MULTISITE ANALYSES OF SPECTRAL-BIOPHYSICAL DATA FOR SORGHUM

Spectral-biophysical functional relations that hold across experiments and environments are needed for economically important crops. Thus, reflectance factors and leaf area index (LAI) measurements for sorghum [Sorghum bicolor (L.) Moench] experiments conducted at Bushland, Texas (102.2-degrees-W, 35.2-degrees-N), Lubbock, Texas (101.8-degrees-W, 33.7-degrees-N), and Weslaco, Texas (98.0-degrees-W, 26.2-degrees-N) over a 7-year period were related using two-parameter power and exponential equations for four common vegetation indices (VI): normalized difference (NDVI), perpendicular (PVI), near-infrared to red ratio (RVI), and transformed soil-adjusted (TSAVI). The objective was to produce recommendable empirical equations for estimating LAI of sorghum from spectral vegetation indices using observations pooled across locations. During both the pre- and post-maximum leaf area index portions of the growing season, power and exponential forms were equally good for estimating LAI from TSAVI and NDVI and gave coefficients of determination, R2, that ranged from 0.76 to 0.83. However, for PVI and RVI the relations were not as nonlinear and the power form accounted for more of the variation (R2 = 0.74-0.82) than the exponential form (R2 = 0.69-0.78). Infinite reflectance for sorghum was estimated from the pooled data to be 4.0% in the RED band and 49.7% in the NIR band. Coefficients of exponential relations for sorghum were similar to those for corn reported earlier. Although soils, agronomic treatments, sun angles, and instruments differed among locations, it was possible to develop general relations for estimating leaf area of sorghum from four of the most commonly used spectral vegetation indices.