Spatial and temporal variations in optimum sowing date of winter wheat in Shijiazhuang City

Global warming can change the ecological environment. The optimum sowing date of winter wheat has changed with increasing accumulated temperature before winter. Thus there was the necessity to analyze the changes in optimum sowing date of winter wheat under warming climate conditions. To this end, the spatial and temporal variations in optimum sowing date of winter wheat were analyzed using daily meteorological data (accumulative temperature before winter was in range of, 495570 ℃·d) for 1963–2012 from 9 meteorological observatory stations in Shijiazhuang. Statistical methods including Trend Analysis, Mann-Kendall Mutation Test, Morlet Wavelet Analysis were used in the study. The results showed that the change in optimum sowing date of winter wheat in the mountain area of Shijiazhuang was not obvious, with a change trend of 0.149–0.296 d·10a^-1. Then the change in optimum sowing date of winter wheat in the piedmont plain was statistically significant, with a change trend of 0.822–1.211 d·10a^-1. The optimum date of sowing of winter wheat delayed 5–7 days in the investigated 50-year period. The Mann-Kendall Mutation Test result showed a temporal process of change in optimum sowing date of winter wheat around 1995, with an obvious delaying trend after 2006. The results of the Morlet Wavelet Test showed the existence of about 5-year periodic change in optimum sowing date of winter wheat during the investigated 50-year period. The cycles of change in optimum sowing date of winter wheat were the same for both the mountain and plain regions. From the cycles, it was deduced that the trend in optimum sowing date of winter wheat relatively accelerated after 2010. There was 3–4 days difference of optimum sowing date of winter wheat from the north to south of Shijiazhuang. The change in optimum sowing date of winter wheat in the mountain area of Shijiazhuang due to climate warming was moderate, which was 0.1–0.4 d·10a^-1 in the investigated 50-year period. However, the change in optimum sowing date of winter wheat in the piedmont plain area was obvious, with a trend of 0.4–1.3 d·10a^-1. The trend of change in optimum sowing date of winter wheat in the plain area was far faster than that in the mountain area. The research result was critical for optimizing local winter wheat production in the Shijiazhuang study area.