overview of the recent progresses in LAI/FPAR estimation algorithms and
resulting biophysical products from the AVHRR, MODIS, SPOT and Landsat
data.
2.1 Introduction
Leaf Area Index (LAI), the one-sided green leaf area per unit ground area, and the
Fraction of Photosynthetically Active Radiation (FPAR; 400–700 nm) absorbed
by vegetation are important biophysical variables for quantifying the cycling of
water, carbon and nutrients through ecosystems (Demarty et al. 2007; Sellers et al.
1996; Tian et al. 2004). LAI characterizes the functioning surface area of a vegetation canopy (Myneni et al. 2002). The interactions between the vegetation
surface and the atmosphere, for example, radiation exchange, transpiration rates,
precipitation interception, momentum and gas exchange, is predominantly determined by leaf area (Monteith and Unsworth 1990). An increase in leaf area, for
example, increases the uptake of CO 2 from the atmosphere due to greater sunlight
absorption and hence results in increased canopy conductance and transpiration
rates (Field and Mooney 1983). Field measurements of LAI include hemispherical
photography and optical instruments like TRAC, LAI-2000 or LI-3000C (Chen
et al. 1997; Weiss et al. 2004). Satellite remote sensing enables retrieval of LAI
globally at different spatial resolutions and temporal frequency with algorithms
based on the physics of radiative transfer. Another parameter that characterizes the
energy absorption capacity of a vegetation canopy is FPAR, defined as the fraction
of photosynthetically active radiation (0.4–0.7 lm) absorbed by the vegetation
canopy. FPAR depends on the incident radiation field, architecture and absorption,
reflectance and transmission spectra of the canopy as well as the reflectance of the
soil and/or understory background. FPAR is well related to NDVI and usually
increases with fractional canopy cover and plant leaf area (Myneni and Williams
1994). It is one of the fundamental parameters used to estimate net primary production and for modeling of terrestrial carbon processes (Knorr and Kattge 2005;
Pitman 2003; Sellers et al. 1986). Similar to LAI, FPAR has also been identified as
one of the fundamental terrestrial state variables in the context of global change
studies (GCOS 2006).
The LAI/FPAR products from the Advanced Very High Resolution Radiometer
(AVHRR), the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor
and the Système Pour l’Observation de la Terre (SPOT) sensor have a large Earth
science community user base and the ease of access, provision of pixel quality and
validation information have greatly aided the use of these products. Recent
research efforts focusing on inter-sensor product consistencies have developed a
foundation upon which mature algorithms and a validation framework can act
synergistically to further refine the accuracy and precision of these existing longterm products (Brown et al. 2006; Ganguly et al. 2008b; Tarnavsky et al. 2008;
44
S. Ganguly et al.
resulting biophysical products from the AVHRR, MODIS, SPOT and Landsat
data.
2.1 Introduction
Leaf Area Index (LAI), the one-sided green leaf area per unit ground area, and the
Fraction of Photosynthetically Active Radiation (FPAR; 400–700 nm) absorbed
by vegetation are important biophysical variables for quantifying the cycling of
water, carbon and nutrients through ecosystems (Demarty et al. 2007; Sellers et al.
1996; Tian et al. 2004). LAI characterizes the functioning surface area of a vegetation canopy (Myneni et al. 2002). The interactions between the vegetation
surface and the atmosphere, for example, radiation exchange, transpiration rates,
precipitation interception, momentum and gas exchange, is predominantly determined by leaf area (Monteith and Unsworth 1990). An increase in leaf area, for
example, increases the uptake of CO 2 from the atmosphere due to greater sunlight
absorption and hence results in increased canopy conductance and transpiration
rates (Field and Mooney 1983). Field measurements of LAI include hemispherical
photography and optical instruments like TRAC, LAI-2000 or LI-3000C (Chen
et al. 1997; Weiss et al. 2004). Satellite remote sensing enables retrieval of LAI
globally at different spatial resolutions and temporal frequency with algorithms
based on the physics of radiative transfer. Another parameter that characterizes the
energy absorption capacity of a vegetation canopy is FPAR, defined as the fraction
of photosynthetically active radiation (0.4–0.7 lm) absorbed by the vegetation
canopy. FPAR depends on the incident radiation field, architecture and absorption,
reflectance and transmission spectra of the canopy as well as the reflectance of the
soil and/or understory background. FPAR is well related to NDVI and usually
increases with fractional canopy cover and plant leaf area (Myneni and Williams
1994). It is one of the fundamental parameters used to estimate net primary production and for modeling of terrestrial carbon processes (Knorr and Kattge 2005;
Pitman 2003; Sellers et al. 1986). Similar to LAI, FPAR has also been identified as
one of the fundamental terrestrial state variables in the context of global change
studies (GCOS 2006).
The LAI/FPAR products from the Advanced Very High Resolution Radiometer
(AVHRR), the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor
and the Système Pour l’Observation de la Terre (SPOT) sensor have a large Earth
science community user base and the ease of access, provision of pixel quality and
validation information have greatly aided the use of these products. Recent
research efforts focusing on inter-sensor product consistencies have developed a
foundation upon which mature algorithms and a validation framework can act
synergistically to further refine the accuracy and precision of these existing longterm products (Brown et al. 2006; Ganguly et al. 2008b; Tarnavsky et al. 2008;
44
S. Ganguly et al.