Sensitivities of Normalized Difference Vegetation Index and a Green/Red Ratio Index to Cotton Ground Cover Fraction
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Vegetation indices based solely on visible reflectance may simplify and decrease the cost of crop growth estimates compared to visible and near-infrared (NIR) indices. Ground-based and aerial visible and visible/NIR vegetation indices based on aerial images were compared for sensitivity to ground cover fraction (GCF) of cotton (Gossypium hirsutum L.) under four irrigation treatments in 2004 and five treatments in 2005 and 2006. In-season cotton imagery was collected using an unmodified Nikon COOLPIX 4300 camera and a COOLPIX 4300 camera modified for NIR imaging attached to a tethered blimp. GCF imagery was collected at 45 to 60 m and compared with normalized difference vegetation index (NDVI) and green/red ratio values from imagery collected at 180 to 250 m. Ground-based (1.5 m) spectrometer NDVI measurements using multiple spectral regions were also evaluated. Spectrometer (r2 = 0.40 to 0.80) and camera (r2 = 0.68 to 0.90) indices were highly correlated with season-wide GCF between fractions of 0.20 and 0.80 and were sensitive to irrigation treatments. Camera green/red ratio was linearly correlated with GCF throughout the 3 yr. The pooled comparison for the 3 yr was strongly linear (r2 = 0.86). Our results suggest that the green/red ratio index might allow quick, simple, and accurate crop growth estimates for production. Crop Science Society of America.