Changing rule and influence of threshold for peak-over-threshold flood series under the changing environment in Wujiang, China

The threshold of peak-over-threshold (POT) flood series has changed for many rivers in the world. Improper selection of threshold will affect the rationality of frequency analysis. The changing rule and influence of the threshold selection for POT flood series under the changing environment in Wujiang River was analyzed using historical flood data. The research results indicate that watershed vegetation and runoff coefficient varied significantly in 1991.After the environment was changed, the threshold increased significantly, and the occurrence number of flood exceeding a certain magnitude also increased. The POT model can flexibly capture the information on changes of flood magnitude and frequency of occurrence. The threshold values, which changed before and after environment, were selected to calculate designed flood peak. The difference degree of designed flood peak for different threshold is up to 19.21% at Pingshi and more than 8.12% at Lishi station when the return period is greater than 200a. High threshold in the selection after the environment was changed can effectively improve the fitting degree of distribution to higher flood flow and improve the accuracy of design flood under the changing environment.