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3
THEORY
SRS Sensors
Figure 5: Relationship between fractional canopy interception and
NDVI, where NDVI is converted to LAI using Nguy-Robinson et al.
(2012). Campbell and Norman (1998) give the relationship between
LAI and fractional interception.
3.4
Canopy Phenology
Like all spectral measurements, NDVI is an indirect measurement.
Over the years, researchers have correlated NDVI to several param-
eters of interest, like LAI and
f s
, biomass, and canopy productiv-
ity, among others. Two of these variables are the focus of Ryu et
al. (2010), who used an NDVI sensor, similar to the SRS-NDVI, to
measure canopy phenology and associated changes in photosynthesis
in an annual grassland over a four year period. Ryu et al (2010).
show an exponential relationship between NDVI and canopy photo-
synthesis, but found that the LAI of grassland never increases above
2.5 m
2
m
−
2
. Ecosystem phenology can also be tracked in the time
series data from their NDVI sensor with errors on the order of a
few days. It should be noted that they filtered their data by lim-
iting NDVI measurements to a particular sun elevation angle (e.g.,
sampling under identical sun zenith and azimuth angles from day to
day).
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