Floods drive groundwater flow in low-lying Tweed Valley cane fields

New research led by Southern Cross University PhD candidate Jackie Webb has shown pulses of groundwater from agricultural floodplains in the Tweed Valley region are only released following flood events, while for the rest of the year groundwater is stored in the floodplain.

The paper ‘Constraining the annual groundwater contribution to the water balance of an agricultural floodplain using radon: The importance of floods’, was published in the journal Water Resources Research, revealing how floods and groundwater control the hydrology of floodplains.

“When put into perspective, our work shows that maintaining very shallow drains helps to keep the groundwater within the landscape and reduce greenhouse emissions,” Jackie said.

“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.”

Jackie used novel measuring technology and discovered groundwater dominated the drains immediately after floods.

“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 flood 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.”

Jackie’s supervisor, Professor Isaac Santos from Southern Cross University’s National Marine Science Centre, said much of the Australian population relied on drained floodplains for their livelihoods, so research addressing their hydrology was imperative.

“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 award-winning farmer Robert Quirk, a sugar cane grower interested in best-practice water and soil management at his property.

“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 shows the eco-system services that sugar cane provides.

“Sugar cane captures about nine tonnes of carbon per hectare per year,” Mr Quirk said.

“Sugar cane also helps draw down the water table, similar to the trees in the Murray Darling.”

Jackie’s research was inspired by previous work where she discovered the implications of flood events in causing large greenhouse gas emissions from agricultural drains.

“Agricultural drains from these carbon rich systems can be hotspots for carbon dioxide and methane emissions,” Jackie said.

“These large greenhouse gas releases were observed following flood events, and in the case of carbon dioxide can now be linked to these coinciding groundwater pulses.

“However, the sooner the floodplain hydrology returns to normal conditions after these flood events the quicker the greenhouse emissions return to insignificant levels, highlighting the importance of hydrology management in these flood-prone landscapes.”

Jackie recently present her research at the Australian Society of Sugar Cane Technologists annual conference in Cairns.

This research project was a collaborative effort between SCU researchers Jackie Webb, Professor Isaac Santos, Dr Damien Maher and CSIRO colleagues Dr Barbara Robson and Dr Ben Macdonald. The research was funded by the Australian Research Council and CSIRO with logistical support from farmer Robert Quirk.

Watch a video of her Jackie’s work here: https://www.youtube.com/watch?v=w-k-PgNEllk