Eddy Covariance Measurements of the Surface Energy Budget and CO2 fluxes over a Reservoir
Inland water bodies (e.g., lakes, reservoirs, wetlands, streams, etc.) play a very important role in biogeochemical cycles at regional and global scales. Total CO2 and CH4 emissions from inland waters are now estimated at 2.1 Pg C /yr and 0.65 Pg C /yr, respectively. CO2 emissions from inland waters are a consequence of the super-saturation of CO2 in the surface water, as quantified by CO2 concentration in the surface water (hereafter pCO2). Such super-saturation is primarily due to respiration of allochthonous organic carbon and transport to aquatic systems of dissolved CO2 by surface runoff and ground water flows. CO2 emission rates are conventionally estimated by indirect methods (e.g., wind-based gas transfer models and the surface renewal model) that rely on pCO2 and a gas transfer coefficient (hereafter indirect methods). Identifying the sources of uncertainties associated with the indirect methods and understanding underlying mechanisms is necessary to reducing uncertainties in the contribution of carbon emissions from global inland waters to regional and global carbon budgets and the response of inland waters to climate change. We conduct long-term eddy covariance measurements of CO2 fluxes over a reservoir to quantify CO2 emission from diurnal to seasonal and annual time scales, aiming to understand physical processes and mechanisms that regulate CO2 emissions.
Inland waters act differently than surrounding lands in the exchanges of radiation, energy, and water vapor between the water and the overlying atmosphere, thus providing unique spots in terms of their influence on the local and even regional climate and hydrological cycle. Shortwave radiation that is not reflected by the water surface penetrates to deeper layers and is directly absorbed by the water from the surface to a certain depth. The amount of solar radiation absorbed by water decreases exponentially with the water depth and is dependent on water turbidity, which is influenced by suspended organic and inorganic material. Water eddy diffusion leads to efficient heat transfer in water layers below the surface. As a consequence, water surfaces respond differently than land surfaces to diurnal and seasonal changes in the solar radiation forcing, in terms of temporal variations in the surface temperature. It is known that sensible heat flux is primarily determined by the air temperature difference between the water surface and the overlying atmosphere as well as the turbulent exchange coefficient. Latent heat flux is dependent upon vapor pressure differences between the water-atmosphere interface and the overlying atmosphere as well as the turbulent mixing intensity. The water-air interface is typically at its saturation point and vapor pressure at this interface is a function of the water surface temperature, thus making the water surface temperature a key element in determining a water-atmosphere exchange. Smooth water surface and wind-induced waves alter turbulent exchange coefficients of energy and water vapor across the water-atmosphere interface. As a consequence, turbulent exchange of momentum and scalars across the water-atmosphere interface remains a challenging task due to lack of eddy covariance measurements of turbulent fluxes over water surfaces. Our research objectives are to understand the processes that regulate different components of the water surface energy budget and evaporation, and to study their diurnal, intra-seasonal, seasonal variations, and interannual variations.
The eddy covariance flux tower is located in the center of the Ross Barnett Reservoir (32°26'17.63''N; 90°1'48.00''W) in Mississippi, USA. The reservoir has a surface area of 134 km2 with depths ranging from 4 to 8 m. An eddy covariance tower is installed over a stable wooden platform, which has dimensions of 3 m x 3 m and is at a height of 1 m above the water. To minimize the impacts of the surrounding land on flux footprints, the distance from the platform to shore ranges from about 2 km to more than 14 km.
The instruments mounted on the tower include a 3-D sonic anemometer (mode CSAT3, Campbell Scientific, Inc.) and an open-path CO2/H2O infrared gas analyzer (mode IRGA, LI-7500, LI-COR, Inc.) at about 2.8 m above the water surface, five temperature and humidity probes (mode HMP45C, Vaisala, Inc.) at 1.2, 2.4, 2.8, 3.5 and 4.7 m above the platform and three wind speed sensors (model 03101, RM Young, Inc.) at 2.4, 3.5 and 4.7 m above the platform. Water skin temperature (Ts) is measured by two infrared temperature sensors (mode IRR-P (sn:2481 and 5395), Apogee, Inc.). Water temperature profiles (107/109, Campbell Sci. Inc.) are measured at depths of 0.001, 0.025, 0.05, 0.1, 0.25, 0.4, 0.55, 0.7, 0.85, 1.0, 1.2, 1.5, 1.8, 2.1, 2.5, 3.0, 3.5, 4.0, and 4.5 m. A net radiometer (CNR 2; Kipp & Zonen Inc.) measures net shortwave and longwave radiation. A tipping bucket rain gauge (TB3, Hydrological Services) is used to collect 5-min accumulative precipitation. A barometer (RPT410F, Druck, Inc.) measures atmospheric pressure. Signals from the eddy covariance system (CSAT3 and LI7500) are sampled at 10 Hz and signals from all other sensors are recorded as 5-min average values of 1-s samples by a data logger (CR5000, Campbell Scientific, Inc.). All sensors/instruments are powered by nine deep-cycle marine batteries that are charged with two solar panels (SP65, 65 W Solar Panel, Campbell Scientific, Inc.).