SEASONALITY OF PRECIPITATION ON THE LOESS PLEATEAU AND CROP WATER STRESS
黄土高原降水的季节性指标及其与作物水分亏缺的关系

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.