| Ninan is a dry region in the inner part of China and located at the northwestern part of loess plateau. It belongs to inland climate, consists semi-moist area with drought stress, semi-arid area and semi-arid area with drought tendency. Semi-arid area is the main part. The annual average rainfall is about 400mm, less than the half of the national average. Due to its unfavorable geographical location and hilly landform, rainfall is the fundamental form for farming water supply. To fight against the ever increasing dry area and deteriorating drought situation, it is necessary and urgent to use sci-tech achievement to improve the utilization of rainfall. And this is the foundation for the sustainable development of dryland farming and rural economy in Ningnan area.Based on the daily precipitation data accumulated in forty years from 1961-2000 in typical dryland areas of Jingyuan, longde, Xiji, Guyuan, Haiyuan, Tongxin and Yanchi counties in Ningnan section of Ningxia Autonomous Region, on the recognition of the precipitation character and rule in this areas, this paper has scientifically estimated the precipitation in drought stressed areas and explored the restrictive factors to rainfall resources. In addition, in virtue of the related scientific theories and approaches of precipitation-crop-ecology, using quantitative and qualitative analysis, combining macro- and micro-studies, this paper also systematically put forward elementary but explicit suggestions on the scientific utilization of precipitation resource in drought stressed areas.The annual precipitation in Ningnan area stably ranges from 270mm to 584mm, and spatially decreases from south to north. With the reduction of regional rainfall or decrease in time scale (year, month, and 10-days), variability is more significant. Within the same year, precipitation mainly concentrates in June, July, August and September, April and November were the turning point of annual rainfall; the dates of effective rainfall are less, and the dates of ineffective rainfall are more. There is significant or highly significantly correlations between annual precipitation and rainfall from July to October. Although the annual rainfall in counties of Ningnan mountainous area fluctuates in time scales (year, month, and 10-days), the rainfall is on the trend of decreasing in forty years. The changes in month rainfall are not consistent with that of the annual rainfall. This recognition could is the premise for the study of various technological measures to regulate the annual rainfall, and has great significance for the full utilization of the limited rainfall in Ningnan areas.Although there exists spatial and annual changes, the average rainfall in different places fits in with normal distribution basically in a long period. Judging from the precipitation data of the 41 years, the occurrence frequency of rainfall >(X +1S)mm in Jingyuan, Longde, Jixi, Guyuan, Haiyuan, Tongxi and Yanchi counties is > 84.13%. Except Haiyuan and Yanchi counties, which are 83%, a bit lower than the theoretical probability value of 84.13%, the rainfall of other five counties is larger than the theoretical probability value. This indicates that, within certain rainfall range, the annual rainfall is much stable. This relatively stable threshold value of rainfall is the prerequisite for the study of effective methods to make a full utilization of the limited precipitation resource in this region. In order to make a full use of the precipitation in this region, we should set up the guiding principle that is to use theabundant precipitation in rainy season to compensate the rainfall shortage in the hard times.Through analysing the correlation between the annual rainfall in years(annual rainfall > ( X. -1S) mm; and annual rainfall > ( X - 1S ) mm) with notable rainfall variation from the average and monthly precipitation, we found that, apparent correlation not only exists between the monthly rainfall in rainy season and the annual rainfall, it also exists between the monthly rainfall in some...
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