Linking tracers and travel time distributions: the emergence of age mixing patterns

Abstract:

Tracers have always played an essential role in the investigation of hydrologic systems, where water flow pathways cannot be directly observed. The use of tracers in catchment hydrology has been constantly increasing, with countless applications including water travel time estimation and hydrograph separation. Moreover, several high-quality datasets now exist and are publicly available, that enable the systematic use of tracers to improve the process representation of hydrologic models. However, tracer information is integrated over space and time and is best interpreted using a coherent and robust theoretical framework. Here, we propose methods derived from the time-variant theory of travel time distributions to interpret tracer measurements and model catchment functioning under different hydrologic conditions. In such a framework, the population of water particles moving within a catchment is considered as a dynamical system, where individuals move and mix inside a control volume before reaching the sampling point. The variability of flow, storage states and mixing experienced by the catchment are explicitly accounted for, providing rigorous tools for tracer investigation. We present applications to highly monitored watersheds (including Hubbard Brook (USA), Hupsel Brook (NL) and Plynlimon (UK)) that show how the transition between catchment states is mirrored by changes in the ages stored and released by the catchment, causing the chemical composition in the discharge to vary accordingly. This can be used to address a number of environmental problems, such as quantifying the interactions between shallow and deep flow systems and understanding nutrient loading persistence.