| Ordos Plateau is in the marginal zone which China’s southwest and southeast monsoon combined effect on. The area is typical environmental degradation and sensitive zone. It is called "environmental critical zone" by domestic and foreign scholars. The Ordos Plateau is a district full of various natural disasters. Include meteorological disaster, biological disaster and geological disaster. Natural disasters in the area are concentrative, and meteorological disaster is the most important type. Droughts, floods, snow, wind, frost, hail disaster and other meteorological disasters seriously affect the normal life of the Ordos Plateau residents. In this paper, the grade scale, stage change, cycle regularity and occurrence cause of flood disaster, frost and snow disasters, hail disaster in Ordos Plateau since Ming Dynasty, which were studied based on the statistical analysis of historical documents, least squares method and wavelet analysis. This reveals the relationship of these disasters and climate change since Ming Dynasty. It has important realistic meaning for the disaster prevention and reduction. The following conclusions can be drawn from the study.(1)The flood disaster of Ming Dynasty was43times, one time every3years in east of Ordos Plateau. It was23times, one time every12years in west of Ordos Plateau. From early stage in Ming Dynasty to the end, the occurrence frequency of the flood disaster was increased in east of Ordos Plateau, downward trend in west, the flood disaster in east is more than which in west of Ordos Plateau, but the flood disaster in west has greater intensity than that in east. The frost and snow disasters of Ming Dynasty were23times, one time every12years in Ordos Plateau. From early stage in the Ming Dynasty to the end, the occurrence frequency of the frost and snow disasters was increased. The majority of the disaster grade is serious disaster. The hail disaster of Ming Dynasty was19times, one time every14.5years in Ordos Plateau. From early stage in the Ming Dynasty to the end, the occurrence frequency of hail disaster was increased in Ordos Plateau. The majority of the disaster is moderate disaster.(2) The flood disaster of Qing Dynasty was78times, one time every3.4years in east of Ordos Plateau, and that was89times, one time every3years in west. From early stage in Qing Dynasty to the end, the occurrence frequency of the flood disaster was increased in east and west of Ordos Plateau, the flood disaster in west of Ordos Plateau is more than which in east, the number of moderate disaster in west of Ordos Plateau is equal to the number in west. The number of moderate disaster in east of Ordos Plateau is more than that in west, the number of serious disaster in west of Ordos Plateau is more than that in east. The flood disaster in Ordos Plateau has obvious periodic oscillations of30～35a,10～20a,5～10a.The frost and snow disasters of Qing Dynasty were36times, one time every7.4years in east of Ordos Plateau. The frost and snow disasters were39times, one time every6.9years in west. From early stage in Qing Dynasty to the end, the occurrence frequency of the frost and snow disasters was increased in east and west of Ordos Plateau, the number of the frost and snow disasters in west of Ordos Plateau were equal to which in east, the moderate disaster is the majority of the disaster grade in east and west of Ordos Plateau. The frost and snow disasters in Ordos Plateau have obvious periodic oscillations of15～20a,10～15a,5～10a. The hail disaster of Qing Dynasty was82times, one time every3.3years in east of Ordos Plateau, and it was62times, one time every4.3years in west. From early stage in Qing Dynasty to the end, the occurrence frequency of the hail disaster was increased in east and west of Ordos Plateau, the number of the hail disaster in east of Ordos Plateau was more than which in west, the moderate disaster is the majority of the disaster grade in east and west of Ordos Plateau. The number of the serious disaster in east of Ordos Plateau was more than which in west of Ordos Plateau. The hail disaster in Ordos Plateau have obvious periodic oscillations of35～36a,15～20a,5～7a.(3) The flood disaster since the Republic of China was57times, one time every1.5years in east of Ordos Plateau, and it was33times, one time every2.7years in west. Since the Republic of China, the occurrence frequency of the flood disaster was slow increased in east of Ordos Plateau, obviously increased in west, the flood disaster in east of Ordos Plateau is more than which in west, severe disaster occur most frequently, follow by moderate disasters, minor disaster is fewer. The flood disaster in Ordos Plateau has obvious periodic oscillations of8～14a,2～5a. The frost and snow disasters since the Republic of China were42times, one time every2.1years in east of Ordos Plateau. The frost and snow disasters were16times, one time every16.8years in west. Since the Republic of China, the occurrence frequency of the frost and snow disasters was increased in east and west of Ordos Plateau, the number of the frost and snow disasters in east of Ordos Plateau were higher which in east, but the frost and snow disaster in west of Ordos Plateau has greater intensity than that in west. The frost and snow disasters in Ordos Plateau have obvious periodic oscillations of8a,12a,25a. The hail disaster since the Republic of China were53times, one time every1.7years in east of Ordos Plateau and it was36times, one time every2.4years in west. Since the Republic of China, the occurrence frequency of the hail disaster was increased in east and west of Ordos Plateau, the number of the hail disaster in east of Ordos Plateau was more than which in west, the serious disaster is the majority of the disaster grade in east and west of Ordos Plateau. The hail disaster in Ordos Plateau have obvious periodic oscillations of25a,8～10a,2～5a.(4)Heavy rain, long period of rain, ice flood are three types of flood disaster in Ordos Plateau. The intense rainfall of summer and autumn and an increasing annual precipitation are the main reasons for the formation of flood disasters. The condition of frost and snow disaster is at low temperatures such as below0℃or0℃. The temperature suddenly drop to below0℃which caused by cold air activities in warmer months is the main reason causing the frost disaster on Ordos Plateau. The strong contrast between wet and dry and thermal which formed by special climatic conditions of the East Asian monsoon and the edge of the desert is one of the main factors causing unstable weather. Under the intense solar radiation, the heat in Mu Us Sandland and Kubuqi desert and its bare ground is intense, which result in a strong convective motion and is the main reason for hail on this area.(5) According to the analysis of frequency and intensity of flood disaster, the humid climate events have occurred on the eastern part and western part of the Ordos plateau since the Ming Dynasty are respectively from1500to1509,1520to1529,1540to1549,1884to1893,1972to1981and from1420to1429,1774to1783,1814to1823,1982to1991. According to the analysis of frequency and intensity of frost and snow disasters, there is one cold climate event from1628to1643in Ming Dynasty. The cold climate events have occurred on the eastern part and western part of the Ordos plateau since the Qing Dynasty are respectively from1754to1763,1962to1971and from1774to1783,1972to1981. |
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