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  • Thumbnail for Carbon fluxes in the SiB3 model : evaluating the response of the terrestrial biosphere to significant drought
    Carbon fluxes in the SiB3 model : evaluating the response of the terrestrial biosphere to significant drought by Shiach, Ian Michael

    This study investigated the behavior of the terrestrial biosphere during times of significant drought, particularly in regard to carbon fluxes. The Simple Biosphere Model Version 3 (SiB3) was used to facilitate an investigation of ecosystem drought response. The Standardized Precipitation Index (SPI) was evaluated from 1983 to 2006 in order to produce historical drought maps, which were used to facilitate a subjective analysis of drought behavior and to identify geographic point locations in the SiB3 model for further temporal study. Standardized maps were produced for modeled physiological variables (gross primary productivity, respiration, net ecosystem exchange, and soil water stress factor) over time in order to determine general regional drought response patterns. Physiological response variable data for particular spatial locations was then analyzed over time during drought years for anecdotal comparison with observational study data. While the SPI, which standardizes precipitation, was predicted to be an indicator of ecosystem drought response, this did not appear to be the case. The droughts modeled in the SiB3 model, which included the droughts in the United States Southwest and Australia in 2002 and in Europe in 2003, were found to respond heterogeneously in terms of carbon fluxes to similar droughts. The U.S. Southwest and Australia appeared to respond to drought in a manner consistent with anecdotal evidence with regard to perturbations in gross primary productivity (GPP), ecosystem respiration, and net ecosystem exchange (NEE), while Europe appeared to respond in a manner dissimilar to published descriptions of that drought. The behavior of the soil water stress factor in Australia and Europe seemed to be incorrect as well. Precipitation input data, derived from a reanalysis dataset from the National Centers for Environmental Prediction (NCEP), the treatment by the SiB3 model of the soil water stress factor, and the possible heterogeneous vegetative response to seasonality between regions were identified as potential causes of these disparities.