期刊名称：ISPRS Annals of the Photogrammetry, Remote Sensing and Spatial Information Sciences
印刷版ISSN：2194-9042
电子版ISSN：2194-9050
出版年度：2007
卷号：XXXVI-7/C50
出版社：Copernicus Publications
摘要：Ground measurements of photosynthetically active radiation (PAR) balance and leaf area index (LAI) are required for validating, and sometimes calibrating biophysical products derived from satellite observations. Non destructive methods are either based on instantaneous observations of the directional gap fraction from which the fraction of PAR absorbed by the canopy (fAPAR) is approximated by the PAR interception efficiency (fIPAR). LAI can also be derived from gap fraction measurements, leading to estimates of the effective LAI, or to an approximation of the actual LAI under a number of assumptions on leaf clumping and presence of woody elements. Current devices dedicated to gap fraction (and LAI) measurements are generally 'instantaneous': hemispherical photos or light transmittance (LAI2000, TRAC, ACUPAR…) systems operate within one 'shot'. Such systems allow easy replications over several places to account for spatial variability and get better representation of the average value of areas corresponding to a single or few high spatial resolution (5-50m) satellite pixels, However, they are very tedious to operate when describing the seasonality of fAPAR and LAI, which is mandatory for a number of studies. Alternatively, classical PAR balance could be installed at the site level by distributing individual PAR sensors on the ground, which are connected to a data logger. However, this system is relatively tedious to install because of the wires, has limited autonomy (both memory and energy wise) and is relatively costly, limiting thus a large spatial coverage
