at a regional level,” Dr Kellas said.
“This is why no one in Australia uses individual plant water use studies as a basis for crop water use estimates, let alone whole-of-catchment estimates.”
He said the SKM report was also a good reality check for putting the issue of land use change into context and determining the real impact of plantations. The report highlights that the development of bluegum plantations has been a relatively minor land use change compared with other agricultural activities.
Between 1990 and 2003, the SKM report said, about 73,000ha of new bluegum plantation was established in south-west Victoria, while over the same period the area under dairy increased by 280,000ha and that under-intensive cropping increased by 130,000ha.
According to Auspine resource general manager Phil Lloyd, the SKM report has important ramifications for South-East water policy.
“SKM confirms that the reality for the South-East community is that climate will continue to be the ultimate driver of changes in the water resource, and the continuing intensification of agriculture over 70% or 80% of the South-East landscape will have an equivalent or greater impact on the water resource compared to future plantation development.”
Timbercorp’s community liaison manager John Kiely said that intensification of agriculture wasn’t limited to south-west Victoria … “it’s happening in South Australia to a similar degree”.
