Anne F. Van Loon (2015) reviews the current scientific understanding of hydrological drought, its definitions, processes, and quantification. Hydrological drought refers to a lack of water in the hydrological system, manifesting as abnormally low streamflow, lake levels, and groundwater levels. It is a complex natural hazard with significant impacts on ecosystems and society. The review highlights the influence of climate and terrestrial properties on hydrological drought characteristics and the role of storage. It discusses the debate over drought indicators, recent advances in monitoring and prediction, and future projections of hydrological drought. The paper also touches on the link between hydrological drought characteristics and impacts, as well as challenges for future research. It emphasizes the importance of understanding hydrological drought for effective management and mitigation. The review outlines four challenges for future research, relating to international initiatives such as the Intergovernmental Panel on Climate Change (IPCC) and the 'Panta Rhei' decade of the International Association of Hydrological Sciences (IAHS). The paper also discusses the different types of drought, including meteorological, soil moisture, hydrological, and socioeconomic droughts, and their characteristics. It highlights the importance of understanding drought propagation, climate control, and catchment characteristics in hydrological drought development. The review also addresses the spatial and temporal scales of hydrological drought, noting that droughts occur on different scales than floods. It discusses the challenges of quantifying hydrological drought and the need for further research to improve understanding and management of drought. The paper concludes with a call for further research to better understand and manage hydrological drought.Anne F. Van Loon (2015) reviews the current scientific understanding of hydrological drought, its definitions, processes, and quantification. Hydrological drought refers to a lack of water in the hydrological system, manifesting as abnormally low streamflow, lake levels, and groundwater levels. It is a complex natural hazard with significant impacts on ecosystems and society. The review highlights the influence of climate and terrestrial properties on hydrological drought characteristics and the role of storage. It discusses the debate over drought indicators, recent advances in monitoring and prediction, and future projections of hydrological drought. The paper also touches on the link between hydrological drought characteristics and impacts, as well as challenges for future research. It emphasizes the importance of understanding hydrological drought for effective management and mitigation. The review outlines four challenges for future research, relating to international initiatives such as the Intergovernmental Panel on Climate Change (IPCC) and the 'Panta Rhei' decade of the International Association of Hydrological Sciences (IAHS). The paper also discusses the different types of drought, including meteorological, soil moisture, hydrological, and socioeconomic droughts, and their characteristics. It highlights the importance of understanding drought propagation, climate control, and catchment characteristics in hydrological drought development. The review also addresses the spatial and temporal scales of hydrological drought, noting that droughts occur on different scales than floods. It discusses the challenges of quantifying hydrological drought and the need for further research to improve understanding and management of drought. The paper concludes with a call for further research to better understand and manage hydrological drought.