Enhancing the Simplified Biosphere Model (SSIB) to Estimate Carbon Fluxes of Terestral Ecosystems for LBA study

Increase in greenhouse gases, in particular CO2, has great impacts on global climate change. Recent studies have shown that terrestrial ecosystems may be an important sink of atmospheric CO2, which indicates a pressing need for accurate assessments of the CO2 fluxes between the terrestrial ecosystems and the atmosphere. The current version of the simplified biosphere model (SSIB) does not consider the photosynthetic activities of CO2 assimilation of plants and thus is unable to model the CO2 fluxes. The work in this paper is an enhancement of the SSIB model for the CO2 flux modeling. The equations and parameters of the photosynthesis-stomatal conductance coupled models by Collatz et al. For C3 or C4 plants are adopted for simulating the uptake or release of CO2 by land surface ecosystems. These models have sound physiological basis and fine description to the interactive effects of environmental factors of plant photosynthesis and stomatal movement. In the current literature, however, the equations of these models were solved numerically with iterations, which consumes a great amount of computational time. In some cases, the iteration process may not be convergent and an appropriate solution may be difficult to obtain. In this study, a new procedure is developed to quasi-analytically solve the equations of these models.

By this procedure, appropriate solutions can be directly computed without iterations. When an appropriate solution of these models does not exist under some night time environmental conditions, the procedure applies simpler empirical equations from literature to simulate the processes because the land surface fluxes in these cases are not significant. Off-line testing of the enhanced SSIB model for modeling the land surface fluxes of CO2, water vapor and sensible heat is conducted against field measurements from the ABRACOS and other LBA experiments.