Attribution of nonstationary changes in the annual runoff of the Weihe River using the de-nonstationarity method

The attribution of nonstationary change of the annual runoff series of the Weihe River is a topic of debate. In order to determine main drivers of nonstationary changes of Weihe River runoff, a de-nonstationarity method is used to remove the nonstationary influence of the potential driving factors and transform the nonstationary annual runoff series into a stationary reconstructed series. Then, the primary causes of the nonstationarity are identified by determining which factor results in the most stationary reconstructed series. The climate change factors (annual precipitation and annual average temperature) and the human activity factors (irrigation area and reservoir index) are selected as the reconstructed factors. The attribution results demonstrate that (1) both temperature rises and human activities have a significant impact on the nonstationary change, (2) the temperature is the most significant contributor to the nonstationary change in the annual runoff, which has a nonlinear impact on the first and second moments, (3) the influence of reservoir index is relatively smaller than irrigation area without considering the regulation rule, (4) the stationary precipitation series still exerts some impacts on the second moment of the annual runoff, (5) the temperature and irrigation area have some overlapped influence on annual runoff because the synchronous increase trend in the two series leads to a certain correlation between them, and the temperature rise may also lead to the adjustment of irrigation planning and management.