Outline

• Approach• Data sets• PAR• fPAR• C4/C3• LUE optimization

Approach

• GPP = LUE × APAR = LUE × fPAR × PAR • PAR = SW × RPB

• C4 and C3 LUE and fPAR algorithm

Data set: Ameriflux Site

• Ameriflux GPP observations were used to minimize the discrepancy between modeled and observed GPP

• Sites with more than one year data during 2004 to 2006 were chosen

• Monthly GPP calculated from half-hourly GPP fluxes

Data set: fPAR products• EVI/NDVI-derived fPAR

– Monthly EVI/NDVI 2004-2006– Original: 0.05 deg– Used in model: 0.5 deg

• MODIS fPAR– Monthly MODIS fPAR 2004-2006– Original: 0.05 deg– Used in model: 0.5 deg

• SeaWifs fPAR– Monthly, 2004-2006 Jan – June– Last 6 months were restructured by monthly mean from previous years– Original: 0.25 deg– Used in model: 0.5deg

Data set: radiation• GEOS-5 surface net shortwave radiation and surface

albedo– 3-hourly; 2004 – 2006; 2×2.5 deg

• SRB PAR and SW– Monthly; 2004-2006; 1 deg

• CERES PAR and SW– Monthly; 2004-2005; 1 deg

• BSRN measured SW– Monthly; 2004-2005

Data set: C4 distribution

• Global C4 percentage– 1 deg

• Africa C4 percentage– 1 km

EVI/NDVI-derived fPAR

• Area distribution of global EVI according to biome types

• Linear function applied to covert EVI/NDVI to fPAR

• Upper bound: top 25% EVI of topical rainforest

• Lower bound: EVI in Sahara desert with annual rainfall less than 25 mm

PAR• Adjustment for GEOS-5 shortwave radiation

– Downward SW = net SW / ( 1 - albedo )– Ratio of SRB SW to GEOS SW

• Conversion from shortwave radiation to PAR– Ratio of SRB PAR to SRB SW– High temporal resolution PAR using GEOS-5 adjusted SW

APAR = PAR × fAPR

C4 photosynthesis

• Algorithm A– Same fPAR for C3 and C4– Separate APAR according to

C3 and C4 spatial distribution

• Algorithm B– Allocate most of the fPAR

seasonal cycle to C4– Separate APAR with different

fPAR for C3 and C4

C4 LUE• Lobell ratio:– Mean C4 LUE: 0.66 g C/MJ– Mean C3 LUE: 0.29 g C/MJ

• Ameriflux ratio:– Cropland rotation sites– Mean C4 GPP/mean C3 GPP = 1.32

LUE optimization

• A range of LUE was used to calculate GPP using the paradigm:

GPP = LUE × APAR• Just account for monthly GPP from all C3 sites and

rotation site with C3 crops• C4 LUE calculated from the Lobell and Ameriflux

ratios