Assessing Groundwater Drought Hazard in Groundwater Depletion Regions: Recommendations for Large-Scale Drought Early Warning Systems

In groundwater depletion (GWD) regions, negative trends in groundwater storage (GWS) are problematic for groundwater drought detection, since they mask climate-induced drought signals. As this is not yet considered in any large-scale drought early warning system (LDEWS), we used GWS from the global hydrological model WaterGAP 2.2e to investigate, for the first time at the global scale, how groundwater drought can best be quantified in GWD regions. We analyzed two methods: (a) Linear detrending of monthly GWS time series and (b) analysis of naturalized GWS computed by assuming no human water use. We found that linear detrending is unsuitable for global-scale groundwater drought monitoring and forecasting as even small deviations from a pronounced linear trend can lead to a systematic over- and underestimation of the drought hazard. In contrast, indicators from naturalized GWS can identify climate-induced GWS anomalies. We recommend to provide, in LDEWS, indicators of the magnitude, duration, and severity of groundwater drought that are based on monthly time series of model-derived GWS simulated with (“ant” variant) and without water use (“nat” variant). In both GWD and non-GWD regions, the “nat” variants inform about the occurrence of climate-induced droughts. In GWD regions, they specify periods in which the negative GWS trend is either exacerbated or alleviated by climate-induced variations of groundwater recharge, while “ant” variants are of very limited informative value. In non-GWD regions, the joint analysis of “nat” and “ant” variants informs whether a climate-induced drought is aggravated or alleviated due to human activities.