Estimation of Terrestrial Carbon Fluxes by Integrating Remote Sensing with Ecosystem Modelling
2. Methodology
The ecosystem model used in the study is known as Sim-CYCLE developed by Ito (2000) and it has been used to retrieve the carbon dynamics of various terrestrial ecosystems in global scale. In Sim-CYCLE, terrestrial carbon dynamics are conceptualised as a five-compartment system, viz., foliage (WP
F), stem (WP
C), root (WP
R), litter (WP
L) and mineral soil (WP
H) as shown in Fig. 1(a). Carbon fluxes in these compartments include gross primary production (GPP), maintenance respiration (ARM), translocation (PT), growth respiration (ARG), litterfall (LF) and heterotrophic respiration (HR).
Fig.1 Schematic diagram of Sim-CYCLE (Ito et al., 2002)
Fig. 1(b) shows the water and radiation schemes from the single leaf to canopy scale. Stomatal conductance (GS), intercellular CO
2 concentration (CD
ICL), light-use efficiency (QE) and photosynthetic carbon assimilation (PC) are determined interactively as functions of various environmental factors such as photosynthetic photon flux density (PPFD), atmospheric CO2 concentration (CDATM), surface temperature (TG), soil-water availability (MS) and ambient vapour pressure deficit (VPD). Precipitation (PR) is partitioned into soil-water storage (MS), evaporation from soil surface (EV), evaporation from canopy (IC), transpiration from canopy (TR) and subsurface runoff (RO). Evaporation and transpiration depend on the input energy to the canopy (RNC) and soil (RNS).
Gross primary production (GPP) is the origin of all organic carbon through which atmospheric CO
2 is fixed into dry matter. The scaling procedure in terms of GPP is one of the characteristics of Sim-CYCLE and GPP is estimated by the dry-matter production theory established by Monsi and Saeki (1953). Sim-CYCLE simulate carbon dynamics of individual leaves of canopy and then scale this leaf level carbon dynamics to the canopy level by accounting for variations in structure and environmental conditions within the canopy. Monsi and Saeki formulated the downward attenuation of photosynthetically active radiation (PAR) irradiance due to mutual shading of leaves in a canopy with accumulating leaf area as follows:
PPFD
IN = PPFD
TOP exp(-KA. LAI
CML)
