黄土高原位于我国东部季风区与西部非季风区之过渡地区,属弱季风区,降水相对较集中。为分析本区降水的时空分布,文中采用矢量合成方法计算了黄土高原296个站的降水季节性指标值及降水季节,前者变化于0.44—0.64 之间,后者集中于7月中、下旬至8月上旬,自东南向西北推迟。降水的季节性指标与降水季节直接影响到作物水分的亏缺量,全区小麦严重缺水100—200mm 不等,玉米缺水30—50mm,而谷子生育期中水分需求基本可以得到满足。最后,讨论了本区降水与土壤水分含量的关系,指出本区水分不足,认为有的作者提出的该区尚有80％的水分潜力没有得到发挥的见解是错误的。

Meteorological elements, such as the atmosphere pressure, temperature, precipitation, humidity, sunshine duration, etc., are usually considered as scalars. We use such a method that mean monthly precipitation at a station can be treated as a vector. The magnitude of the vector is the amount of rain, and the direction angle of the vector is the angle expressed in units of one representing that month. When monthly figures are added vectorially, the resultant vector can be used to compute the seasonality index, which is the ratio of the magnitude of the resultant vector to the mean annul rainfall. The direction of the resultant vector represents the seasonal concentration of precipitation distribution. Data from about 296 stations on the Loess Peateau are analyzed by this method. The maps of seasonality index are presented in Fig.3.The seasonality index characterizing the degree of seasonality of precipitation and the seasonal concentration of precipitation distribution are more useful parameters to describe the agricultural respect of climate of a district. They provide a simple method which condenses the 12 monthly rainfalls into an expression containing 5 digits only.The seasonality index of precipitation on the Loess Pleateau is between 0.44 and 0.66. The distribution of the isolines is closer in the Weihe River basin and the lower reaches of thet Yellow River and sparser in the northern part. It means that the precipitation is more concentrated in the northern part and decreases toward the south.The seasonality index of precipitation has good relations with the crop water stress. The precipitation is concentrated in the mouths of July and, August. So the crop of wheat fells severe deficiency of water, the millet fells almost enough and the situation of corn is between them.