Use of Spectral Vegetation Indices to Infer Leaf Area, Evapotranspiration and Yield: I. Rationale

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Abstract
Observations of plant canopies in visible and near‐infrared wavelengths and the calculation of spectral vegetation indices from them are now routine, but a unified rationale for interpreting the measurements has been lacking. Herein we present a rationale that links spectral observations to plant processes and express the relationships in equation form. Specifically, the equations relate vegetation indices or cumulative daily vegetation indices to leaf area index, fraction of photosynthetically active radiation that is absorbed, economic yield, leaf mass, aboveground dry phytomass, and evapotranspiration. Data from the literature demonstrate the relationships. The approach, termed spectral components analysis, assumes that crop canopies display the net assimilate achieved in response to growing conditions experienced and that vegetation indices are a measure of the amount of photosynthetically active and transpiring tissue present. It is expected to hold as long as the size of the canopies achieved by the time reproduction begins and the duration/persistence of the canopies limit economic yield, the situation typical of commercial agriculture. The approach provides a basis for using spectral observations to estimate large area yields and to model effects of multiple stresses on crops and forages.