Analyzing the Seasonally Nonstationary Discharge in the Wei River Basin (WRB) from Temporal to Spatial

Traditional hydrological analyses often rely on the assumption of stationarity, which assumes that the probability density function of a time series remains constant over time. However, due to climate changes and intensification in human activities, non-stationarity has been detected in various hydrological variables in many basins around the globe. Therefore, relying on the stationarity methods to investigate hydrological processes may introduce bias and significant issues in resource management and risk assessment. This could potentially result in environmental degradation or economic loss. Therefore, it is important to accurately identify the non-stationarity in hydrological relevant time series and employ appropriate approaches for analysis to adapt to the changing environment. This study investigates non-stationarity in the annual and seasonal discharges of the Wei River Basin in China. Four non-stationarity detection approaches (MK test, Pettitt test, BG algorithm, and Sen’s slope) were used to systematically detect trends and abrupt changes in the mean and maximum discharge time series as well as the precipitation and temperature. The results indicated that for most catchments on the Wei River non-stationarity signal have been found and the Baojixia channel was considered to be the most important driver of the reduction in discharge. The GAMLSS model was also used to develop stationary models and non-stationarity time-varying models. Precipitation, temperature, population, irrigation area, and drainage area were used as covariates in the non-stationarity models. The results show that the non-stationarity models performed better than the stationarity models and time-varying models. It was also noted that the attributions of drivers had seasonal patterns. The impact of climate change on the mean discharge, low flows, and high flows was also explored, using four different Shared Socioeconomic Pathways (SSPs). The results indicate that while the upper reach of the Wei River Basin will face increases in water resources (discharge), the lower reaches will face decreases. However, all across the Wei River Basin the risk of droughts and floods will increase in the future. It was also found that the impacts due to Baojixia channel will become more pronounced in the future. This thesis presents the first study on the non-stationarity of annual and seasonal discharge in the Wei River Basin, covering both historical and future perspectives. The research reveals non-stationarity behaviors within the basin and quantifies the contributions of various drivers, providing valuable insights for water resource management and disaster prevention in the region.