Simulating catchment scale transit-times using an age-ranked storage approach in a south-eastern Australian catchment

The amount of time water spends in the subsurface is highly informative of the hydrology and water quality of catchments. Recent developments to catchment transport theory have resulted in a modelling framework that can concurrently model water and solute transport within a catchment volume through the use of an age-ranked storage (also known as StorAge Selection or SAS). This formulation allows non-stationary travel-time distributions to be estimated, providing insight into the functioning of catchments. Effectively, each precipitation event is added to the age-ranked storage and is tracked until it is removed by either streamflow or evapotranspiration. Studies utilising this framework are sparse globally and have generally been limited to wetter and energy-limited environments. We tested the age-ranked storage approach to estimate catchment mean transit-times in the Corin catchment in south-eastern Australia, an important water supply catchment for capital city Canberra. A beta sampling distribution is used to simulate the selection preference of discharge and evapotranspiration from storage. A time-series of conservative tracers (oxygen-18) is used to constrain parameter selection to probable combinations that best reproduce streamflow tracer concentrations. The results suggest a strong preference for older water in streamflow and a milder preference for younger water by evapotranspiration from the age-ranked storage. Estimates of the mean transit time (MTT) of discharge over the study period ranges from 3-9 years, highlighting the potential lasting impact of pollutants on catchment water quality and the existence of long flow paths within the Corin catchment that may provide a buffer against drought. The modelling framework and implementation is discussed.