SOLAR ANGLE INDEPENDENCE IN THE RELATIONSHIP BETWEEN ABSORBED PAR ANDREMOTELY-SENSED DATA FOR ALFALFA

Multispectral vegetation indices are often used to estimate the propor tion of incident photosynthetically active radiation (PAR, 0.4-0.7 mum ) that is absorbed by plants for potential use in photosynthesis. Fiel d experiments were conducted near Phoenix, Arizona to establish such p redictive capabilities for alfalfa and also to determine the effect Of varying solar zenith angles (theta(s)) on the relationships. The frac tion of absorbed PAR (fA(PAR)) was measured using a 1-m long line quan tum sensor. Canopy reflectance measurements (red, 0.61-0.68 mum; near- infrared, 0.79-0.89 mum) were obtained with a hand-held radiometer. Da ta were collected for theta(s) from 27-degrees to 72-degrees. Statisti cally significant relationships were observed between fA(PAR) and red reflectance factors (r2 = 0.97) and several commonly used vegetation i ndices (ratio, r2 = 0.96, normalized difference, r = 0.96; and soil ad justed, r2 = 0.93). Actual values of these parameters varied with time of day, but the relationships between fA(PAR) and various indices der ived from reflectance observations were independent of theta(s), exten ding the potential usefulness of remote sensing approaches for inferri ng changes in fA(PAR) at various times of the day and different season s and latitudes.