Surface boundary layer of cattle feedlots: Implications for air emissions measurement

Air quality issues at cattle feedlots are a growing concern, and micrometeorological techniques have potential for measuring air emissions from these operations. However, eddy covariance and related methods rely on assumptions about the boundary layer that might not hold above the complex, non-uniform, and fetch-limited surface of a feedlot. The objective of this study was to characterize the surface boundary layer of an open-air cattle feedlot to provide insight into how micrometeorological techniques might be applied to these non-ideal sites. An open-path eddy covariance system was used to collect high-frequency time-series data of wind speed, CO2, and H2O above a large commercial feedlot in central Kansas in 2006 and 2007. This site, like many High Plains locations, was characterized by windy conditions with daytime average wind speed of 5 m s−1, and near-neutral atmospheric stability was common, even at night. Using a modeled displacement height of 0.65 m, the roughness length ranged from 2 to 6 cm with a median of 3.6 cm. Ogives showed no signs of low-frequency transport (i.e. periods> 30 min). Eddy covariance measurements of CO2 fluxes averaged 0.4 kg m−2 d−1 while H2O fluxes averaged 2.3 kg m−2 d−1, both of which agreed with other studies measuring cattle respiration or water consumption. The tower was located along the north edge of a rectangular-shaped feedlot so the fetch was over 1600 m when winds were southerly. However, the length of fetch encompassed by feedlot pens decreased as winds became more southeasterly or southwesterly. Using the sharp contrast in CO2 fluxes from the pens versus the surrounding fields, the outer edge of the sampling footprint could be determined by observing abrupt changes in CO2 flux as wind directions shifted to the southeast or southwest. This provided a way to measure the footprint requirement using the respired CO2 from the cattle as a tracer. Under neutral atmospheric stability the required fetch was about 360 m when the sensor height was 6 m. The fetch requirements and the source area were predicted with a footprint model. Results showed that, on average, the three pens directly south of the tower contributed 61% of the measured flux. Roads, feeding bunks, and transfer alleys (i.e. surfaces within the footprint other than pens) accounted for 21% of the total area. Thus, accounting for the diluting effect of these spaces in the source area was important when attempting to compute a flux per unit animal or per unit pen area.