中文摘要：

本文对月保证率设定法进行改进,不仅考虑了不同保证率年份河流生态需水的差异,而且对不同的生态需水推荐流量等级进行了区分。随后采用改进月保证率设定法研究了青海湖流域的河流生态需水特征,结果表明：布哈河和沙柳河径流量的年际变化幅度较大,但增加或减少趋势不明显,夏季径流量年际波动最为明显,而径流量年内分布均呈单峰变化态型,峰值都出现在7月-8月份;伴随着保证率的降低,河流生态需水量不断增加,而且需水等级越高、增加幅度越大,同时不同保证率年份河流逐月生态需水呈现不同的变化规律;平水年（P=50%）青海湖流域两条主要河流布哈河和沙柳河适宜的生态需水量分别为2.18亿m3、1.36亿m3,枯水年（P=90%）二者分别为1.16亿m3和0.70亿m3。

英文摘要：

Rivers in the Qinghai Lake watershed have been environmentally degraded seriously over the past several decades due to a combined effect of climate change and human activities.In order to provide reference for optimal allocation of water resources,it is critical to evaluate the environmental flow requirements（EFRs） of rivers in this watershed.On the basis of the improvement of the monthly guaranteed frequency method,the authors not only considered the difference of EFRs for different guaranteed frequency years,but also distinguished different recommended grades of EFRs.Monthly river discharges of the Buha River and Shaliu River,two largest rivers in the study watershed,during the period 1958-2007 were used to quantify the EFRs.Subsequently,the EFRs of the two rivers were examined based on the improved monthly guaranteed frequency method（IMGFM）.Results indicate that the mean annual discharge of the Buha River and Shaliu River varied significantly during the past fifty years,without showing an obvious increasing or decreasing trend.Variation in the summer discharge was most significant for the four seasons.The annual distribution of the river discharge exhibited a single peak curve with the maximum peak in July or August,and the cumulative discharge of the Buha River and Shaliu River from May to October accounted for roughly 93.55% and 83.93% of the total annual discharge,respectively.In general,the EFRs of the rivers increased consistently,corresponding to decreasing guaranteed frequency.The higher EFRs grade,the greater magnitude of the increase in EFRs.For the recommended grade of ＂perfect＂,the EFRs of the Buha River at normal year（P = 50%） was 152.76% larger than that at dry year（P = 90%）,whereas the percentage was only 27.91% for the recommended grade of ＂minimum＂.Corresponding differences for the Shaliu River were found to be 91.84% and 17.65%,respectively.Moreover,monthly EFRs for different guaranteed frequency years varied significantly,which was jointly affected by month discharge,m