Nonstationary Frequency Analysis of the Weihe River Annual Runoff Series Using De-Nonstationarity Method

Abstract

Climate change and human activity has increased the degree of nonstationarity in the annual runoff series of the Weihe River Basin, making it difficult to conduct a traditional hydrological frequency analysis in this study area. Most of the existing nonstationary frequency analysis methods are still controversial and cannot be applied in practice. Furthermore, the nonstationarity of the annual runoff series is caused by nonstationary changes in some specific influencing factors. In this study, we remove the nonstationarity from the annual runoff series by quantifying the relationship between the annual runoff and its driving factors. This denonstationarity process removes the nonstationary influence of the driving factors and is not limited to some specific types of nonstationarity, such as trend and abrupt change. For the annual runoff series of the Weihe River, the annual average temperature (Tave), irrigation area (IA), and reservoir index (RI) are used as the reconstruction factors, respectively. And the decreasing trends in the first and second moments, and an abrupt change in 1993 are removed simultaneously by this denonstationarity method. The stationarity of the denonstationarity series reconstructed by Tave is the best, followed by the denonstationarity series reconstructed by the IA and the RI. Climate change has a more significant impact than human activity on the nonstationary change of the annual runoff. Once the reconstructed series are stationary, the traditional hydrological frequency analysis method is adopted. Reconstructed series with a higher degree of stationarity result in lower uncertainties in the design quantile of the annual runoff, whereas the significant nonstationarity in the original series results in large design value uncertainties.