Climate Characteristics of Different Levels of Precipitation Days during the Flood Season in Baiyin, Gansu Province and Their Relationship with Precipitation

利用1961~2020年白银地区4测站逐日降水资料，采用线性趋势估计、相似系数、子波分析等数理统计方法，对白银地区不同等级降水日数气候特征进行分析。结果表明：随着降水等级的上升，对应该量级的降水日数迅速减少，并呈现“南多北少”的空间分布特点，地域上南部较北部稳定，不同级别上总降水日数最稳定，小雨次之，再次中雨。北部总降水日数、小雨日数、中雨日数的年代际变化趋势均不明显；南部年代际下降趋势比较显著，总降水日数变化较显著，小雨次之、再次中雨。白银市汛期总降水日数和小雨降水日数的年代际变化非常相似，21世纪10年代呈下降趋势，20年代呈上升趋势；中雨降水日数年代际变化北部呈上升趋势，南部呈下降趋势。降水总日数和小雨日数具有10~15年左右的周期变化，目前正处在偏多的正位相；在较大的时间尺度上具有20~25年左右的周期，目前正处在偏多的正位相。汛期年降水量不仅与总雨日数、小雨日数、中雨日数的变化特点及趋势非常相似，而且不同等级降水日数对汛期年总降水的贡献关系均非常密切，其正相关程度随着降水等级的升高而增大，在异常年份，中雨及以上降水日数的变化对旱涝的影响更大。Using daily precipitation data from four stations in Baiyin area from 1961 to 2020, this study analyzed the climatic characteristics of different levels of precipitation days in Baiyin area using mathematical statistical methods such as linear trend estimation, similarity coefficient, and wavelet analysis. The results show that as the precipitation level increases, the number of precipitation days corresponding to that level rapidly decreases, and presents a spatial distribution characteristic of “more in the south and less in the north”. Geographically, the south is more stable than the north, and the total number of precipitation days is the most stable at different levels, followed by light rain and then moderate rain. The decadal trends of total precipitation days, light rain days, and moderate rain days in the northern region are not significant; The decadal decline trend in the southern region is relatively significant, with a significant change in the total number of precipitation days, followed by light rain and moderate rain again. The decadal changes in the total number of rainy days and light rain days during the flood season in Baiyin City are very similar, showing a downward trend in the 2010s and an upward trend in the 2020s; The decadal variation of the number of days with moderate rainfall shows an upward trend in the north and a downward trend in the south. The total number of rainy days and the number of light rainy days have a periodic variation of about 10~15 years, and are currently in a positive phase with more precipitation; On a larger time scale, it has a period of around 20~25 years and is currently in a biased positive phase. The annual precipitation during the flood season is not only very similar to the characteristics and trends of total rainy days, light rainy days, and moderate rainy days, but also closely related to the contribution of different levels of precipitation days to the total annual precipitation during the flood season. The degree of positive correlation increases with the increase of precipitation level. In different years, the change in the number of moderate and above rainy days has a greater impact on droughts and floods.