Towards an improved understanding of hydrological alteration under global change

This dissertation proposes a comprehensive approach to explain the transitions in discharge regime in a more practical way using Indicators of Hydrological Alteration (IHA) and to disentangle the effects of individual factors on the detected changes in hydrological alteration. To identify the role of each environmental factor in the transition from local to global scales, the method uses discharge time series, both historical observations and projected data, as target variables. This allows a proper assessment from multiple case studies and at different scales.

The method is first applied to an anthropogenically influenced river basin and then transferred to simulated runoff based on climate input at the multi-basin scale under climate factors until the end of the 21st century. Finally, it was used to detect the transitions in runoff regimes globally in the period 1960-2010. More than ten environmental variables, including climatic factors and anthropogenic factors, categorized by nine climate zones, are used as input to investigate systematic changes in the historical river regime using machine learning.

The main findings of the thesis are that abrupt changes in the discharge regime are clearly evident in all case studies, and that these coincide visibly with human intervention (diversion dam) at the local scale. On a global scale, two-thirds of catchments worldwide experienced significant changes in discharge between the 1980s and 1990s. The top, highest, and lowest ranking variables that contributed to the change in flow regime have been tentatively identified.

Overall, an improved understanding of the influence of environmental factors on hydrological change has been demonstrated. This highlights the contribution of individual environmental factors to global runoff regimes and can be used to provide a good guidance for further strategies applied to the river basin management.