Gains made in floodplain drains

Research reveals impact of floods on groundwater


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Researcher Jackie Webb is studying the impact of drain depth on farming landscapes. She is pictured here in the Tweed Valley.

Researcher Jackie Webb is studying the impact of drain depth on farming landscapes. She is pictured here in the Tweed Valley.

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Research reveals impact of floods on groundwater

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NEW hydrology  research shows for the first time the impact floodwater has on groundwater, delivering relevant flow-on benefits to floodplain drain design.

Southern Cross University PhD student Jackie Webb’s work aims to quantify the complete ecosystem carbon budget of agricultural floodplains. Her study area is a sugarcane farm in northern NSW, near Tweed Heads.

She showed that when floodwater inundates a paddock it flushes into underlying aquifers, pushing stored groundwater into drains.

Her work demonstrated that 75 per cent of the groundwater beneath a coastal floodplain is discharged into floodplain drains after the area is inundated, while under average conditions groundwater is not present in drains.

Farmers are aware of the effects of fertilising before flooding, but the new findings demonstrate the extent to which floodwater can transfer through soil into aquifers, draining nutrients. 

The water level in the drain will match the water table, meaning the deeper the drain, the greater its capacity to discharge groundwater. 

Jackie Webb says deep drains are a cause of greenhouse emissions.

Jackie Webb says deep drains are a cause of greenhouse emissions.

Conversely, shallow drains limit this effect, preventing the floodwater passing through to aquifers and pushing out its groundwater.

Ms Webb said her findings were relevant across other reclaimed floodplain land.

“The principle would apply to any low-lying, artificially drained farmland. After flood events there would likely be a similar effect.” 

Ms Webb determined the amount of groundwater present in a drain by measuring for the gas Radon. It occurs naturally in high concentration in groundwater, but only in low concentration in surface water. 

A mass balance of radon in drainwater was also measured. Coupled with data from a sensor measuring water table depth to gauge run-off from the aquifer, Ms Webb calculated the rate of groundwater discharge. 

Her supervisor Professor Isaac Santos said drain depth was a difficult, but important balance for farmers to strike.

Nutrient loss, wasted soil carbon and environmental health are at stake when runoff hits rivers and the sea.

“Deeper drains mean drier soils, which can be good for crops, but it’s important to prevent groundwater discharge,” Prof Santos said.

Discharge often causes water quality problems, as groundwater is quite acidic and has low levels of dissolved oxygen. With no oxygen, that means no fish.”

“Floodplain soils have lots of carbon content. It takes thousands of years to build up. But if the soil is flushed with water, that carbon escapes.”

It’s a gas

IT’s not just water and soil quality to consider when looking at floodplain drains.

Deeper drains, which provoke high levels of discharge from aquifers, can also be a significant source of greenhouse gas emissions, Ms Webb said.

“Drains from these carbon rich (floodplains) can be hotspots for carbon dioxide and methane emissions. 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.”

“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.”

Prof. Santos, said managing emissions would be of future financial benefit.

“We don’t have a mechanism to reward good soil practices, but it is quite likely there will be in the near future,” he said.

The story Gains made in floodplain drains first appeared on Farm Online.

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