Local research sinks dam greenhouse gas emission models

Carbon emission estimates for some Australian dams have been massively overestimated, new research reveals.

Lead researcher Jackie Webb, who examined 38 farm dams in the Murrumbidgee Valley. Picture: Rachael Lenehan

Greenhouse gas emissions from farm dams in Australia’s irrigation regions may have been massively over-estimated using existing international models, requiring a re-evaluation of the agriculture sector’s carbon footprint.

New research into the emissions of irrigation dams in the NSW Riverina district found the Intergovernmental Panel on Climate Change guidance over-estimated their nitrous oxide and methane emissions by four and fives times respectively.

The preliminary results show the carbon footprint of irrigation dams, which make up a large proportion of water storages in Australia, is smaller than expected, possibly requiring revisions of ­national emissions accounting.

A quarter of the dams surveyed were found to be net greenhouse gas sinks, while half were carbon dioxide sinks and more than 70 per cent were nitrous oxide sinks.

The research raises questions about how Australia’s emissions inventories are calculated and whether models being used are a true reflection of an industry’s carbon footprint.

Lead researcher Jackie Webb, a freshwater and agricultural scientist from Deakin University’s Centre for Regional and Rural Futures, said the findings could lead to a complete revision of the global emissions budget for human-made water bodies.

“N2O (nitrous oxide) emission factors probably need to be revised down for these systems, because our understanding has been challenged,” she said. “No one’s wrong, it’s to do with the lack of data and there had been no other studies of farm dams in irrigation regions.

“The (IPCC) do great work but they rely on the data that exists ­already … but it was quite off in terms of estimating emissions from these semi-arid irrigation dams.”

The Australian models currently used to calculate the carbon footprint of farm dams in the ­national greenhouse gas inventory, which is provided to the UN, mostly represent livestock dams from other climates.

“A lot of agricultural industries are committing to net-zero emission goals for 2030 and 2050 so we need to get it right in terms of where the emissions are happening, where they can be reduced and where there might even be carbon storage happening. Everything counts,” Dr Webb said. The study was confined only to a sample in the Riverina, and Dr Webb said results could be different in other regions, but the majority of Australia’s irrigation dams are within the Murray-Darling basin.

Irrigation dams in the Riverina have a smaller carbon footprint than previously thought. Picture: Rachael Lenehan

“Given that 41 per cent of farm dams exist within the Murray-Darling Basin which accounts for 60 per cent of irrigated land in Australia, the contribution of irrigation dams to countrywide farm dam emissions needs to be reconciled,” Dr Webb said in her report.

The research project, funded by federal statutory corporation ­AgriFutures, was the first detailed assessment of the carbon footprint of irrigation dams in Australia.

Previous research has suggested Australian farm dams, of which there are about 1.76 million, contribute between 1 and 3 per cent of methane emissions attributed to Australian agriculture each year.

Earlier studies looked at the emissions of dams on livestock farms which emit higher levels of greenhouse gases, particularly methane, because of the higher presence of nutrients in the water and the effect this has on algal growth and decaying organic ­matter.

Irrigation dams were assumed to produce similar emissions, but Dr Webb’s research has refuted that assumption by showing that semi-arid irrigation dams, which usually don’t receive as much overland flow, had significantly less output than global averages.

The 38 irrigation dams tested in the Murrumbidgee Valley had ­estimated yearly methane emissions about five times lower than the IPCC’s emission factor of 183kg. “This vast overestimation would be an unfair assumption for some of Australia’s irrigation growers and industries,” Dr Webb said in her report.

Previous studies had shown small water storage bodies around the world were significant contributors of methane. “Given Australia recently joined the Global Methane Pledge to reduce methane emissions by 30 per cent, this is a promising finding to the irrigation sector that their farm dams are negligible methane emitters,” Dr Webb said in her report.

A baseline of irrigation dam carbon footprints will give agricultural industries the ability to better refine their emissions budgets.

The study will embolden farmers seeking to reduce their emissions by reducing nutrient run-off into their dams.