Floods’ hidden fury
Insight: Groundwater driven out by draining floodwaters
FLOODING such as that caused by recent record rainfall in northern NSW drives groundwater out of drained floodplains, according to Southern Cross University researchers.
PhD candidate Jackie Webb found that groundwater stored in floodplains, such as those farmed by cane growers in the Tweed Valley, is “released” following flood events, seeping out when the flood waters drain away.
Shallow drains maintained by farmers help to keep the groundwater within the landscape all year round, but also enhance the rapid removal of flood waters – and consequently the groundwater. Ms Webb’s paper on the subject of the impact of floods on groundwater and water balance was published in the journal Water Resources Research, revealing how the impact of floods and groundwater control the hydrology of floodplains.
“Cane farms in the Tweed Valley are near sea level and during floods they become fully inundated, so when that drains away it makes the groundwater seep out as well,” Ms Webb said.
The project also showed that by keeping groundwater in place, the canefields’ shallow drains helped reduce greenhouse emissions.
It also provided information about the effects of run-off from plantations.
“This research gives us insights into how to manage some of the pollutant impacts on our local rivers and estuaries.
“The results of this study can also help inform farmers on how to best manage their properties to reduce nutrient losses from the soil,” she said.
“We just witnessed one of the largest floods of the century. If these events become more frequent then we are likely to see more groundwater released from agricultural floodplains, which could have implications for downstream water quality and water storage.” Ms Webb’s supervisor at SCU’s National Marine Science Centre, Isaac Santos, said the research had huge significance for the many Australians whose livelihood depended on floodplains.
“The success of this method to calculate groundwater flows annually can be applied across other agricultural floodplains, which would greatly assist in water quality management and farm management practices,” Professor Santos said. Most of the research was conducted at the Tweed property of Robert Quirk, a sugar cane grower concerned with best practice water and soil management.
“Mr Quirk’s farming practice is cutting edge – his property has been given multiple awards for managing the landscape,” Professor Santos said.
Mr Quirk said the research showed the eco-system services that sugar cane provided.
“Sugar cane captures about nine tonnes of carbon per hectare per year,” Mr Quirk said.
It also helped to draw down the water table, he said. Ms Webb’s research was inspired by previous work, in which she discovered the implications of flood events in causing large greenhouse gas emissions from agricultural drains.