Historical changes of Ningxia natural disasters and its reason analysis.

A quantitative model is built and the historical variation of natural disasters is analyzed in this paper,by using materials as affected area(damaged area),affected population(dead and injured),casualty of livestock,collapsed houses(damaged houses),decrease of crops yield,economic loss(direct and indirect) and price index over the same period of Ningxia natural disasters(include drought,flood,gale and hail,frost,pest disasters and other disasters) during 1978-2007,and applying gray correlation analysis method.The long-term changes trend of natural disasters is analyzed by the application of the least squares method for linear trend,and the oscillation period is analyzed by using the maximum entropy statistical method.It is found that natural disasters have basically 2 to 3 years of variation period either in the whole region or in the individual regions;from the mid 1980s to the late 1990s,Ningxia is in an oscillation period with disasters attacked frequently in decadal and interannual scale under the background of climate;the increase of damage intensity of natural disasters slow down and tends to decrease since 2000.