Estimation of Terrestrial Carbon Fluxes by Integrating Remote Sensing with Ecosystem Modelling
where PPFDTOP denotes the photosynthetic photon flux density at the canopy top. The PAR irradiance attenuates exponentially with the light attenuation coefficient KA, such that a leaf underlying the cumulative leaf area index (LAICML) receives light equivalent to incident photosynthetic photon flux density (PPFDIN) on leaf surface. Daily GPP increases linearly with increasing leaf area index (LAI) and saturates to LAI when KA is high, suggesting that radiation use efficiency depends on plant productive structures (Saeki, 1960). Thus, LAI is a key parameter of Sim-CYCLE and it is defined as the one-sided green leaf area per unit ground area in broadleaf canopies and projected leaf area for needle leaf area in coniferous canopies. In Sim-CYCLE, LAI is estimated on basis of specific leaf area assumed for each biome types. However, such estimation can not be accurate for ever-changing landuse and land cover around the globe. Satellite remote sensing is the most effective means of retrieving LAI in global scale
at a regular interval. Advances in remote sensing technology and radiative transfer modelling (Myneni and Ross, 1991) have improved the possibility of accurate estimation of LAI from spectral and angular dimensions of remotely sensed data. MODIS sensor in TERRA platform provides rich spectral and angular sampling of reflected radiation field for estimating LAI in global scale. MODIS-LAI product is currently being produced operationally by NASA and it is available for user community.
LAI estimated by Sim-CYCLE are first compared with the MODIS-LAI for identifying the actual biome responses and then MODIS-LAI is used to set the initial conditions and boundary values of vegetation structures for further simulation of Carbon budgets.
3. Results and Discussions
3.1 Annual carbon fluxes
Global annual GPP, NPP and AR are estimated as 131.2, 62.7 and 68.5 PgCyr
-1 respectively, after reaching an equilibrium through iterative calculations, using the Sim-CYCLE model. Incorporating the MODIS-LAI into the Sim-CYCLE model as described in the methodology section, the global annual GPP, NPP and AR are re-estimated as 122.1, 59.6 and 62.5 PgCyr
-1 respectively.
Fig.2 Latitudinal distribution of annual NPP
Latitudinal distribution of NPP from the original and MODIS-LAI incorporated Sim-CYCLE follows similar trends as shown in Fig.2. The highest NPP occurs around the equator with an another smaller peak occurring around 60ºN and in both cases, NPP estimated by MODIS-LAI incorporated Sim-CYCLE exceeds the NPP estimated by original Sim-CYCLE. These two regions are tropical and boreal forest respectively. Other than these two regions, overall estimation of NPP is higher in case of original Sim-CYCLE than MODIS-LAI incorporated Sim-CYCLE.
