Trees Offer Early Warning Of Water Shortages
Sinking the local groundwater table by as little as 10 meters below the surface can kill an Australian landscape, researchers at the National Centre for Groundwater Research and Training (NCGRT) warned today.
And in a groundbreaking advance, a team from NCGRT and the University of Technology Sydney (UTS) has come up with an early warning system that can potentially tell Australian groundwater managers and users when they are in danger of taking too much water away from the trees.
“All across Australia there are vast tracts of woodland, forest and riparian vegetation that depend on groundwater for survival, particularly during drought,” says Professor Derek Eamus of NCGRT and UTS.
“We know that if we extract too much water from under these ecosystems, there is a good chance the trees will die. But until now we didn´t know clearly what the cut-off was — how much water you can take before it adversely affects the forest that depends on it.”
Working in pristine woodland in the Sydney catchment, Prof. Eamus and researchers Sepideh Zolfaghar and Randol Villalobos-Vega have made a discovery of profound importance to our ability to pass a healthy Australia on to our grandchildren.
By measuring tree girth, leaf area, and other factors associated with vegetation they have for the first time managed to establish a clear relationship between the depth of groundwater and the health of the forest — one of the ℠holy grails´ of groundwater science.
“Up to a groundwater depth of 6-9 meters the forest does fine, the trees grow well and develop thick trunks and heavy leaf canopies. The diversity of tree species is high.” he explains. “But when the water table sinks below ten meters you begin to see a steep decline in all these things.
“Even though some tree roots may go much deeper in some parts of Australia, it appears that ten meters is the point at which tree health is clearly suffering. We think that this rule broadly applies to most types of groundwater dependent ecosystem across Australia.”
The research is helping to establish sustainability limits that will eventually be incorporated in all licenses to extract groundwater. “Basically it is an easy way to tell if we are over-extracting water — which is the case in many of Australia´s major aquifers, especially in the Great Artesian Basin and parts of the Murray-Darling Basin.”
The research has highlighted one of the great difficulties of managing groundwater sustainably: both people and trees compete for the same water, and both tend to draw it down – so it is much easier to over-extract that we previously thought. “When we think about the sustainability and recharge rate of an aquifer, we also need to think what the trees will be taking out of it, as well as what we need,” Prof. Eamus says.
The team has been using a tiny heat sensor to measure the amount of water used by trees — it injects a small amount of heat, which the rising sap then carries up the tree trunk, enabling the researchers to measure the sap flow and calculate how much water the tree is using .
“The answer is a surprisingly large amount. For example a lot of people talk about using trees to lock up carbon — well for every molecule of carbon you lock up, the trees will use between 1000 and 3000 molecules of water.
“Given the long-term water scarcity issues facing Australia, we may need to consider alternative ways to store carbon than by only planting trees.”
The team´s pioneering of indicators of forest health has also given Australians a new way to look at their landscape: using satellites to scan large areas of the continent for tree health will enable us also to ℠read´ the underlying state of the groundwater, even though it is out of sight and not well monitored.
“It also means that anyone who is doing the wrong thing and extracting too much water can be spotted from space, being ℠dobbed in´ by their trees — and this is likely to become as regular a part of water management as speed cameras are of traffic management.”
The National Centre for Groundwater Research and Training is an Australian Government initiative, supported by the Australian Research Council and the National Water Commission.
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