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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.
3.3
Canopy Phenology
Like all spectral measurements, NDVI is an indirect measurement.
Over the years, researchers have correlated parameters of interest,
like LAI and f s, to measurements made at 630 nm and 800 nm.
Researchers have estimated other variables using NDVI besides these
relationships. Two of these variables are the focus of Ryu et al.
(2010), who used a simple two-band LED-based 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 photosynthesis, but found that the LAI of grassland never
2
2
increases above 2.5 m
m
. 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 limiting 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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