Canopy Reflectance at the Oklahoma ARM-CART Research Site

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Photos Updated April 14, 1999 

Canopy Bidirectonal Reflectance and Related Field Measurements in Native Tallgrass Prairie and Wheat Ecosystems in Oklahoma

Oklahoma ARM-CART Field Sites: The study is being conducted at two AMERIFLUX tower sites in the DOE ARM-CART region in north central Oklahoma: (a) a wheat site (36.77 N; 97.13 W) near Ponca City, Oklahoma; (b) a native tall grass prairie site (36.93 N; 96.68 W) near Shidler, Oklahoma. The 20km x 20km area surrounding the cultivated wheat site is approximately 75% in wheat and approximately 85% of the 20km x 20km area surrounding then tall grass prairie site is in tall grass prairie. Photos

Canopy & Soil Reflectance. Reflected radiation from vegetated surfaces is measured in the solar principal plane (SPP) and in the plane perpendicular to the SPP (PSPP) using a Spectron Engineering SE-590 spectroradiometer (output in the 400-1000 nm range at a 5 nm interval) mounted on a hand-held pointable mast. For canopy measurements, view zenith angles range from 75 to 0 (nadir) degrees at 15 deg intervals on both sides of nadir in the SPP and PSPP. For soil measurements, view zenith angles range from 60 to 0 (nadir) degrees at 15 deg intervals on both sides of nadir in the SPP and PSPP. Reflected radiation is measured from two vegetated plots at each site during each solar zenith angle period (an apparent angle is given, indicating that this was the desired targeted solar angle, but measurements may vary within 5 deg of the given angle). In addition, reflected radiation in the nadir-view is measured from a bare soil plot during each apparent solar zenith angle period. Reflected radiation measured from the vegetated and soil plots are bracketed by measurements of reflected radiation from a field reference panel (reflectance is calculated as the ratio of the reflected radiation from the target to the reflected radiation from the panel). The measurement sequence per solar zenith angle typically takes 25 minutes. The current measurement schedule permits canopy reflectance measurements approximately once per month.  Photos

Canopy Element Optical Properties. Dominant vegetative elements of the canopy are selected for measurement. Average leaf reflectance and transmittance (adaxial and abaxial surfaces) are derived from a suite of measurements from four leaves selected from the research site in the vicinity of the plots. In the case of solid components, such as stems and grain heads, only an average reflectance is derived; the average is derived from a suite of measurements from four samples of each canopy element. Photos

fAPAR. The fraction of absorbed photosynthetically active radiation (fAPAR) is derived from a set of measurements of incoming, canopy reflected, canopy transmitted and soil reflected PAR values scanned every 5 seconds from which 5-minute averages are computed. The fAPAR components are measured using Li-Cor line and point quantum sensors near the canopy reflectance plots during the period of canopy reflectance measurements. Additionally, fAPAR is measured throughout the year at each site near the flux towers (scanned every 5 sec from which 30-minute averages are computed). A hand-held line quantum sensor is used to measure fAPAR at each canopy reflectance plot and near the fAPAR plot at approximately solar noon on days when canopy reflectance is measured.  Photos

Additional Notes- Leaf area index (LAI) and plant phenology are measured approximately every two weeks throughout the growing season from plots surrounding the flux tower stations. Canopy status at the time of canopy reflectance can be inferred from these data. In addition, LAI in the canopy reflectance and fAPAR plots is inferred from measurements of light penetration using a Li-Cor Plant Canopy Analyzer; inferred LAI values are available for conditions within a day of canopy reflectance measurements.

Copyright 1998-2000.

Information contacts:
Dr. Elizabeth Walter-Shea: or 402-472-1553 or 402-472-3679
Mr. Mark A. Mesarch: or 402-472-5904

School of Natural Resource Sciences; University of Nebraska-Lincoln
246 LW Chase Hall
Lincoln, NE 68583-0728