USE OF A GREEN CHANNEL IN REMOTE-SENSING OF GLOBAL VEGETATION FROM EOS-MODIS

Most animals use a ''green'' spectral range to remotely sense the pres ence and vitality of vegetation. While humans possess the same ability in their eyes, man-made space-borne sensors that sense evolution of g lobal vegetation, have so far used a combination of the red and near i nfrared channels instead. In this article we challenge this approach, using measurements of reflectance spectra from 400 nm to 750 nm with s pectral resolution of 2 nm, with simultaneous determination of pigment concentrations of mature and autumn senescing leaves. We show that, f or a wide range of leaf greenness, the maximum sensitivity of reflecta nce coincides with the red absorption maximum of chlorophyll-a (Chl-a) at 670 nm. However for yellow-green to green leaves (with Chl-a more than 3-5 mu g/cm(2)), the reflectance near 670 nm is not sensitive to chlorophyll concentration because of saturation of the relationship of absorptions versus chlorophyll concentration. Maximum sensitivity of Chl-a concentration for a wide range of its variation (0.3-45 mu g/cm( 2)) was found, not surprisingly so, around the green band from 520 nm to 630 nm and also near 700 nm. We found that the inverse of the refle ctance in the green band was proportional to Chl-a concentration with correlation r(2) > 0.95. This band will be present on several future s atellite sensors with a global view of vegetation (SeaWiFS to be launc hed in. 1996, Polder on ADEOS-1 also in 1996, and MODIS on EOS in 1998 and 2000). New indexes that use the green channel and are resistant t o atmospheric effects are developed. A green NDVI = (rho(nir) - rho(gr een)/(rho(nir) + rho(green) was tested for a range of Chl-a from 0.3 m u g/cm(2) to 45 mu g/cm(2), and found to have an error in the chloroph yll a derivation at leaf level of less than 3 mu g/cm(2). The new inde x has wider dynamic range than the NDVI and is, on average, at least f ive times more sensitive to Chl-a concentration. A green atmospherical ly resistant vegetation index (GARI), tailored on the concept of ARVI (Kaufman and Tanre, 1992), is developed and is expected to be as resis tant to atmospheric effects as ARVI but more sensitive to a wide range of Chl-a concentrations. While NDVI and ARVI are sensitive to vegetat ion fraction and to rate of absorption of photosynthetic solar radiati on, a green vegetation index like GARI should be added to sense the co ncentration of chlorophyll, to measure the rate of photosynthesis and to monitor plant stress. (C) Elsevier Science Inc., 1996.