中文摘要：

采用1960—2011年西北地区111个观测站逐日气象资料,利用FAO Penman-Monteith模型,计算出各气象站的潜在蒸散量,由此计算出各站的湿润指数,并对其进行标准化,统计极端干旱发生频率。对西北全区和不同自然区的降水量及极端干旱发生频率的时空变化特征进行了探讨分析。结果表明：近52年来西北全区年降水量变化倾向率表现出微弱的上升趋势,平均每年上升0.17 mm。由该区年降水量变化的空间差异,可将研究区划分为3个部分：明显增加区、轻度增加区、减少区。西北全区极端干旱发生频率的平均值为3.8月/a,气候变化倾向率总体上表现出明显的下降趋势,平均每年下降0.011月。根据西北地区极端干旱发生频率变化的空间差异,也可将研究区划分为3个部分：明显减少区、轻度减少区、增加区。极端干旱发生频率与降水量和平均气温表现为显著的负相关性,与无雨总日数呈现出显著的正相关性。

英文摘要：

Based on the daily data observed at 111 meteorological stations in Northwest China from 1960 to 2011,the potential evaporation was calculated using the FAO Penman-Monteith model,and then the humid index was derived,as well as the extreme drought frequency. The temporal and spatial variations of the precipitation and extreme drought frequency were discussed in this paper. The results indicated that the precipitation in Northwest China has increased by 0.17 mm per year in the last 52 years. According to the spatial variation of precipitation,the study area could be divided into three subregions including obvious increasing,light increasing and decreasing. The mean extreme drought frequency was 3. 8months per year during 1960—2011,at a decreasing rate of 0.011 months per year. The variation of the extreme drought frequency also showed a spatial diversity and could be divided into three subregions including obvious decreasing,light decreasing and increasing. The extreme drought frequency showed a negative correlation with precipitation and average temperature,and a positive correlation with rainless days.