Quantifying the effects of direct human activities and climate change on the spatial propagation of hydrological drought in the Yellow River Basin, China

The spatio-temporal evolution and propagation of flood and drought events are important in predicting their occurrences. This study attempted to investigate into the spatial propagation characteristics of hydrological droughts and their dynamics by identifying the spatial propagation probability (PP) and propagation time (PT) of different drought levels in the Yellow River Basin (YRB), China, based on copula functions and conditional probability. Then, the effects of direct human activities and climate change on the hydrological drought propagation were quantified by comparing the propagation characteristics of hydrological drought under observed and natural runoff. Results indicated that: (1) overall, the propagation of hydrological drought in the YRB showed a generally decreasing trend in PP and an increasing trend in PT with the increase of distance, presenting a gradually attenuating propagation along the river channel, the spatial propagation process of hydrological drought is similar to the domino effect; (2) when extreme hydrological drought occurred in the area represented by the Tangnaihai station located in the upstream basin, the probability of downstream areas experiencing moderate to severe hydrological drought showed an approximately decreasing trend along the river channel, with the PP of the observed sequence being slightly less than the natural sequence and the fluctuation of the hydrological drought propagation being more apparent; (3) taking 1986 as the dividing year, the average relative contribution rates of direct human activities to the changes in drought PT in spring, summer, autumn, and winter were 39.7%, 62.9%, 55.3%, and −67.2%, respectively, indicating a significant regulatory effect of reservoirs on hydrological drought propagation. In general, this study provided new insights into the spatial propagation mechanism of hydrological drought and offered a promising pathway for drought early warning and mitigation.