The winter wheat late frost disaster (WFD) occurs mainly in the Yellow and Huaihe River area, of which Henan Province covers the most part. Henan is the major area of wheat production in China, but it is severely hit by the WFD. In this study, we construct a WFD index based on the minimum temperature and the winter wheat development period (WDP). The WFD degrees and days at 30 agrometeorological stations in Henan Province during the period of 1981-2004 are calculated. For the large-scale temporal variation analysis of WFD, the 24-yr WDP observation series is relatively short, so it is expanded by using the relation between the turning 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 critical value of 2.5 • C. The WFD data are also expanded for the last 50 years and are analyzed by using the empirical orthogonal function (EOF) and the Morlet wavelet methods. Characteristics in the spatial distribution and temporal variation of WFD are revealed. The results show that the frequency of WFD is generally high, exceeding 40% in parts of Henan, and exhibits a rising trend in the period of 1970-1990. The variation trend of WFD degrees is similar to that of WFD days, and the areas with higher WFD degrees coincide the areas with more WFD days. Moreover, the WFD degree has a greater impact on the winter wheat yield than the WFD days. The areas with high WFD degrees lie in the southeast and southwest of Henan, and the areas with low WFD degrees lie in the south of the Huaihe River and parts of western Henan. Temporal variations of the first and second EOF modes of the WFD degree display 16-and quasi-22-yr periodicities, respectively. The areas of high (low) WFD frequency are distributed in the northern Henan and the southwest border of Henan (the northeast Henan and the middle part of southwest Henan). The temporal variation of the first (second) EOF mode of WFD days exhibits a periodicity (periodicities) of quasi-4 yr (quasi-3 and quasi-6-7 yr).
In the context of global warming, extreme drought climate events show a trend of frequent occurrence. Studying the relationship between climate change and drought disasters by using the performance characteristics of climate change has become a new strategic focus of global change science. In this paper, we collected meteorological data from 14 meteorological stations from 1956 to 2015 in Heilongjiang Province and used a standard precipitation index (SPI) of meteorological drought to analyze temporal and spatial characteristics of droughts in the province. Multiple methods such as Linear regression analysis, Mann-Kendall trend test and Kriging interpolation were applied and analyzed to reveal temporal and spatial distribution patterns of drought frequency and drought intensity in different parts of the province. The results show that: (1) the precipitation in Heilongjiang Province has shown a downward trend in the past 60 years and the correlation between the monthly average precipitation and the monthly mean temperature is also apparent. (2) Since 1990s, the intensity of higher drought has escalated in the whole province and has gradually strengthened from south to north. (3) The frequency distribution of drought is the lowest in the central and southern regions, and the highest in the west. (4) The uneven precipitation, sandstorm and uneven distribution of evaporation caused by the monsoon in Heilongjiang Province are the main meteorological factors for the formation of drought in the province.
Soil moisture and vegetation growth are the most important and direct index for drought. The interpreting to vegetation and spectrum analysis of soil are two important elements in the judgment of drought. Recently, Abduwasit Ghulam and other researchers, on the basis of the spatial distribution characters of soil moisture in near infrared spectrum, adopt the expansion analysis method and establish PDI. Later, vegetation index is introduced by establishing a new drought monitoring method MPDI after comprehensively considering about the soil moisture and vegetation growth characters. The article, directing against the drought in different periods of Henan Province, adopts the MODIS image data to undertake PDI and MPDI calculations and compares with the soil moisture data with that of the same period, concluding that: PDI and MPDI are closely related with the original data from land observation, among which the relations between MPDI and 0-20cm calculation is the closest; PDI and MPDI are all close to the drought situation concluded from the calculation of bared land and the early growth period of vegetations; MPDI is more suitable for the areas with vegetations.
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