Precipitation Characteristics of Yaan in Drought and Flood Years

Multiple-time-scale variations of precipitation amount and precipitation frequency in Yaan region in four typical drought years and four typical flood years are investigated by using the daily precipitation data from 1951 to 2008 and the hourly precipitation data from 1969 to 2000.The result showed that the average annual precipitation in drought years was 1242.9mm Precipitation of flood season in drought years accounted for 70.4% of the total precipitation in drought years.The precipitation of flood season in flood years is twice of it in drought years, and it accounted for 81.1% of the total.Seasonal characteristic of precipitation distribution in drought and flood is obvious, especially in flood years.Rainfall intensity is similar to the seasonal changes in precipitation, reaches its maximum in summer, and the difference between annual rainfall intensity and rainfall intensity of flood season was obvious both in drought and flood years.The difference of wet days between drought and flood was much smaller, either was the seasonal differences.The maximum and minimum value of precipitation occurred in different months.In drought years, precipitation reached the maximum value in July, and the minimum value appeared in January.While, in flood years, the maximum value appeared in August, and the minimum value appeared in December.In addition, the maximum value the precipitation and wet days in the day and night appeared in different time, so was the rainfall intensity.The monthly change of precipitation and wet days was also quite different in drought and flood years.Characteristics of diurnal variations of precipitation and mid-night rainfall were prominent.Both the amount and frequency of mid-night rainfall is much larger than that of in the day.But the appearance time of peak and valley value were different in drought years and in flood years.In drought years, the peak of precipitation usually appeared at 1 a.m., and there was a valley at 2 p.m..In drought years, mid-night precipitation showed a bimodal structure, maximum value appeared at 11 p.m.and at 3 a.m., and the valley was at 4 p.m..The maximum frequency in drought years and in flood years both occurred at 0 a.m.and the minimum occurred at 2p.m.and 3 p.m.respectively.The increase rate from valley to peak is much faster than the decrease rate from peak to valley.Further analysis revealed that with the rainfall intensity increasing, the mid-night precipitation was more prone to form multi peaks, and the difference of amount and frequency of mid-night rainfall between drought years and flood years was more obvious.The amount and frequency of moderate and heavy rain in drought years are more than that of in flood years, but the amount and frequency of rainstorm in drought years is far less than that of in flood years.