Satellite Estimation of the Global Distribution and Interannual Variability of Photosynthetically Active Radiation.
Abstract
Photosynthetically active radiation (PAR, 400 -700 nm) is a fundamental resource for global primary production, but existing data sources have been insufficient to provide a reliable, quantitative account of global PAR distribution and variability. The objectives of this research were: (1) evaluate the accuracy of a new satellite remote sensing method (Rem. Sensing Environ., 38:135-146) for estimating and monitoring incident PAR on a global basis, and (2) evaluate the magnitude, distribution, and interannual variability of global incident PAR to determine its potential significance as a factor contributing to variability in global patterns of vegetation-absorbed PAR (APAR) and primary production. The method employs ultraviolet (370 nm) reflectance data from the Nimbus-7 Total Ozone Mapping Spectrometer (TOMS) with a maximum spatial resolution of 1^circ x 1.25^circ. The study covers the 11-year (132 month) period between January 1979 and December 1989, and latitudes between 66^ circN and 66^circS. Validation of monthly PAR estimates at sample points produced coefficients of determination (r^2) between.863 and.996, and RMS differences between 2.8% and 16.1% (average RMS difference = 9.2%). Possible sources of error in both the satellite estimates and reference data are identified. The global analysis identified distinct regions of substantial interannual variability in incident PAR. Coefficients of variation (CV) as high as 18-24% for growing season months and 4-7% for annual PAR were observed in Europe, China, Southeast Brazil and Northern Australia, among other regions. CVs as high as 20% for monthly PAR and 8% for annual PAR in the western tropical Pacific Ocean region may be associated with El Nino-Southern Oscillation (ENSO) conditions. Deviations in global average PAR observed in 1982 suggest possible global-scale effects of the El Chichon volcanic eruption and/or ENSO. The results from the study confirmed that many regions of the globe are subject to substantial interannual variations in monthly and annual amounts of PAR received. Based on understanding from current physical models and evidence from recent field investigations, this variability represents a significant, potential source of variability in vegetation-absorbed PAR (APAR) and primary production.
- Publication:
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Ph.D. Thesis
- Pub Date:
- January 1992
- Bibcode:
- 1992PhDT.......125D
- Keywords:
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- Physical Geography; Physics: Atmospheric Science; Biology: Ecology; Remote Sensing