Even wet woodlands at risk from groundwater over-extraction

Scientists at Australia’s National Centre for Groundwater Research and Training (NCGRT) are warning nations around the world to cut back on their water use if they want to save their woodlands and rivers.

It is currently estimated that the world is extracting around 1000 km³ of groundwater every year - often far more than is naturally replaced from rainfall - and this is causing water shortages. The work of PhD researcher Sepideh Zolfaghar reveals that even trees in areas with abundant rainfall are at risk from over-extraction of groundwater.

Research supervisor Professor Derek Eamus, of NCGRT and University of Technology, Sydney (UTS), said, “Even trees growing in wet climates are far more sensitive to water stress than we originally thought and may be less likely to survive if too much groundwater is extracted.”

Zolfaghar explains that in regions with high rates of rainfall, if the water table is sufficiently shallow, forests will use this source of water. “But this is also why they can’t cope with a sudden water shortage,” she said.

“They’ve grown in an environment where water supply is continuous and plentiful, so they haven’t evolved to cope with droughts. Trees in dry areas, in comparison, have developed drought resistance and are more resilient.”

Zolfaghar studied how groundwater depth affects eucalypts in a humid forest in NSW. Currently, the NSW government has reserved the groundwater beneath this forest for future use in Sydney in case of drought.

“We found that once the groundwater depth exceeded 10 metres, the trees became less productive,” said Zolfaghar. “When groundwater depth increased from 5.5 metres to 9.8 metres, the trees were significantly shorter and tree density - the number of trees per hectare of land - was much lower than when groundwater depth was shallow.

“When water supplies are reduced, such as when too much groundwater is extracted, trees absorb less carbon dioxide. This means they can’t grow as fast or as tall as when water supply is abundant.”

Professor Eamus said this is the first study to have tracked how groundwater depth affects trees across multiple scales, including small scale (cellular, leaf), medium scale (branches and whole tree) and larger scales (entire stands of trees). “We found that the whole system is tightly linked - one change can affect everything else.

“Not only that; since a lot of groundwater flows into rivers, we can also expect to see changes in river flow when groundwater extraction is excessive.

“Studies like these allow us to better understand how ecosystems use groundwater and how sensitive they are to changes in groundwater depth,” Professor Eamus said.

“Based on this study, we can also develop a mathematical model to help find out how much groundwater we can extract from woodlands growing in wet environments. This can potentially be adapted for use in forests worldwide.”