Measuring methane emission rates of a dairy cow herd by two micrometeorological techniques

Two well-known micrometeorological techniques were used to measure methane emission rates from a herd of dairy cows freely grazing within a fenced paddock. The integrated horizontal flux technique (IHF) was preferable to the flux-gradient technique (FG) because it does not rely on similarity assumptions, and because it had the smaller measurement error. Both techniques were comparable, within estimated errors, with scaled-up animal emissions, determined by a tracer gas technique. The cows’ emissions, by IHF including 243 half-hourly data obtained during three campaigns in the midsummer, late summer and spring of 2002, averaged 330 (±153) g CH4 per day per cow (±S.D.). For FG, source strength was proportional to measurement height, after application of a conventional source area weight function. This implies that the turbulent diffusivity of CH4, a passive tracer, cannot be substituted by that of heat or momentum, as assumed by FG. We show that in our experimental situation, where the passive tracer has a finite source area, the ratio of FG to IHF results approximates the square of the turbulent Schmidt number (the ratio of momentum diffusivity to tracer diffusivity). This ratio increased with height and decreased with distance from the source area. We conclude that the Schmidt number is a function of position in the downstream airflow, and argue that the cause is vertical transport efficiency being affected by horizontal plume spread. FG may thus include systematic errors if one does not account for the position dependence of the Schmidt number.