Estimating alfalfa low leaf area indices from red (660 nm) and far-red (730 nm) hemispheric reflectance measurements.

The leaf area index of a crop is the main driver of dry matter production. We propose to use the differences between leaf and soil reflectance in the red and far-red wavebands, in order to estimate the leaf area index. Reflectance measurements are made with low-cost sensors measuring hemispheric fluxes around 660 and 730 nm. The reflectance normalized difference, empirically related to the leaf area index of an alfalfa canopy, allows. to estimate low (< 2) leaf area indices; the relationship, however, is dependant on soil optical properties, and experimental precautions must be envisaged. The low cost of the sensors enables to propose to use them for crop permanent monitoring.