Global-scale drought propagation and the drivers and patterns of multi-year groundwater drought

Abstract. Groundwater stores a third of all global freshwater and supports water supply, irrigation and ecosystems across the world. As such, groundwater drought can have wide-reaching financial, social and environmental impacts, particularly when drought events are prolonged or multi-year. Although recent work has made significant progress in understanding the drivers and patterns of multi-year meteorological droughts, we do not know how this signal translates into multi-year groundwater drought, where subsurface processes, anthropogenic influences and abstractions can alter the meteorological signal. This is particularly true at the global-scale, where a major barrier to understanding large-scale groundwater drought dynamics is the difficulty of obtaining consistent and comprehensive groundwater data. In this research, we use a new global hyper-resolution (∼1 km) groundwater dataset to investigate the global patterns and drivers of groundwater drought from 1960–2019, with a specific focus on multi-year events. We start by characterizing the propagation of meteorological (represented by SPEI-12) to groundwater drought, evaluating how and to what extent the sub-surface plays a role in modulating the meteorological drought signal. Subsequently, we define three global groundwater response types that provide a framework for understanding the processes and geo-physical drivers of normal versus multi-year groundwater droughts. We find that 35 % of the world has an average groundwater drought duration which is multi-year. In 83 % of these locations, the subsurface extends the meteorological drought signal, whereas in the remaining 17 %, groundwater drought duration appears to be primarily driven by meteorological anomalies. Our analysis offers new insights into global-scale drought exposure by identifying regions which have been most vulnerable to multi-year groundwater drought in the past, as well as those which might be more vulnerable in the future. Importantly, our typology also highlights areas where multi-year groundwater droughts can be anticipated based on meteorological drought anomalies and can therefore inform strategies for managing and mitigating future water scarcity risks.