咸潮作用下上海水资源风险极值统计分析

Analysis of Water Deficit Risk in Shanghai Caused by Salt Intrusion by Extreme Value Statistics

长江口咸潮灾害使上海陈行水库和青草沙水库面临水资源匮乏的风险。本文从灾害风险角度,以极值统计原理拟合水库河段年最大连续不可取水时间超越概率分布。水库不可取水极值时间和日供水量作为水库缺水的风险因子,蒙特卡罗方法随机模拟分析陈行和青草沙水库的缺水风险。结果表明长江口盐水入侵造成的水库河段不可取水极值分布属于Gumbel-II(Frechet)型极值分布,陈行水库每年缺水事件的最大概率高于80%,青草沙水库仅为3%,与陈行水库实际运行情况及青草沙水库设计要求较一致;发生概率小于1%的巨灾事件中,青草沙水库水资源风险大于陈行水库。

Chenhang and Qingcaosha Reservoirs are faced with the risk of fresh water deficit as a result of brackish water sources when salt water intrusion from the Yangtze Estuary.Examining its mechanism is of vital importance to sustainable development of Shanghai Municipality.Based on the conception of hazard and disaster risk,the authors investigated the annual exceedance probabilistic distribution of fresh water resource risk of the two reservoirs.Annual extreme failure times of fresh water intake of the reservoirs from the Yangtze Estuary were investigated by the extreme value statistics in order to highlight the extreme hazard risk.Data on fresh water intake failure time were utilized to examine the correlation between Datong discharge and extreme failure time of fresh water intake in the Yangtze Estuary.By taking the extreme failure time of fresh water intake and the daily water supply as two risk factors,the deficit risks of the Chenhang and Qingcaosha Reservoirs in water supply were explored by the Monte Carlo simulation method.Results demonstrated that annual exceedance probabilistic distribution of extreme water intake failure time from the Yangtze Estuary can generally be categorized as the Gumbel-Ⅱ （Frechet） distribution.The fitting results of fresh water intake failure time for Chenhang and Qingcaosha Reservoirs by extreme value statistics showed that the possibility of fresh water intake failure time more than 35 days and 70 days is relatively low.It was also found that the maximum failure time of fresh water intake of Qingcaosha Reservoir is 68 days by Shanghai Water Authority,which means that the situation of fresh water intake failure time more than 68 days would rarely happen.The fitting results of Qingcaosha Reservoir are essentially consistent with that from the Shanghai Water Authority.The maximum annual water deficit probability of Chenhang Reservoir could be more than 80%,while that of Qingcaosha Reservoir is less than 3%.Results from the Monte Carlo simulation are basically accordant with practical operation of Chenhang Reservoir during the period 2004-2009.If the reservoir was required to be sustained under the condition of the fresh water intake failure time of 68 days,the annual maximum water deficit of Qingcaosha Reservoir shows a significantly low probability due to a huge capacity.However,the water resource risk of Qingcaosha Reservoir could be higher than that of Chenhang Reservoir under catastrophic events with a probability less than 1%.