Computer Science
Scientific paper
Jun 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002esasp.474e...2c&link_type=abstract
Proceedings of the SPECTRA Workshop - The concept of a space-borne Earth Observation Mission addressing the terrestrial componen
Computer Science
Scientific paper
Explaining observed regional-scale variability of carbon fluxes is critical for increasing the credibility of predictions of future ecosystem changes. Combining direct and remote observations is not straightforward because the observations are taken at different spatial and temporal scales and previous land satellite missions have not been designed for use together with existing measuring networks. Although ecosystem models, which estimate relevant components of the carbon cycle at different spatial and temporal scale, can serve as an integrative tool, their estimates of the state variables have uncertainties related to the poorly understood processes. Nevertheless, our first attempts to combine the three tools show interesting relationships between satellite indices and flux measurements as well as outline some issues, which can be resolve by the type of data provided by SPECTRA. A new technique of CO2 and energy fluxes measurements on tall (200-600 m) towers allows studying ecosystem-atmosphere interactions at the regional scale using a combination of CO2 observations, ecosystem modeling, and remote sensing. Signal measured at a tall tower integrates both daily biogeochemical cycles and small-scale heterogeneity of the land surface. For 1997-99 we compared the "preferred" annual net ecosystem exchange (NEE) measured at the tall tower in Wisconsin to the simulated annual NEE (BIOME-BGC model) and to adjusted normalized difference vegetation index (NDVI) aggregated over each year. Most of the differences between measured and modeled fluxes occurred in the beginning and the end of growing season. The length of the carbon uptake period defined by CO2 exchange observations was better captured by remote observations (NDVI) than by the model. Small deviations in growing season length resulted in significantly different annual NEE from measurements and model simulations. Analysis of the carbon uptake period by terrestrial vegetation and the annual net ecosystem exchange suggests linear correlation between the two. The growing season length derived from AVHRR NDVI data is closely related to the carbon uptake period and consequently can be used to approximate annual NEE values of the ecosystems. Remote detection of growing season length in arid and cloud-covered ecosystem was however problematic using data from AVHRR. The upcoming satellite mission should meet the following requirements: temporal resolution of the new sensor has to resolve subtle changes in the early and late growing season and detect pulses in a seasonal arid systems; angular resolution should be sufficient to detect canopy structural changes at the seasonal transitions; spectral information should allow to observe canopy chemical changes (Chl, etc); spatial resolution to observe the individual responses of distinct phases of the landscape and vegetation types. Acquisition strategy for remotely sensed data has to be coordinated with field measurement activities.
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