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 production. 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
two 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 yr⁻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.