THE USE OF SATELLITE NDVI DATA FOR THE VALIDATION OF GLOBAL VEGETATION PHENOLOGY MODELS - APPLICATION TO THE FRANKFURT BIOSPHERE MODEL

An algorithm based on a three-spline function fitted to measured NDVI courses (normalized difference vegetation index) was developed to anal yze a given NDVI annual course with respect to leaf shooting and leaf abscission times of deciduous vegetation. in contrast to algorithms wh ich are based on modified second derivatives of the NDVI time course t o detect shooting or abscission, the proposed algorithm takes into acc ount the whole annual time course and is therefore less sensitive to n oise in the NDVI-signal. In the present study this algorithm was used to validate the phenology results for the deciduous vegetation of a gl obal equilibrium run of the prognostic Frankfurt Biosphere Model (FBM, spatial resolution 0.5 degrees x 0.5 degrees) driven by a climatology which represents a mean seasonality of the driving variables. The mea n value of the area-weighted frequency distribution of the difference between the shooting date deduced from NDVI and the shooting date calc ulated by the FBM for the deciduous vegetation types is -4 days, indic ating that in the global mean the FBM predicts leaf shooting less than one week too late. A 75% fraction of the area under consideration sho ws predicted shooting dates lying within a range of +/- 30 days compar ed to the satellite-derived dates. The distribution has its maximum at a difference of 0 days (i.e. the FBM exactly fits the NDVI deduced sh ooting day for these areas). This result supports the general assumpti on that at least in global scale models phenology can be successfully deduced from carbon flux balance considerations.