Fencing the Flux: Seasonal Trends, Environmental Drivers, and Mitigation Opportunities of Methane Emissions From Farm Dams.

Farm dams are significant methane (CH4) sources in agricultural landscapes. Fencing them to limit livestock access reduces organic matter and nutrient inputs, thereby limiting CH4 emissions. However, existing studies on the benefits of fencing are constrained by short durations, omission of ebullitive fluxes, limited spatial and temporal coverage, and small sample sizes. Here, we report a large-scale, multi-season assessment of total CH4 (diffusive + ebullitive) and carbon dioxide (CO2) fluxes from fenced and unfenced farm dams, along key environmental drivers. We monitored 113 farm dams in temperate mainland south-eastern Australia over 2 years, amounting to 39,552 and 45,408 hourly observations of total CH4 and CO2 fluxes, respectively. We integrated field-measured emissions with Sentinel-2 indices, topo-climate variables, and geostatistical models to identify flux drivers, quantify temperature sensitivity, and spatially extrapolate mitigation potential across Local Government Authorities (LGAs). We found that fencing reduced CH4 fluxes by 66%-82% across seasons while also significantly lowering the temperature sensitivity of CH4 fluxes, slowing the exponential rise in emissions under warming conditions. Specifically, CH4 fluxes in fenced dams increased by 71% per 10°C warming (Q10 = 1.71, EM  = 0.4 eV), compared to unfenced dams increasing by 275% (Q10 = 3.75, EM  = 0.98 eV). CH4 fluxes were driven by temperature, rainfall, and hydrological proxies (Modified Normalized Difference Water Index, MNDWI; Floating Algae Index, FAI), while CO2 fluxes responded to rainfall and Normalized Difference Water Index (NDWI). Extrapolating our findings across the study area (~526,296 km2), fencing all farm dams could cut CH4 fluxes by 1.16-1.35 kt year-1. By combining high-resolution emission data with scalable management strategies, this study offers a framework to improve greenhouse gas inventories and guide targeted climate mitigation in agriculture.