关键词: 粮食产量, 波动, 经验模态分解, 多尺度, 本征模函数

Abstract: Grain security is a very important issue which has a significant influence on the social stability of both a country and a region.Meanwhile,the fluctuation of grain output has a close relation with grain security.Thereby,the researches on the periods,amplitudes and characteristics of the fluctuation of grain output and its causes are propitious to reduce the intensity of the fluctuation,to eliminate unfavorable factors,and further to keep the stability of grain output to insure grain security.Empirical Mode Decomposition(EMD) is a powerful method for analyzing the nonlinear and non-stationary time series.Different from the traditional method in doing integral transformation to signal,it decomposes signal into several intrinsic mode functions (IMFs),which contain and extrude the local characters of signal.So the characteristic information of the original signal can be well held by analyzing the IMFs.Therefore,EMD method was introduced to study the fluctuation of grain production in China and its causes at multi-time scale,and the data have been decomposed into two IMFS and a residual trend term.The conclusions are drawn :1)The fluctuations of grain output have 3-year and 9-year time scales.Hereinto,fluctuation of the 9-year time scale is more prominent.Shown from the residual trend term,the grain output has increased continually since 1949,but stagnated after 1996. 2)We have decomposed the data of per capita grain yield and cultivated land by EMD.We found that per capita grain yield is the control factor of the 3-year fluctuation of grain output while cultivated land is the main influencing factor of the 9-year fluctuation of grain yield,moreover,the national policy and natural disasters also influence the 9-year fluctuation of grain yield to a certain degree.Therefore,in order to eliminate the short-term unfavorable factors,we must improve the foundation of grain production,add the investment in science and technology and enhance the ability of disaster prevention so as to stabilize per capita grain yield.With regarded the elimination of the long-term unfavorable factors,we should control the decrease in cultivated land.3)Analysis on the periodic fluctuation of the two time scales revealed that the grain output will decrease in the recent 2～3 years,but it will soon reverse to increase for a long time.Meanwhile,the amplitudes of the oscillation of grain output is increasing,which indicates that it is possible for the grain output to increase or decrease with big amplitudes.So it is urgent to take measures to guarantee the grain security in the case of crop failure.

Key words: grain output, fluctuation, empirical mode decomposition, intrinsic mode function