Improvement of reasoning formula method for design flood based on runoff generation and confluence theory

The establishment, improvement and successful application of the IWHR reasoning formula method, an optimization of the reasoning formula method, is one of the important achievements in hydrology. It considers the time-history variation of rainstorm，introduces the concept of runoff duration in the analysis of net rainfall，and adopts rainstorm and average loss intensity in the calculation of flood. In recent years, the practice and comprehensive analysis show that the determination of the average rainfall loss intensity, a key parameter of the IWHR reasoning formula method, is uncertain. In fact，the problem of the generalization and determination of the rainfall loss intensity is a reduction in the process from design rainstorm to net rainfall, which could be resolved by runoff generation and confluence theory. By using the method of rainfall-runoff correlation, the net rainfall in the maximum confluence duration and the net rain process can be calculated, so the relationship between the net rain process and the flood at the exit section can be formed. Thus, the uncertainty of the generalized loss intensity can be avoided and the flood calculation result is more closer to the reality. Therefore, the reasoning formula method is perfected.