Spatial and temporal distribution characteristics of extreme area rainfall in Hanjiang River basin

Based on the daily precipitation data of 1961—2018, the spatial and temporal distribution characteristics of the extreme area rainfall calculated by the relative threshold method in the five sub-basins of the Hanjiang River are studied by using the sliding average and linear tendency. Pearson Ⅲ distribution is used to reproduce annual maximum extreme area rainfall (P_max). The results show that: (1) The spatial distribution of mean annual extreme area rainfall days (D) and the mean annual extreme 5-day area rainfall days (D_5d) decrease from upreaches to downreaches in the Hanjiang River basin. The mean annual extreme area rainfall (R) and the mean annual extreme 5-day area rainfall intensity (I_5d) are more in upreaches and downreaches, and less in middle region. (2) There is a positive correlation between D and R in Shiquan and its upper reaches, Shiquan Ankang and Danjiangkou Huangzhuang, but it shows an anticorrelation in Ankang Danjiangkou and Huangzhuang and its lower reaches. (3) The interdecadal variations of D, R, D_5d and I_5d are similar. In Shiquan and its upper reaches, Shiquan Ankang and Ankang Danjiangkou the variation shows a double-peak distribution, and in Huangzhuang and its lower reaches it shows a single-peak distribution. The interdecadal characteristics are not obvious in Danjiangkou to Huangzhuang. (4) The peak (valley) values of D, R, D_5d, I_5d, and Pmax are consistent, which indicates that the persistent and extreme rainfall events are relatively high (low) in the year with high (low) frequency of extreme area rainfall in the same sub basin, but the probability of persistent extreme precipitation events occur in all the sub-basins in the same year is small. (5) Pmax shows an increasing trend in most sub basins. Before the return period of 50 years, Pmax increases rapidly, and then slows down. The growth rate of Pmax is the largest in Huangzhuang and its lower reaches in any return periods.