Abstract: Pi River located in the west of Anhui Province is the largest tributary of the Huaihe River at its south bank. The spatial and temporal change of precipitation is very obvious in Pi River Valley. Meanwhile, droughts and floods occur frequently in this region. Analysis on the characteristics and changes of different-class precipitation is of great importance for revealing the laws of climate changes fully in the study area, taking precautions against and fighting natural adversities as well as utilizing water resources reasonably. Based on the daily precipitation data from 1958 to 2012 of 12 meteorological stations in Pi River Valley, daily precipitation were classified into four grades according to precipitation intensity, namely, light(0.1-9.9 mm/d), moderate(10.0-24.9 mm/d), heavy(25.0-49.9 mm/d), hard(≥50 mm/d) rain. The regional amount and frequency of different-class precipitation in the year and seasons were established on the basis of corresponding series of each station by using the arithmetical mean method. We analyzed the seasonal distribution as well as changing trend and abrupt changes of regional different-class precipitation, together with probability distribution of the continuous 3 days maximum precipitation in main rainy season (May to September) in Pi River Valley in this paper. The results showed that the mean proportion of light, moderate, heavy, hard rain precipitation to annual total precipitation were 21.2%, 29.7%, 26.4% and 22.7%, respectively, and those of different-class precipitation frequency were 73.4%, 16.9%, 7.0% and 2.7% in turn. Amount and frequency of light rain in seasons was relatively uniform. Precipitation class was higher, the frequency of it in a year was less and time distribution was more concentrated. The increasing trend of annual rainfall was not obvious, but frequency of annual precipitation exhibited a decreasing tendency at the 0.001 significance level, resulting in the increased annual average precipitation intensity. The amount and frequency of all grades both exhibited increasing tendency in summer and the increasing trend of hard rain were more significant than that of other precipitation classes. There wasn't obvious tendency in winter precipitation frequency except moderate rain, at the same time amount of light, moderate rain in winter exhibited increasing trend. The amount and frequency of all grades decreased consistently in spring and autumn and the decreasing tendency of light rain frequency were very significant exceeding 0.001 level. The abrupt changes in amount and frequency of annual hard rain occurred both in 1968. The jump of decreasing at the 0.01 significance level took place in 1975 in annual light rain frequency. There existed an abrupt increasing change in annual total rainfall, while there existed an abrupt reduction change in annual total precipitation frequency. In comparison to 1958-1977, the probabilities of the continuous 3 days extreme precipitation in main rainy season in Pi River Valley were higher in 1978-2012 and the extreme precipitation in different return periods increased obviously. Facing increasing flood frequency and intensity, we should strengthen the flood control and disaster mitigation efficiently.

Key words: different-class precipitation, rainfall, precipitation frequency, continuous heavy rain, Pi River Valley