TEMPORAL VARIATIONS IN SATELLITE REFLECTANCES AT FIELD AND REGIONAL SCALES COMPARED WITH VALUES SIMULATED BY LINKING CROP GROWTH AND SAIL MODELS

The objective of this article is to present a methodology for modeling shortwave satellite measurements over crops. This step is a prerequis ite before we can apply inversion techniques of satellite data in orde r to retrieve crop yields. Temporal profiles of satellite data were si mulated for an agricultural area, at field (20 m, SPOT HRV) and region al (4 km, NOAA AVHRR) scales, using a vegetation process model (AFRCWH EAT2) and a radiative transfer model (SAIL). Reflectances for individu al winter wheat fields observed in SPOT HRV scenes on five dates durin g the cereal growing season were simulated accurately. Leu; resolution NOAA AVHRR data used for the regional study included various vegetati on blages. The fractional area for each type of vegetation and its ref lectances were taken into account. Winter cereals reflectances were si mulated by the methods validated at the field scale and empirically mo deled reflectances were used for other species. Although an AVHRR pixe l consists of the intermingled signals characteristic of several veget ation categories, the simulated and observed NOAA AVHRR scale reflecta nces agreed well.