A Spectral Reflectance Index of Developing Crops Largely Independent of Cloud Cover and Soil Surface Wetness

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Abstract
Ratios between reflectance in a near infrared and a photosynthetically active band (NIR/PR) were determined frequently each day in spring barley, winter wheat, spring rape and perennial ryegrass during the 1986 and (for barley) 1988 growing seasons. For the central four hours of day NIR/PR was insensitive to changing solar elevation and largely independent of soil surface wetness and cloud cover, responding almost entirely to varying green crop cover. For barley a very close and linear correlation (r 2=0.982) was found between the NIR/PR ratio and green leaf area index. A very close and linear correlation (r 2=0.975) was found between NIR/PR and relative PAR-interception in plots with rows running north-south, whereas in a plot with rows in the east-west direction the relationship was non linear.