水是作物生长发育不可缺少的条件,俗语说:“有收无收在于水,收多收少在于肥”。我国地域辽阔,又处于季风的影响之下,气候的干湿情况不但各地差异很大,而且一年中的变化也很悬殊,对农业的影响极不相同。只有考虑干湿的季节变化才能反映我国气候的实际情况,才能为农业生产提供可靠的依据。

In this paper, the geographical and seasonal distributions of the wet and dry periods and their influences on the agriculture of China are discussed. By using the meteorological data during 1951-60 of 310 stations, the durations of the wet and dry periods are de- termined by the histogram of the monthly aridity. The results of the calculations (Fig. 3-8) are in good agreement with facts. Based on the indices of aridity, they are distinguished into the moist, humid, arid and dry periods. The indices of aridity(D) are obtained from the following formula: D=Σd/r，where Σd is the sum of the daily saturation deficit(mb) and r the monthly precipitation (mm). The criteria for wet and dry periods are shown in table 1.