中文摘要：

过去50多年来，环境脆弱的北方农牧交错带中部区域增温明显且干旱频发，对生态系统和农牧民的生计造成极为严重的影响。认识该区域降水过程的持续性特征对准确预测未来可能面临的灾害事件及政策调整有重要意义。本文采用时序距平分析法与基于分形理论的重标极差分析法相结合的方式，详细分析了北方农牧交错带中部地区6个站点1956年-2009期间降水变化过程及其持续性特征。结果表明，研究区过去54年间的年降水量总体呈下降趋势，夏季降水量年际变化曲线与年降水量变化曲线基本一致，但春季降水量略有上升；在1年与5年时间尺度上，年降水量和各季节降水量变化均呈剧烈波动态势。值得注意的是，在5年时间尺度上，Hurst指数在整个研究区域上的平均值比其在1年时间尺度上的平均值更接近于1．0，即随着时间尺度的增大，降水过程的自相关性、持续性增强明显。在10年尺度上，研究区全年少水期的出现即距平值为负值的持续时间长约10年，而春季少水期出现的持续时间则可能长达20．30年。

英文摘要：

The environment is extremely fragile in the central region of the northern agro-pastoral zone. There were obvious temperature increase and frequent drought over the past 50 years, which have profound and harsh effects on ecosystem and the livelihood of farmers and herdsman. Knowing the persistent characteristics of precipitation series in this region is of great importance to the forecast of future disaster and policy adjustment. With the combination of time series anomaly analysis method and rescaled range analysis method based on fractal theory, this paper has expounded the precipitation change process and its persistent characteristics with data of 6 stations in the central region of the northern agro-pastoral zone from 1956 to 2009. The results show that, in the past 54 years, the annual precipitation was overall in a continuously decreasing trend; the inter-annual variations curve of summer precipitation was similar with the variations curve of annual precipitation, but the spring precipitation had slight increase; On 1-year and 5-year time-scales, the variations of annual and seasonal precipitation were all in a severely fluctuant situation. Remarkably, on 5-year time-scale, the average value of Hurst index for the whole area is much closer to 1.0 than that on 1-year time-scale, that is, the auto-correlativity and persistence enhance obviously along with the increase of the time-scale. On the ten-year time-scale, the low precipitation period when the anomaly is negative will last for 10 years, while the low precipitation period of spring will last for 20-30 years. In order to deal with the drought effectively, we need to adjust both agriculture and animal husbandry in agro-pastoral zone. It is necessary to reduce agricultural irrigation water, adjust the planting structure and way in agriculture, solve problems of livestock drinking water, strengthen the raising management, enhance the crisis awareness of farmers and herdsmen and keep improving their ability to adapt to the disaster.