Variability of the phenological stages of winter wheat in the North China Plain with NOAA/AVHRR NDVI data (1982-2000)

Vegetation phenological stages such as the start of season (SOS), the end of season (EOS) and the growing season length (GSL) are key indices to depict vegetation growth. Crop phenological characteristics are generally more complex than natural vegetation because they are affected by both climate variability and human activities. The North China Plain (NCP) is an intensively cultivated agricultural area where winter wheat is widely planted. In this study, the SOS, EOS, and GLS of winter wheat in the NCP from 1982 to 2000 were extracted at daily level with 8km NOAA/AVHRR 10 days MVC NDVI data by considering human activities like harvesting and sowing. The major findings of this study are as the follows: 1) The SOS1 of the winter wheat has large inter-annual variation. The SOS is very sensitive to climate variation. GSL has significant increase. 2) The EOS2 decreased in most areas, which means the winter wheat harvesting date became earlier and earlier. Consequently the GSL has a significant decreasing trend from the year of 1982 to 2000. 3) The decrease of GSL is well connected to the temperature increase and precipitation decrease over the NCP during the past 20 years.