Continuing water catchment trials
From the Summer 2007 edition of Bush Telegraph Magazine.
The Crabaple Creek v-notch weir, is one of eight weirs constructed in 1975 as part of the Karuah River project, that continues to be used today for research. Photo by Ashley Webb.
A water monitoring project set up in the 1970s to evaluate the possible effects of forestry activities upstream of a proposed dam on the Ka-ruah River has been conscripted into service to demonstrate the effects of forest ages on water flow.
The dam, intended to augment the water supply for Newcastle, was never constructed, but Forests NSW has maintained the project ever since.
“The infrastructure already in place will allow us to determine the water flow properties of eight small catchments that flow into the Karuah River,” said Forests NSW water monitoring manager, Dr Ashley Webb.
“It includes eight v-notch weirs constructed in 1975, and there are two meteorological stations and ten rain gauges.”
The initial work on the project was carried out by Dr Peter Cornish who has published the results up until 2000.
Six of the eight catchments were harvested in 1983. Two catchments were replanted as eucalyptus plantations, two were regenerated into native forest after burning and two regenerated into native forest without burning.
“As far as we can tell there is no difference today in stream flow between the native forest catchments that were burnt and left unburnt,” Ashley said.
“They are all now unthinned. This operation will mean that we will leave two mature forest catchments unlogged, and we will leave two native forest regrowth catchments and one plantation catchment unthinned.
“We will thin two native forest catchments and one plantation catchment.
“The result will be a set of quantitative data that will show how we can interact through silviculture to affect water yield in forested areas.”
At the same time Dr Geoff Smith from the plantation improvement branch at Coffs Harbour will take an inventory of the current status of the stands before and after thinning so that the water data can be related to size and age class of the trees on site.
“Previous data has shown us that after logging the water yield increases proportionally to the area harvested in catchments,” Ashley said.
“After harvest there was higher runoff which lasted for two to three years, and then peaked.
“As regrowth became more vigorous, and plantations older, by an average of seven years after logging the water yield had returned to the equivalent of before logging, and then as the trees grew even bigger the yield fell to below previous levels.”
That had continued in the regrowth forests but it could be many years before the water yield returned to the levels of mature forest.
“We are trying to show that the thinning program can get yields back to pre-harvest levels sooner than previous published opinions have proposed might be the case,” Ashley said.
“The same infrastructure had been used for water quality testing, including the effects of harvesting and haulage on water turbidity and sus-pended sediment.
“We expect current harvesting practices to keep that to a minimum, but we will still measure for it.”
Howard Spencer Public Affairs & Media, Coffs Harbour
