Comparison of AVHRR and SMMR data for monitoring vegetation phenology on a continental scale

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Abstract
Advanced Very High Resolution Radiometer (AVHRR) normalized difference vegetation index (NDVI) data for a one-year period were compared with contemporaneous, registered data sets of Scanning Multichannel Microwave Radiometer (SMMR) microwave polarization difference temperature (MPDT) for the study of vegetation phenology. Sample areas were selected for the major land-cover types of Africa and South America. Comparison of the data showed the MPDT response to differ considerably from the seasonal pattern exhibited by the NDVI. The MPDT showed a general decrease with increasing vegetation density, whereas the NDVI showed a positive relationship with the amount of green leaf material. In contrast to the NDVI, the MPDT showed a generally poor response to seasonal variations in green leaf material. Only those vegetation types exhibiting a substantial seasonal variation in the areal extent of vegetated cover showed strong seasonality in the MPDT data, e.g. annual grasslands. Several cover types with large proportions of trees and shrubs displayed seasonal variation in the NDVI but no equivalent change in MPDT. The previously stated relationship between leaf water content and MPDT is brought into question. Emphasis must now be placed on determining the relationship between MPDT and vegetation structure at 37 GHz, in particular through field measurement programmes.

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