Changes in Rainfall and Flood Characteristics under Nonstationarity in a Mountain Basin of Northwest China

Nonstationarity in hydrology has aroused great concern among people, and various influence factors have been proved to challenge the assumptions of the traditional hydrological analysis. For the selection of a reasonable flood frequency model and rationality of hydraulic engineering design, it is of great significance to study the variation characteristics of rainfall and floods and their relationship under the nonstationary condition. Taking the Yue River Basin, a mountain basin in northwest China, as an example, the nonparametric Mann-Kendall test and moving t-test methods were used to test the temporal trends and mutation of hydrological variables during the recent 53 years. The recession curve was derived from the historical flood series by the genetic algorithm, and the flood hydrographs were separated by the constant-slope base flow separation method. The results showed that the annual rainfall and runoff in the Yue River Basin during the recent 53 years exhibited insignificant increasing and decreasing trends, respectively. The abrupt change point of annual runoff series occurred in 1985. The average proportion of underground runoff changed from 21% before 1985 to 17% after 1985, and the relationship between rainfall and floods became weakened after 1985, indicating that the stationarity of flood was challenged.