Technical note: Transit times of reactive tracers under time-variable hydrologic conditions

Abstract. Water transit time distributions (TTDs) have been widely used in hydrology to characterize catchment behavior. TTDs are also widely used to predict tracer transport, but the actual transit times of tracers, which may differ from those of water because of different physical processes and tracer input patterns, remain largely unexplored. Here, we address the TTDs of tracers transported by water and subjected to linear processes of sorption, degradation and interaction with evapotranspiration. We focus on the special case of randomly sampled systems (which are mathematically similar to well-mixed systems), for which analytical solutions can be derived. Through the analytical solutions and their numerical implementation under time-variable flow conditions, we explore how reactive transport parameters impact tracer TTDs. Results show that sorption delays tracers as much as a larger water storage does. Evapotranspiration can both increase tracer transit times (in the case of evapoconcentration) or decrease them (in the case of net evaporation extraction), while degradation can be seen as an additional output flux that always shortens tracer transit times. Combinations of randomly-sampled systems are widely used as transport models and we show how tracer TTDs may differ from water TTDs in the building blocks of such models. Distinguishing the TTDs of tracer from those of water is important for an improved understanding of water quality dynamics and the circulation of solutes at the catchment scale.