an increase in asymmetry in the red reflectance as indicated by a MND of 6% with a decrease in asymm

1
Diurnal Trends in Nadir-based Reflectance and
NDVI under Clear Sky Conditions E.A. Walter-Shea,
K.N. Nang, and M.A. Mesarch University of
Nebraska-Lincoln J.L. Privette NASA Goddard Space
Flight Center

• 2000.
• an increase in asymmetry in the red reflectance
  (as indicated by a MND of -6) with a decrease in
  asymmetry in the NIR reflectance (MND of -6),
  and an increase in asymmetry in fAPAR (MND of
  -3)
• symmetry in NDVI values
• morning values were lower than afternoon values.

Introduction The usefulness of remotely sensed
data depends on the consistency of relations
between detected radiation and biophysical
properties. Diurnal changes in factors affecting
reflected radiation and biophysical properties
can thus result in potential changes in relations
between remotely-sensed data and vegetation
properties.

• xam reflectance, fAPAR or NDVI value at a
  comparable morning value
• xpm reflectance, fAPAR or NDVI value at a
  comparable afternoon value
• n number of value pairs across time periods
• Subtle variations in clear sky conditions
  existed for the days about solar noon

Study Area The research presented here was
conducted in 1999 and 2000 at a native tallgrass
prairie AmeriFlux tower site in the DOE ARM-CART
region in north central Oklahoma (36.95 N
96.68 W) near Shidler, Oklahoma.
DOY 149-151
DOY 221-228
1999
2000
Reflectance,NDVI or fAPAR
PPFD (?mol m-2 s-1)
Time prior to/after solar noon (hr)
Canopy and soil moisture condition during the
1999 and 2000 study periods.
DOY 236-239
Time prior to/after solar noon (hr)

• Methodology
• Canopy reflectance.
• Two Exotech radiometers - incoming and outgoing
  radiation
• MSS spectral bands
• Mounted at a height of 2m on an A-frame after
  the burn (mid-April through October in 1999 and
  2000)
• Queried every minute in a 10-minute window on
  the half-hour during daylight hours
• Continuously for every ½ hour regardless of
  sky and weather condition
• Outgoing Exotech in nadir direction, 15E field
  of view
• (spot size of 0.5m), hemispherical reflected
  flux densities
• derived
• Incoming Exotech with hemispherical field of
  view, spectral irradiances
• Reflectance and NDVI (band4-band2)/(band4band2)
  were derived
• fAPAR.
• Two Li-COR LI-190SA quantum sensors for incident
  and canopy reflected PAR (on the A-frame)
• Two LI-191SA sensors for canopy transmitted and
• soil reflected PAR.
• Data reduction

Reflectance,NDVI or fAPAR

• Results
• Mean reflectance (red -9- and NIR -)-), NDVI
  (-"-) and fAPAR (-"-) (with standard deviation)
  were plotted as a function of time about solar
  noon
• Plotted in this manner, asymmetry about solar
  noon can be detected.
• Results varied by year.
• 1999.
• Asymmetry about solar noon (mean normalized
  difference of 2 for red and 12 for NIR)
• Morning values higher than afternoon values
• Variation about the mean diminished the
  asymmetry effect in some cases
• Variation in NDVI resulted but not always
  following the asymmetry pattern exhibited in the
  reflectance values (mean normalized difference of
  6).
• fAPAR followed a fairly symmetrical pattern.

Time prior to/after solar noon (hr)
A-frame with incoming and outgoing Exotech
radiometers, incoming and reflected PAR sensors.
DOY 206-211
DOY 189-196
Reflectance,NDVI or fAPAR
Conclusions While diurnal differences in
nadir-viewed reflectance and NDVI for clear sky
days were detected and indicate a possible bias
with limited temporal measurements, the detected
differences were small and may perhaps lie within
measurement error, especially for satellite data.
However, diurnal changes in nadir viewed
reflectances are often small while changes in
off-nadir reflectances, particularly in the
backscatter direction of the solar principal
plane, can be large (Deering et al., 1992
Walter-Shea et al., 1997). The small changes
observed with the measurements described in this
study may imply stronger changes in off-nadir
reflectances as a result of diurnal changes in
vegetation with implications for off-nadir
satellite-based measurements. Investigation into
diurnal effects on relations with off-nadir
measurements is recommended.
Time prior to/after solar noon (hr)
Approximate view of canopy from nadir directed
Exotech
DOY 235-239
DOY 265-267
Reflectance,NDVI or fAPAR
References Deering, D.W., Middleton, J.R. Irons,
B.L. Blad, E.A. Walter-Shea, C.J. Hays, C.L.
Walthall, T.F. Eck, S.P. Ahmad, B.P. Banerjee.
1992. Prairie grassland bidirectional
reflectances measured by different instruments at
the FIFE site. J Geophys. Res. 9718,887-18,903.
Walter-Shea, E.A., B.L. Blad, C.J. Hays, M.A.
Mesarch, D.W. Deering, E.M. Middleton. 1992.
Biophysical properties affecting vegetative
canopy reflectance and absorbed
photosynthetically active radiation at the FIFE
Site. J. Geophys. Res. 9718,925-18,934. Walter-S
hea, E.A., J. Privette, D. Cornell, M.A. Mesarch
and C.J. Hays. 1997. Relations between
directional spectral vegetation indices and leaf
area and absorbed radiation in alfalfa. Remote
Sens. Environ. 61162-177.

Li-Cor LI-191SA line quantum sensors installed in
the tallgrass prairie vegetation
Time prior to/after solar noon (hr)