Global patterns, drivers and trends of multi-year groundwater drought

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 and anthropogenic pumping or 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 (30 arc-seconds or ~1 km) groundwater dataset produced by the global groundwater model GLOBGM to investigate the global trends, patterns and drivers of groundwater drought from 1960-2019, with a specific focus on multi-year events. We start by characterizing the relationship between meteorological drought (represented by SPEI-12) and groundwater drought, evaluating how and to what extent the sub-surface plays a role in modulating the meteorological signal. Subsequently, we categorize the global groundwater data based on its relationship with the meteorology to provide a framework for understanding the processes and geo-physical drivers of normal versus multi-year groundwater drought events in each category. We find that 35% of the world has an average groundwater drought duration which is multi-year. In 84% of these locations, the subsurface extends the meteorological drought signal, whilst in the remaining 16% the groundwater appears to be primarily driven by SPEI-12. We found that pooling of meteorological droughts, the presence of abstractions and lag in groundwater response time are the main drivers for multi-year groundwater droughts. Our analysis offers new insights into global-scale drought exposure and can help inform strategies for managing and mitigating future water scarcity risks.