Reducing Uncertainty in Biospheric CO2 Exchange

Our understanding of terrestrial CO2 fluxes comes primarily from flux towers which measure CO2, H2O, and energy exchange between the vegetated surface and the atmosphere with the eddy covariance (EC) technique. The proliferation of flux towers began in the late 1990's - early 2000's with flux towers rapidly erected in the United States (AmeriFlux), Canada, Europe and Australia/New Zealand. Today there are over 400 active flux sites across the globe, as part of a broad network called FLUXNET, spanning nearly all biomes and climate zones, including more recent deployments in Mexico/South America, Asia, and Africa (http://www.fluxnet.ornl.gov/fluxnet/index.cfm). Given that hundreds of towers across the globe directly measure biospheric CO2 exchange, the question then becomes, why is uncertainty in the land CO2 fluxes so high? Flux towers suffer from logistical problems including instrument failure and power outages which lead to data gaps as well as random and systematic errors inherit to the EC technique: rain, lack of turbulence, inhomogeneous terrain, and inhomogeneous vegetation lead to erroneous flux estimates. These periods of missing or erroneous data must be gap-filled in order to estimate whether an ecosystem is a net annual carbon sink or source. Current gap-filling techniques are inadequate and over the course of a year can cause estimates to be off by 30-50 g C m-2 yr 1 or more. For an ecosystem that is a small annual sink or source of carbon (typical for a disturbed forest, grassland, or savannah ecosystem), this uncertainty is then of the same magnitude as the measured flux! In this study we describe a novel approach to gap filling – one which combines state-of-the-art instruments with an advanced soil-canopy-atmosphere model – to reduce the uncertainty in the biospheric CO2 flux. We will test our gap-filling model at two contrasting flux towers: Wind River, a 60 m tall conifer forest, and at Site 300, a one m tall grassland. Contrasting ecosystems will provide very different test sites for validating the CO2 flux model.