Link between residence times and groundwater renewability obscured by hydraulics

Aquifer residence times are commonly used to make inferences about groundwater system behaviour. However, the link between average aquifer residence times and hydraulic response times which control groundwater storage changes relevant to sustainable pumping, remains unclear. Here, we show that water levels in some aquifers containing fossil groundwater are controlled by modern climates while many others are affected by past climates over periods shorter than their aquifer residence times.