Monitoring and evaluation of the vegetation state of agricultural crops using satellite - derived radiative products

The topic of the paper focuses on satellite remotely sensed monitoring of the Earth surface and, in particular, of the vegetation. The main task of the study is to model the vegetation reflected field in the visible and near infrared spectral domains, based on ground measurements and simulated data, gathered in different natural conditions, as well as on satellite data. The paper tries to establish connections between the biophysical parameters of a vegetation canopy, the reflected radiation flux in different spectral channels and the geometrical and radiometrical factors that characterize the interaction processes in the atmosphere-vegetation-soil system. The vegetation reflectance model, used in the study, is based on the radiative transfer theory and on the concepts developed by the turbid models. The model uses the approximation of four radiative flows (diffuse upwards and downwards flows and direct upwards and downwards flows) expressed by linear differential equations in order to evaluate the radiative flow in any point on a vegetation cover. This model computes the bidirectional reflectances as a function of the optical proprieties of the vegetative elements (especially the leaves), such as the factors that define the architecture of vegetation: density, orientation and spatial distribution of these elements. The algorithm takes into consideration the illumination conditions and the parameters associated with the geometry of observations. The model estimates the reflectances of vegetation in the visible and near infrared channels, operated by some of the most used operational satellite sensors: SPOT, LANDSAT - TM, NOAA - AVHRR, TERRA - MODIS, TERRA - ASTER, etc. The paper presents the derived radiative products, like vegetation indices, spectral signature characteristics, moisture or thermal stress indices issues from the model and the application for the vegetation state monitoring of the main agricultural crops in the Romanian Plain, situated in the South part of the territory. The benefits of the practical use of the radiative products in studies of the vegetation development dynamics or for determining the vegetation state criteria and the biological potential are also emphasized.