Retrievals of chlorophyll fAPAR/LAI for GPP modeling

Qingyuan Zhang, GESTAR/USRA, qingyuan.zhang-1@nasa.gov (Presenter)
Tian Yao, GESTAR/USRA, tyao@usra.edu (Presenter)
Yujie Wang, UMBC, yujie.wang-1@nasa.gov
Alexei Lyapustin, NASA GSFC, alexei.i.lyapustin@nasa.gov

Gross primary production (GPP) of vegetation through the photosynthesis (PSN) process has widely been estimated as the product of photosynthetically active radiation (PAR) absorbed by chlorophyll (APARchl) and the photosynthetic light use efficiency (LUEchl). In the literature there are three groups of approaches to estimate absorbed PAR (APARchl): (1) directly utilizing leaf area index (LAI), e.g., MOD15A2 LAI; (2) directly using fraction of APAR of the whole canopy (fAPARcanopy), e.g., MOD15A2 FPAR; and (3) using LAI/fAPARcanopy estimated with vegetation index (VI), e.g., the normalized difference vegetation index (NDVI), the enhanced vegetation index (EVI), the green band wide dynamic range vegetation index (WDRVIgreen), and the green band chlorophyll index (CIgreen). However, only the PAR absorbed by chlorophyll is utilized for photosynthesis, and PAR absorbed by non-photosynthetic components of the canopy is not used for photosynthesis.

We have developed an algorithm to derive fAPAR of chlorophyll (fAPARchl), fAPAR of foliage (fAPARfoliage) and chlorophyll LAI (LAIchl) for globally selected flux tower sites with the radiative transfer model PROSAIL2 and the MODIS surface reflectance. GPP estimates with fAPARchl and with MOD15A2 FPAR were compared to tower flux GPP. GPP simulated with fAPARchl was corroborated with tower flux GPP. Improvements in GPP estimation were achieved by replacing MOD15A2 FPAR with fAPARchl. MOD15A2 LAI and the retrieved LAIchl were implemented into CLM to simulate GPP, respectively. Simulated GPP series with both MOD15A2 LAI and LAIchl were compared to tower flux GPP. GPP simulation with LAIchl was significantly improved, compared to GPP simulation with MOD15A2 LAI. Our results show that the fAPARchl/LAIchl products significantly improve the accuracy and reduce the uncertainty of GPP estimates, compared to MOD15A2 FPAR/LAI.

Presentation Type:  Poster

Session:  Theme 3: Future research direction and priorities: perspectives relevant to the next decadal survey   (Mon 4:30 PM)

Associated Project(s): 

• Zhang, Qingyuan: Carbon Monitoring and Ecosystem Feedbacks Prediction Using fAPARchl and the Community Land Model (CLM) ...details

Poster Location ID: 228