Semi-arid irrigation farm dams are a small source of greenhouse gas emissions

Small artificial waterbodies are larger emitters of carbon dioxide (CO₂) and methane (CH₄) than natural waterbodies. The Intergovernmental Panel on Climate Change (IPCC) recommends these waterbodies are accounted for in national emission inventories, yet data is extremely limited for irrigated landscapes. To derive a baseline of their greenhouse gas footprint, we investigated 38 irrigation farm dams in horticulture and broadacre cropping in semi-arid NSW, Australia. Dissolved CO₂, CH₄, and nitrous oxide (N₂O) were measured in spring and summer, 2021–2022. While all dams were sources of CH₄ to the atmosphere, 52% of irrigation farm dams were sinks for CO₂ and 70% were sinks for N₂O. Relationships in the linear mixed effect models indicate that CO₂ concentrations were primarily driven by dissolved oxygen (DO), ammonium, and sediment carbon content, while N₂O concentration was best explained by an interaction between DO and ammonium. Methane concentrations did not display any relationship with typical biological variables and instead were related to soil salinity, trophic status, and size. Carbon dioxide-equivalent emissions were highest in small (< 0.001 km²) dams (305 g CO₂-eq m⁻² season⁻¹) and in those used for recycling irrigation water (249 g CO₂-eq m⁻² season⁻¹), with CH₄ contributing 70% of average CO₂-eq emissions. However, irrigation dams had considerably lower CH₄ emissions (mean 40 kg ha⁻¹ yr⁻¹) than the IPCC emission factor (EF) of 183 kg CH₄ ha⁻¹ yr⁻¹ for constructed ponds and lower N₂O EF of 0.06% than the indirect EF for agricultural surface waters (0.26%). This synoptic survey reveals existing models may be severely overestimating (4–5 times) farm dam CH₄ and N₂O emissions in semi-arid irrigation areas. Further research is needed to define these artificial waterbodies in emissions accounting.