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The climatic characteristics of length of extreme drought period and its relation with NDVI in Northwest China

The maximum continuous no rain days is defined as the length of extreme drought period (LEDP hereafter) in this paper. In the present study, using the daily precipitation data ( 1960-2004) of 107 stations, the GIMMS NDVI data set (1982- 2003) of Northwest China, and the methods of EOF, REOF, correlation and wavelet analysis, the basic temporal-spatial distributions of the LEDP and its cycles and abrupt mutation time as well as its relationship with NDVI are analyzed. The results show that the areas with shortest LEDP are located in southern Qinghai and Gannan tableland of Gansu province. The first load vector of the LEDP variations has the characteristics of spatial consistency, and the second one has adverse variation trend in the east and west part. The rotated load vector reflects 5 anomaly climatic regions of the LEDP, i.e. northeast of Qinghai Plateau, southern Xinjiang, Qinghai, northern Xinjiang and southeast part of Northwest China. There is obvious 2-3a high frequency cycle of LEDP in 5 climate anomaly regions, 16a and 8-1 la low frequency cycle in southeast of Northwest China and other 4 regions, respectively. There have two abrupt points in northeast of Qinghai Plateau, Qinghai and northern Xinjiang, whereas just one abrupt point in southern Xinjiang and southeast part of Northwest China. The correlation coefficient between the LEDP and the NDVI is notable negative value in Northwest China. From the distribution of land vegetation cover and the LEDP, it can be found that the area of longest LEDP almost overlaps the area of the sparsest vegetation cover, and the same for the shortest LEDP and the densest vegetation cover. Therefore, either NDVI or LEDP distribution can be used to express the character of drought in Northwest China.