Earth Surface Dynamics
Although lakes occupy only a small fraction of Earth's surface area, study of the CO2 exchange between them and the atmosphere is needed to better define the carbon cycle and to improve our understanding of air-water gas transfer over much larger water bodies such as oceans. Williams Lake (part of the USGS's Interdisciplinary Research Initiative in the Mississippi River headwaters area of north-central Minnesota) was selected for a 3 year (1992-4) study of air-water CO2 exchange. Detailed measurements of lake-water CO2 concentrations were made over the course of the experiment. Eddy covariance instruments were deployed from a tower that was inserted into lake sediments and guyed underwater.
A raft tethered to the tower supplied data logging and logistical support for the equipment. Eddy covariance measurements of CO2 flux and measured change in lake CO2 storage were compared with state of the art models of flux over water. Agreement was inconsistent between the two measurements and the models, bringing into question whether some other mechanisms, not represented in the models, were at work. The models tested are often used to estimate global air-sea CO2 exchange. (See Anderson et al. 1999; Limnology and Oceanograph vol. 44 for further details.)
Researcher installing micrometeorological instruments on a communications-type tower in a Canadian coniferous forest. |
In addition to establishing that jack pine forests are a weak, perhaps ephemeral, sink for atmospheric CO2, the USGS team found age dependent differences of carbon sequestration with younger forests drawing about 20% more CO2 than mature stands. The USGS and other BOREAS science teams found that evapotranspiration from these forests was considerably less than originally thought while heat flux to the atmosphere was considerably more. This finding has bearing on the parameterization of these forests in numerical weather prediction models (see J. Geophys. Res. (1997), BOREAS Special Issue, Vol. 102).
Subalpine forest site west of Boulder, Colorado. The Rocky Mountain range in the background is part of the Continental Divide. |
Infrastructure is currently being emplaced near the Niwot Ridge LTER (Long term Ecological Research) site. This site operates as a part of Ameriflux -a group of individual investigators making eddy covariance measurements in North and South America (see Science (1998) Vol 281. p.506, for more information). Detailed eddy covariance flux measurements, and wind and CO2 concentration profile measurements will be conducted from two 33 m (110') tall towers, individually operated by the University of Colorado and the USGS, in collaboration with NCAR (National Center for Atmospheric Research) researchers and with the cooperation of the U.S. Forest Service. This will be the first Ameriflux site capable of directly measuring nocturnal drainage flow of CO2 flux originating from ecosystem respiration. CO2 flux carried by this flow can be a substantial fraction of the total diurnal and annual forest-atmosphere flux.
In a unique application of the eddy covariance method, diffuse gas emissions from the volcano's flank were continuously measured in three pilot studies (1996,'97,'98). These week-long studies afforded the opportunity to evaluate the feasibility of this methodology in the measurement of a strong, heterogeneous, CO2 source in complex terrain. These were the first direct measurements of CO2 flux of volcanic origin. Results indicate that the basic assumptions underlying the eddy covariance technique were preserved in this experiment and that long term, continuous measurements may be made at the site. (Procedures and results are to appear later this year in Chemical Geology.)
Gas or heat flux is determined by calculating a covariance statistic of the fluctuations in vertical wind velocity and the entity of interest (gas concentration or temperature) from a series of sequential measurements made over time (about 30 minutes, typically). To sufficiently capture significant turbulent motions, eddy covariance instrumentation must be capable of accurately sampling wind or gas concentration at least several times per second. The instrumentation may be costly (depending upon needs) and considerable care is needed to ensure that environmental conditions will not violate assumptions underlying the methodology (see Kaimal and Finnigan, 1994, Atmospheric Boundary Layer Flows, Oxford Press, N.Y.). Instruments are typically deployed from fixed platforms (e.g. towers or scaffolding) meters to 10's of meters above the surface. Depending upon instrument height above the surface, eddy covariance measurements are representative of a surface-atmosphere flux occurring over an upwind area on the order of hectares to 100's of hectares. Usually the higher the instruments are placed, the larger the upwind area sampled. In some instances, eddy covariance instruments have been flown aboard aircraft, enabling researchers to make comparative flux measurements over 100's of kilometers.
Dean E. Anderson USGS, WRD MS413, Box 25046, DFC Denver, CO 80225 303-236-5691 deander@usgs.gov |
Dave Stannard USGS, WRD MS413, Box 25046, DFC Denver, CO 80225 303-236-4983 distanna@usgs.gov |