Fuchs, M., Evapotranspiration Laboratory, Kansas State University, Manhattan, KS 66506, United States
Asrar, G., Evapotranspiration Laboratory, Kansas State University, Manhattan, KS 66506, United States
Kanemasu, E.T., Evapotranspiration Laboratory, Kansas State University, Manhattan, KS 66506, United States
Hipps, L.E., Utah State University, Logan, UT, United States

Transmission of photosynthetically active radiation (PAR) measured with quantum sensors in three wheat cultivars during periods of clear sky were used to indirectly determine the leaf area index (LAI). The exponential description of direct and diffuse radiation penetration, which depends on leaf inclination, was correlated to the transmission measurements at various solar zenith angles. The highest correlations were found for fixed leaf inclinations of 60 and 75°, and for the spherical leaf angle distribution. Angular shape coefficients corresponding to these leaf inclinations were then used to solve the exponential equation for LAI taking incoming PAR to be direct. This was shown to be an acceptable approximation when the proportion of direct radiation in the incoming PAR was higher than 0.85. Predictions of LAI were found to be within 0.7 units for a range of LAI from 1.5 to 5.0. A statistical analysis of the results indicated that the most suitable leaf inclination was a combination of 60 and 75° fixed angles, and spherical distribution. © 1984.