Abstract:As an important factor during crop growing period, phenology is widely used in growing monitoring, field management and output forecasting. Winter wheat is one of the most important crops in Heibei China and its phenology will be affected by not only climate but also farmers′ planting habit and so on In this paper, temporal series MYD09A1, the 8-day reflectivity composite productions provided by NASA, which derived from AQUA-MODIS (Moderate Resolution Imaging Spectroradiometer) images from January to July in 2005 were used to calculate enhanced vegetation index (EVI). Based on winter wheat′s whole growing stages, three key phenologies were defined from temporal series EVI as following: greenup begins with the second continually increasing in EVI, heading stage begins with the maximum of EVI and maturity begins with the second decreasing of EVI before the minimum.
Winter wheat flourishes in the first ten days in May generally, so the 16th 8- day EVI image which is relatively cloud-free was used to density sliced to show different growing conditions. From the south part of winter wheat area to the north in Hebei China, temporal series images of five areas of interesting (AOI) with the same EVI level (0.3-0.4) were analyzed, and greenup, heading stage and maturity were abstracted from the images. Results show that EVI could be used to estimate crop phenology successfully. From the south to the north, with the increasing of latitude, greenup, heading stage and maturity of winter wheat in Hebei China became later and later. Winter wheat in the south of Hebei turned green at the 8th 8-day in 2005 (about the first ten days in March), the middle part of winter wheat turned green about in the second ten days in March, which was 8 days later than that in the south, and the north part of winter wheat′s greenup was deferred about 8 days than the middle part to the last ten days in March. Periods among the three key phenologies of winter wheat were almost stationary, and phenology of wheat in the south of Hebei could be 8-16 days earlier than that in the north.
From the east to the west in the middle part of winter wheat in Hebei Province, three AOIs with about the same latitude but different EVI value (0.3-0.4, 0.4-0.5, 0.5-0.6) were used to analyze phenology spatial character. On the condition that different EVI level could instead different growing conditions and managements of crop, we could get conclusions that from the east to the west, winter wheat with different growing conditions turned green at about the same period; heading stage and maturity of winter wheat with the best growing condition came at the 15.5th 8-day and the 20.5th 8-day, respectively, which could be about 4 days earlier than those of wheat with common growing conditions.