Evaluating an eddy covariance technique to estimate point-source emissions and its potential application to grazing cattle

Abstract

Measurement of gas emissions from grazing cattle presents a challenging application of the eddy-covariance (EC) technique. A cattle herd represents point sources on the landscape, violating the assumptions of spatial homogeneity made in typical EC applications. A proper evaluation of EC fluxes in this case requires an analysis based on the overlap between the EC flux footprint and animal positions. A controlled gas release study was conducted to evaluate the potential of a Lagrangian stochastic (LS) dispersion model to interpret EC fluxes and estimate emissions from point sources. Methane (CH4) gas was released from eight fixed points within a confined area (representing animals in a paddock) while two EC systems monitored CH4 fluxes at two distances downwind of the source area (a near and far tower). Overall accuracy was greater at the far tower location with estimates within 3% of the actual emission rate. The near tower overestimated total emissions by 16%. Deviations from the true emission rate were greatest for night-time and morning periods and least for mid-afternoon to early evening periods when neutral stability and favorable wind directions prevailed. We also investigated the effect of treating the simulated paddock as a homogeneous area emission source. The near tower emission estimate improved with the area source approach (9% overestimation). The far tower suffered a loss of accuracy (17% underestimation), but this was substantially improved (7% underestimation) by reducing the source area to the minimum required to contain the eight release points. Our study suggests that EC can be used to measure animal emissions from grazing cattle on pasture with a level of accuracy similar to other micrometeorological approaches.