Investigating the impacts of climate change and human activities on hydrological drought using non-stationary approaches

Climate variation and human activities are two prime drivers influencing the hydrological cycle and the stationarity of hydrologic systems. Hydrological drought is caused by significant negative variation from normal hydrologic conditions, influenced by climate variation and human activities. Traditional methods employed for quantifying the impacts of climate change and human activities are based on the assumption of stationarity, which is no longer valid under the current changing environment. In this study, a heuristic segmentation method was used to identify the change point in streamflow time series by considering its non-linearity, and trend analysis was performed on hydro-meteorological variables before and after the change point. A non-stationary Standardized Runoff Index (SRINS) was constructed using Generalized Additive Models in Location, Scale, and Shape (GAMLSS). Finally, the influences of climate change and human activities on hydrological drought were quantified at various time scales. The results reveal change points in the streamflow time series after the 1990s. Significant decreases in precipitation and streamflow were also observed after the change points in all study watersheds, whereas the trend of potential evapotranspiration increased at a higher rate after the change points. The impact of climate change on the seasonal (three-month) time scale was greater in the winter and spring seasons, whereas the impact of human activities was significantly higher in the summer and autumn seasons. The influence of climate change on hydrological drought was dominant at longer (6- and 12-month) time scales, whereas human activities accounted for 25.6% and 20% of the changes in watersheds #1018 and #1019 on the 12-month time scale, respectively.