西安市极端降水频数的变化特征及旱涝趋势

利用西安市1951—2006年的逐日降水资料和1992～2006年的逐时降水资料，采用数理统计、5年滑动平均等方法研究了西安市大降水和极端降水的年际、年内变化特征；并运用马尔可夫模型对未来5a旱涝状况进行了预测。结果表明：近56a来西安市大降水频数呈减少趋势；其月际变化呈双峰型，主要集中在7-9月，7月最多，1月和12月无大降水事件发生；旬际变化峰谷波动较多，主要集中在7月和9月上旬，7月上旬最多。而极端降水频数则呈现明显的增加趋势，主要集中在6—8月，7月最多；从日变化看，主要发生在7：00，其次为17：00；晚上偏多，且由多到少的时间排序为：后半夜→下午→前半夜→上午。该市2007年降水量“偏涝”的概率较大，2008～2011年处于“正常值”的概率较大。图3。表5．参17。

Extreme Precipitation Frequent Change and Drought and Flood Prediction in Xi'an City

Based on the daily precipitation data from 1951 to 2006, and hour precipitation data between 1992 and 2006, the change of heavy rain and extreme precipitation are analyzed by mathematical statistlcs,string analysis and a five-year moving average method.The prediction states of drought and flood are determined by Markov in future 5 years. The results indicate: the frequence of heavy rain is decreasing;month change as-sumes the bimodal type,mainly concentrates in July to September, the Largest in July, non-heavy rain events occur in January and in December;There are more valley fluctuations in ten-day periods, whieh mainly concentrates in July and early September,the largest in the first ten-day pe-riod of July. The frequency of extreme precipitation is increasing,mainly concentrates in June to August,the largest in July ,too;which mainly occurs in 7: 00, secondly in 17:00 in a day;the times of extreme precipitation in the evening is greater than that in the daytime;the wee hours the most,the morning hours the fewest.In the next 5 years,the probability of year precipitation is larger in weak flood state in 2007,while it is bigger in normal states from 2008 to 2011.