The relationship between threshold-based and inflexion-based approaches for extraction of land surface phenology

The inflexion-based and threshold-based approaches are the two most popular approaches for extracting land surface phenology (LSP). The first approach uses inflexion point of the vegetation growth curve to determine the start-of-season (SOS) and end-of-season (EOS), while the second approach identifies SOS and EOS with a predefined percentage of vegetation growth amplitude. These two approaches have long been thought to be independent. In this letter, we investigated the relationship between the two approaches for extracting SOS. It was found that the thresholds at several key points in the inflexion-based approach are constant. The threshold at the inflexion point for SOS and EOS is 9.18% of vegetation growth amplitude. This threshold can link the SOS derived from the two approaches, which made it possible to determine the significant vegetation growth transitions for the threshold-based approach. Other thresholds for SOS retrieval, such as commonly-used 10% and 20% of vegetation growth amplitude, were also analyzed using a global reference NDVI. A high SOS difference between the two approaches indicates a slow vegetation growth.