Hydrochronology and watershed-scale reactive solute transport

Watershed hydrochronology - how past precipitation events are stored and discharged through streamflow or evapotranspiration - causes variability of streamflow ages and chemistry. The age-ranked storage selection framework (StorAge Selection or SAS) framework suggests that the age of water in the subsurface (the time since rain or snowmelt infiltration) is the primary factor that predicts whether water will leave the watershed as streamflow or via evapotranspiration. In most watersheds, streams remove more of the youngest water from subsurface storage when flows are high and relatively more of the older water when flows are low. This tendency to preferentially release younger versus older water under different flow conditions is the underlying cause of observed dynamic concentration-discharge relationships.

The SAS framework is also a useful means for understanding patterns of reactive solutes involved in subsurface biogeochemical reactions. A SAS analysis at the Hubbard Brook Experimental Forest, situated in a glacial till, showed that the dynamics of sodium and silicon (produced by weathering) are mostly determined by the contact times between water and mineral interfaces. Combining the SAS model of Providence Creek with sodium release from granite weathering, we simulated the observed sodium concentrations using a kinetic dissolution constant of 166 days and an equilibrium concentration of 4.5 mg/L. This kinetic dissolution constant is two orders of magnitude slower than derived at Hubbard Brook, which could be a consequence of the differences in geology, yielding valuable information on chemical weathering processes. Watershed hydrochronology can therefore predict the reactive solute transport behavior of weathering solutes and subsurface characteristics of watersheds, if the storage selection behavior is well constrained.