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Title:   Author  item
                                            ZHANG, Q. - Collaboratoritem
                                            CHENG, Y. - Collaboratoritem
                                            LYAPUSTIN, A. - National Aeronautics And Space Administration (NASA)item
                                            WANG, YUEJIE - National Aeronautics And Space Administration (NASA)item
                                            Gao, Fengitem
                                            SUYKER, A. - University Of Nebraskaitem
                                            VERMAN, S. - University Of Nebraskaitem
                                            MIDDLETON, E.M. - National Aeronautics And Space Administration (NASA)
  Submitted to: Remote Sensing of Environment
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 7/16/2014
                                    Publication  Date: 8/14/2014
                                    Publication URL: http://handle.nal.usda.gov/10113/59906Citation: Houborg, R., McCabe, M., Cescatti, A., Gao, F.N., Schull, M.A., Gitelson, A. 2015. Joint Leaf chlorophyll and leaf area index retrieval from Landsat data using a regularized model inversion system. Remote Sensing of Environment. 159:203-221.Interpretive Summary: Crop gross primary production (GPP) is an essential indicator for monitoring crop carbon exchange and growth. The Moderate Resolution Imaging Spectroradiometer (MODIS) GPP product has been available globally since early 2000. It is computed based on the fraction of absorbed photosynthetically active radiation (FPAR) at the canopy level, which does not separate between chlorophyll and non-chlorophyll contents. This paper examines the MODIS FPAR and GPP products in three AmeriFlux crop fields in Nebraska. The FPAR absorbed by chlorophyll content (fAPAR_chl) was retrieved and used to estimate crop GPP. Results show that fAFPAR_chl can improve GPP estimations for both corn and soybeans as compared to the in-situ measurements, especially during the spring and fall seasons. The accurate estimation of crop GPP is important for crop growth monitoring and yield estimation as required by the National Agricultural Statistics Service and Foreign Agricultural Service.
                                Technical Abstract: Within leaf chloroplasts chlorophylls absorb photosynthetically active radiation (PAR) for photosynthesis (PSN).  The MOD15A2 FPAR (fraction of PAR absorbed by canopy, i.e., fAPARcanopy) product has been widely used to compute absorbed PAR for PSN (APARPSN). The MOD17A2 algorithm uses MOD15A2 FPAR in gross primary production (GPP) estimation. In this study, the fraction of PAR absorbed by chlorophyll throughout the canopy (fAPARchl) was retrieved from the Moderate Resolution Imaging Spectroradiometer (MODIS) images for three AmeriFlux crop fields in Nebraska. There are few studies in literature that compare the performance of MOD15A2 FPAR versus fAPARchl in GPP estimation. The MOD15A2 FPAR and the retrieved fAPARchl and were compared with in-situ fAPARcanopy and in-situ fraction of PAR absorbed by green leaves of the canopy (fAPARgreen). MOD15A2 FPAR overestimated in-situ fAPARcanopy in spring and in fall, and underestimated field fAPARcanopy in midsummer while fAPARchl captured the spring on and fall off phenology. The retrieved fAPARchl matched well with in-situ fAPARgreen in early crop growth in June, and was lower than field fAPARgreen in late July, August and September. The fAPARchl and MOD15A2 FPAR were used to simulate GPP, respectively, and the GPP estimates were compared to the in-situ GPP. GPP simulated with fAPARchl was corroborated with in-situ GPP. Replacing MOD15A2 FPAR with fAPARchl resulted in improvement of GPP estimates, and reduction of uncertainties.


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