Fast-responding near-natural hydrological systems as early markers of socio-economic drought impacts

Improving drought early warning requires indicators that capture not only precipitation deficits but also how quickly hydrological systems respond and when societal consequences emerge. Here, we assess whether drought propagation pathways and antecedent groundwater conditions in near-natural hydrological systems can serve as early-warning signals for the timing and emergence of drought impacts. We analyze drought propagation in 132 near-natural catchments (areas < 500 km², with no noticeable direct human influence from reservoir storage or abstractions) located within 72 administrative regions in Germany. Using daily precipitation, streamflow, and biweekly groundwater observations spanning almost 70 years, we identify droughts in each hydrological compartment using the variable threshold level method. This allows the reconstruction of event-specific propagation sequences and lag times, which are then linked to Drought Impact Statements (DIS) extracted from news media between 2000 and 2024, documenting the timing and type of reported socio-economic drought impacts. Our results show that drought propagation varies in space and time, with catchments exhibiting different propagation pathways (defined by the order and timing with which drought conditions propagate from precipitation to streamflow and groundwater) and with pathways changing across events. Fully propagated droughts (reaching both streamflow and groundwater) are preceded by prolonged periods of below-average groundwater levels, indicating strong hydrological preconditioning. Linking propagation pathways to reported impacts shows that the timing and composition of socio-economic drought impacts differ across pathways, suggesting that drought propagation through hydrological compartments influence the timing of impact emergence and the sectors affected. Overall, our results highlight how monitoring groundwater levels as indicators of system preconditioning, together with propagation dynamics characterized by short propagation lags, provides impact-relevant information for drought early warning, helping to anticipate impacts.