The relationship between contrasting ages of groundwater and streamflow

Tracer data demonstrate that waters in aquifers are often much older than the stream waters that drain them. This contrast in water ages has lacked a general quantitative explanation. Here we show that under stationary conditions, the age distribution of water stored in a catchment can be directly estimated from the age distribution of its outflows, and vice versa. This in turn implies that the storage selection function, expressing the catchment's preference for the release or retention of waters of different ages, can be estimated directly from the age distribution of outflow under stationary conditions. Using gamma distributions of streamflow ages, we show that the mean age of stored water can range from half as old as the mean age of streamflow (for plug flow conditions) to almost infinitely older (for strongly preferential flow). Many streamflow age distributions have long upper tails, consistent with preferential flow and implying that storage ages are substantially older than streamflow ages. Mean streamflow ages reported in the literature imply that most streamflow originates from a thin veneer of total groundwater storage. This preferential release of young streamflow implies that most groundwater is exchanged only slowly with the surface and consequently is relatively old.