中文摘要：

基于两变量分析方法推求分期设计洪水，对于给定的联合重现期水平，存在着多种洪水组合均可满足规定的防洪标准，现有方法无法评价不同组合发生的可能性和合理性．采用归一化的联合概率密度函数值作为洪水发生的相对可能性指标，提出了分期设计洪水组合的可行区间，并基于条件概率原理推导了3种具有统计意义的联合重现期水平下的计算公式，即最可能组合、条件最可能组合和条件期望组合．丹江口水库应用结果表明：以7日洪量为例，最可能组合和条件最可能组合的估计值均位于可行区间内，条件期望组合的估计值在可行区间外．建议采用条件最可能组合方法计算丹江口水库7日洪量分期洪水设计值，该法不仅具有较强的统计理论基础，而且能够较好地描述汛期洪水特征；在满足防洪标准的前提下，通过适当抬高（降低）夏（秋）汛期的洪水设计值，有利于水库汛末蓄水和提高综合利用效益．

英文摘要：

The current bivariate methods for seasonal design flood estimation usually derive infinite combi- nations for given joint return period, but cannot assess the possibility or rationality of each combination. The normalized joint probability density function value is adopted to measure the relative occurrence likeli- hood of flood events. A methodology is proposed to derive the feasible interval of seasonal design flood~ and then three statistically-based formulae of combinations, i.e. most likely composition （MLC）, condi- tional most likely composition （CMLC） and conditional expectation composition （CEC） are derived based on conditional probability principle. The Danjiangkou reservoir is selected as a case study; and the 7-day flood volume is analyzed for example. It is demonstrated that the joint design values of 7-day flood volume estimated by the MLC and CMLC are within the feasible region, and by the CEC are beyond the feasible region. It is suggested to estimate seasonal design flood of Danjiangkou reservoir using CMLC that has strong statistical basis and can describe flood seasonality well. Under the condition of satisfying flood pre vention standards, the design floods of summer （or autumn） flood season can be raised （or lowered）, which is beneficial for refill operation and comprehensive utilization.