SRS Sensors
3
THEORY
3.5
Photochemical Reflectance Index (PRI)
As described above, researchers use NDVI primarily 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 gross primary productiv-
ity (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
t
), intercepted
light (
f
s
), and light use efficiency (
).
S
t
can generally be estimated
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
s
. The light use efficiency term (
) remains to be
quantified in order to make accurate predictions of GPP.
Gamon et al. (1990, 1992) proposed a dual band vegetation index
(similar to the NDVI) that could be used to estimate
. The founda-
tion of the measurement is based on the absorbance of xanthophyll
pigments in a fairly narrow spectral region around 532 nm. The xan-
thophyll cycle signal seen in reflectance at 532 nm has been shown
to be well correlated with LUE in many plant species (Gamon et al.,
1997).
The Photochemical Reflectance Index (PRI) uses reflectance at 532
nm and is calculated using Equation 3.
P RI
=
ρ
532
−
ρ
570
ρ
532
+
ρ
570
(3)
where,
ρ
532
and
ρ
570
are percent reflectances at 532 and 570 nm, re-
spectively.
In addition to LUE, PRI has also been shown to correlate with nu-
merous other physiological variables associated with plant photosyn-
thetic performance from the leaf to the ecosystem levels (Gamon et
al., 1992, 1997, 2001). Numerous studies correlate PRI to various
ecophysiological variables including the epoxidation state of xantho-
phyll, maximum photochemical efficiency of photosystem II, effective
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