Analysis of the spatiotemporal characteristics of the danger degree of the torrential rain in the Huaihe River basin

To explore the temporal and spatial characteristics of the torrential rain in Huaihe River basin,this article analyzed the average trend and period of the rainstorm frequency and intensity by the empirical mode decomposition method,and then analyzed the comprehensive danger and return period by the Copulas function based on the torrential rain days and maximum daily precipitation data for each year at 68 meteorological stations from 1951 to 2017. The results are as follows. (1) The rainstorm frequency of the whole basin firstly decreased and then increased in the study period,and the turning point occurred around 1980. The intensity has been showing a significant linear upward trend. The main periods of rainstorm frequency and intensity are both subject to 2 a. (2) For more than 90% of the torrential rain,the frequency is 3 days or less and the intensity is less than 100 mm. The increasing rate slows down significantly when exceeding these thresholds. The return period of torrential rain increases with the two single indexes increasing,and the rate gets faster gradually. (3) The high values of the spatial pattern of the average change trend for the two indicators are consistent. And the area ratios of the rising and falling trends of the two indicators are similar. (4) The spatial pattern of the mean and minimum values of the joint probability of the rainstorm is consistent,while the distribution of its maximum values is relatively dispersed. Although the frequency and intensity of rainstorms in some cities show downward trends,the mean and minimum values of the danger of the rainstorm hazard evaluated by joint probability are very high.