A STUDY OF THE VARIATION OF DRYNESS/WETNESS INBEIJING AND ITS FORECASTING kXPERIMENT
北京地区旱涝变化规律及其预测试验

Water supplement is .very important in Beijing where drought has frequently occurred since the 1960s. The climatic prospect about drought or flood is studied in this paper. The-study will focus the attention on three problems: first, how is the climate situation about dryness and wetness described? Second, how do they vary with time? Third, which method will be better for forecasting them? The yearly dryness/wetness index for describing climate situation is proposed in this paper. It is the sum over twelve of the monthly moisture index which is the standardized monthly precipitation. It is more meaningful and representative than the index of yearly precipitation. The series of the yearly dryness/wetness index in Beijing for the period from 1869 to 1987 have been built. Furthermore, it is investigated using power spectral analysis and! variance analysis for its characteristics of the periodic variation, and is found that the mainr periods are 5-yr and 22-yr. The period patterns, for 5-yr and 22-yr are extracted, by means of the period pattern method proposed in this paper. The patterns are represented by the eigenvectors of the cross-product matrix of the period matrix whose elements are from the original series arranged as, an array according to the length of the period. The forecasting of the dryness/wetness index is completed through the regression equation in which the period patterns are considered as the predictors. The results show that the water supplement will probably get improvement in Beijing by the end of this century.