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The Photochemical Reflectance Index from Directional Cornfield Reflectances: Observations and SimulationsThe two-layer Markov chain Analytical Canopy Reflectance Model (ACRM) was linked with in situ hyperspectral leaf optical properties to simulate the Photochemical Reflectance Index (PRI) for a corn crop canopy at three different growth stages. This is an extended study after a successful demonstration of PRI simulations for a cornfield previously conducted at an early vegetative growth stage. Consistent with previous in situ studies, sunlit leaves exhibited lower PRI values than shaded leaves. Since sunlit (shaded) foliage dominates the canopy in the reflectance hotspot (coldspot), the canopy PRI derived from field hyperspectral observations displayed sensitivity to both view zenith angle and relative azimuth angle at all growth stages. Consequently, sunlit and shaded canopy sectors were most differentiated when viewed along the azimuth matching the solar principal plane. These directional PRI responses associated with sunlit/shaded foliage were successfully reproduced by the ACRM. As before, the simulated PRI values from the current study were closer to in situ values when both sunlit and shaded leaves were utilized as model input data in a two-layer mode, instead of a one-layer mode with sunlit leaves only. Model performance as judged by correlation between in situ and simulated values was strongest for the mature corn crop (r = 0.87, RMSE = 0.0048), followed by the early vegetative stage (r = 0.78; RMSE = 0.0051) and the early senescent stage (r = 0.65; RMSE = 0.0104). Since the benefit of including shaded leaves in the scheme varied across different growth stages, a further analysis was conducted to investigate how variable fractions of sunlit/shaded leaves affect the canopy PRI values expected for a cornfield, with implications for 20 remote sensing monitoring options. Simulations of the sunlit to shaded canopy ratio near 50/50 +/- 10 (e.g., 60/40) matching field observations at all growth stages were examined. Our results suggest in the importance of the sunlit/shaded fraction and canopy structure in understanding and interpreting PRI.
Document ID
20120017353
Acquisition Source
Goddard Space Flight Center
Document Type
Preprint (Draft being sent to journal)
Authors
Cheng, Yen-Ben (Earth Resources Technology, Inc. Laruel, MD, United States)
Middleton, Elizabeth M. (NASA Goddard Space Flight Center Greenbelt, MD, United States)
Zhang, Qingyuan (Universities Space Research Association Columbia, MD, United States)
Corp, Lawrence A. (Sigma Space Corp. Lanham, MD, United States)
Dandois, Jonathan (Maryland Univ. Baltimore County Baltimore, MD, United States)
Kustas, William P. (Department of Agriculture Beltsville, MD, United States)
Date Acquired
August 25, 2013
Publication Date
January 1, 2012
Subject Category
Earth Resources And Remote Sensing
Report/Patent Number
GSFC.JA.6714.2012
Report Number: GSFC.JA.6714.2012
Distribution Limits
Public
Copyright
Public Use Permitted.

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