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SRS Sensors
3.4
Photochemical Reflectance Index (PRI)
As described above, the NDVI is primarily useful as a proxy for
canopy structural variables. Although structural properties are crit-
ical, sometimes it is useful to have information about canopy func-
tional properties. For example, estimating the gross primary produc-
tivity (GPP) of ecosystems is critical for modeling the global carbon
balance. The simple model presented in Equation 2 can be used to
predict GPP from three variables: incident light (S
light (f
), and light use efficiency ( ). S
s
depending on geographic location and time of day or measured with
a PAR sensor or pyranometer. Considering the near linear relation-
ship between NDVI and fractional interception noted above, a simple
two-band spectral reflectance sensor like the SRS-NDVI can provide
an estimate of f
quantified in order to make accurate predictions of GPP.
Gamon et al. (1990, 1992) proposed a dual band vegetation index
(similar to the NDVI) to predict . The foundation of the measure-
ment is based on the absorbance of xanthophyll pigments at 531 nm
that correlates with LUE in many plant species (Gamon et al., 1997).
This ratio is called the Photochemical Reflectance Index (PRI) and
is calculated with Equation 3.
Where, ρ
and ρ
531
respectively. When combined, you can use NDVI and PRI to predict
biomass accumulation or GPP of an ecosystem without the expense
and work of some of the other approaches (Gamon et al., 2001). Be-
cause of the low cost, light weight, small footprint and low power use
of the sensors, they can be deployed very quickly, over long periods
of time, or in a spatially distributed network to quantify spatiotem-
poral variations in canopy productivity (Garrity et al., 2010).
In addition to LUE, the PRI has also been shown to correlate with
numerous other physiological variables associated with plant photo-
synthetic performance from the leaf to the ecosystem level (Gamon et
al., 1992, 1997, 2001). Xanthophylls absorb radiation at 531 nm and
. The light use efficiency term ( ) remains to be
s
P RI =
are percent reflectances at 531 and 570 nm,
570
can generally be estimated
t
ρ
ρ
531
570
ρ
+ ρ
531
570
11
3 THEORY
), intercepted
t
(3)
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