Monitoring Phenological Key Stages of Winter Wheat with NOAA NDVI Data
利用NOAA NDVI数据集监测冬小麦生育期的研究

The objective of this study was to monitor phenological stages of winter wheat in HHH plain of China. Three key phenological stages were defined based on NOAA/AVHRR NDVI time series data collected in the HHH plain of China from 1990 through 2000. In a winter wheat_summer maize rotation system, the recovering, heading and maturity stages of winter wheat were recorded using 6 km resolution decadal NDVI. Recovering of winter wheat was identified by two consecutive NDVI increments after a period of stagnating values. Similarly, maturity of winter wheat was identified by the last two consecutive NDVI decrements. The heading stage was identified by maximum NDVI value. The satellite_derived data proved to be consistent with the 'green wave' moving through the HHH Plain in spring. Through analysis of historical climate data, the recovering stage of winter wheat recorded by satellite was closely correlated to the temperatures measured in February, significant correlation between phenological stages and rainfall could not be detected. Comparison with synchronous phenological observations on the ground confirmed the coherence of satellite_derived phenology data. The inter_annual variability of recovering stage is higher than that of heading and maturity stages. Application of irrigation and the relatively short ten_year period for which data were available explain why inter_annual fluctuation of crop phenology was relatively small. The 6 km spatial resolution decadal NDVI limited the prediction accuracy. It is expected that satellite data with greater spatial and temporal resolutions and the improved smoothing methods will further increase the precision of the estimated data.