1. 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. Exceedingly low soil moisture in 2000 resulted in poor vegetation condition which may have affected canopy composition and architecture, and leaf optical properties (Walter-Shea et al., 1992). Further study will investigate the frequency or repeatability of these observations in individual clear sky days throughout the growing season with consideration of soil moisture and wind speed and direction. 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)/(band4+band2)] 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 3-4 clear sky day periods within a 7 day period selected (total of 7 for 1999 and 2000) 3.5-4.5 hours prior to and after solar noon average reflectance and NDVI values at1/2 hour intervals represents the mean reflectance and NDVI as functions of time (and thus solar zenith angle,  i ) Solar zenith angle ranged from ±60E to the solar noon angle (17-39E, depending on day of year) fAPAR averaged yielding 30 -minute means Mean values of reflectance, NDVI and fAPAR plotted as a function of time about solar noon (where time =0) 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 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. Li-Cor LI-191SA line quantum sensors installed in the tallgrass prairie vegetation Band Wavelength Range (nm) 1 501 - 608 2600 - 698 3 703-804 4 812-1025 Approximate view of canopy from nadir directed Exotech x am =reflectance, fAPAR or NDVI value at a comparable morning value x pm =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 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. Year Time PeriodNo. of Days LAI Approx.  i Range (deg.) DatesDOY 1999 7/8 - 7/15189 - 19642.618 - 60 7/25 - 7/30206 - 21143.621 - 60 8/23 - 8/27235 - 23943.429 - 60 9/21 - 9/24264 - 26733.640 - 60 2000 5/28 - 5/30149 - 15132.517 - 60 8/8 - 8/15221 - 22844.025 - 60 8/23 - 8/26236 - 23944.229 - 60 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. 97:18,887- 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. 97:18,925- 18,934. Walter-Shea, 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. 61:162-177. 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. PPFD (  mol m -2 s -1 ) 1999 2000 Reflectance,NDVI or fAPAR DOY 235-239 DOY 265-267 DOY 149-151DOY 221-228 DOY 236-239 Time prior to/after solar noon (hr) Reflectance,NDVI or fAPAR Time prior to/after solar noon (hr) DOY 189-196 DOY 206-211 Time prior to/after solar noon (hr) Reflectance,NDVI or fAPAR A-frame with incoming and outgoing Exotech radiometers, incoming and reflected PAR sensors. Time prior to/after solar noon (hr) Canopy and soil moisture condition during the 1999 and 2000 study periods. Reflectance,NDVI or fAPAR 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.