Estimating land vegetation conditions using surface radiation flux measurements

The hourly radiation flux measurements above land vegetation surfaces from flux towers are used to calculate surface reflectances in the visible and near-infrared wavebands to describe daily changes of vegetation conditions. Three main assumptions are adapted as follows: 1) solar radiation flux density is partitioned into 55% near-infrared and 45% visible radiation; 2) reflected photosynthetically active radiation (PAR) is equated to visible radiation; and 3) isotropic conditions are imposed on radiation measurements on clear sky days at small solar zenith angles. The normalized difference vegetation index (NDVI) is successfully estimated based on the reflectance calculations. Consistent relationship is observed between NDVI and the land vegetation characteristics, and the patterns show a clear evidence of site-to-site differences as well as annual and intraseasonal variations. The simple calculations show good agreement with both observed and satellite derived reflectances for a variety of vegetation environments, where maximum NDVI averaged about 0.8 for agricultural fields and deciduous forests, and 0.6 for grasslands and pine needles. These results suggest the potential of using real-time radiation flux measurements to derive dynamic site parameter requirements for soil-vegetation models