A sensitivity study of energy fluxes and evaporation from a waste lagoon to different stability model formulations

Emissions from agricultural and animal operations directly impact the quality of life and health of people that live and work in proximity to these sources of pollution. One known source of malodorous gases and particulate matter are anaerobic lagoons which are widely used in the United States to collect, treat and store effluents from concentrated animal feeding operations (CAFOS).

The present research is part of an ongoing air quality research program whose general goal is to establish phenomenological relationships between emissions from anaerobic waste lagoons and the meteorological and atmospheric stability conditions that control it. As a first step towards that, it is proposed to test the sensitivity of evaporation and sensible heat fluxes estimates from three methodologies.

The research site is a 60m x 60 m waste lagoon located in Simpson County, Kentucky. Meteorological data was collected from at 5 minute intervals from an automated weather station located 20 m from the lagoon's bank during June 2008. At the center of the lagoon an instrumented raft measured temperature, relative humidity and wind speed at 0.5 m and 1.5 m above the surface. In addition, measurements of solar radiation, surface and below the surface temperature were available from which the net available energy at the lagoon's surface was estimated.

The first method computed Bowen ratio from which latent and sensible heat fluxes were calculated using the profile method (Oke 1987). The second method involved estimating fluxes directly using a bulk method approach in which different measures of atmospheric stability are utilized, namely, the Monin-Obukhov length and the Richarson number (Oke, 1987, Brutsaert, 2005). A third methodology is presented based on the approach of Katul and Parlange (1992).

The original data was smoothed initially with hourly averages and later used with 5, 9, and 13 point moving averages to filter out high frequency variations. The results were then compared to estimations of both the latent and sensible heat flux for determining the variability and sensitivity of each set of methods. The results show a significant range in evaporation estimates particularly at low wind speeds when the atmosphere is unstable during at midday. The significance of these results is assed when comparing to measurements of green gases and malodor emissions from the lagoon.