Spatial-temporal distribution and multiple-temporal scale variation analyses of winter wheat late freezing injury

The winter wheat late freezing injury (WFI, or frost damage) mainly occurs in the Yellow River and the Huaihe River area, of which Henan Province possesses the most part. Henan is both the major producing area of wheat and the harder -hit area of WFI. In this paper, we present a WFI index based on minimum temperature and winter wheat development period (WDP), and calculate the WFI degrees and days at 30 agrometeorological stations of Henan Province in 1981-2004. For the large-scale variation analysis of WFI, the 24-year WDP observation sequence is relatively short, therefore it is expanded by using the relation between the tuning green date of winter wheat and the 5-day running mean temperature and that between the stem elongation phase and the effective cumulative temperature above a base of 2.5 ℃. And then the expanded data of WFI in the last about 50 years are analyzed by EOF and Morlet techniques to reveal the regulations of spatial temporal distribution and multiple-temporal scale variability of WFI. The results show that the frequency of WFI was higher, above 40% in highincidence areas in Henan Province; and the WFI exhibited a rising trend in the 1970s-1990s. The trend of WFI degrees was similar to that of WFI days, and the areas of heavier WFI degrees usually corresponded to the areas of more days of WFI. The impact of winter wheat frost damage degree on the winter yield was more remarkable than that of the frequency of WFI. The incidence areas of heavy WFI degree lay in the southeast and southwest of Henan, and the areas of light WFI degree in the south side of the Huaihe River and parts of western Henan. The first/second EOF mode of WFI degrees displayed 16 years/quasi 2 years multiple time scale variability, respectively. The high/low frequent areas of WFI were distributed in the north of northern Henan and the southwest border of Henan/the northeast Henan and the middle part of southwest Henan, respectively. The first/second EOF mode of WFI days exhibited quasi 4 years/quasi 3 years and quasi 6-7 years multiple time scale variability, respectively.