Recent Developments and Emerging Challenges in Tracer-Aided Modeling

During the last decade, tracer-aided hydrological models (TAMs) have been applied in numerous studies and have successfully evolved for different purposes. Such studies confirmed the value of tracer data in hydrological modeling, offering insights into internal storages, water sources, flow pathways, mixing processes, and water ages, which cannot be derived from hydrometric data alone. The direct coupling of tracers into flux tracking and the water balance of hydrological models can reduce model uncertainty through increased hydrological and biogeochemical process knowledge. More specifically, such models can simultaneously capture the celerity of hydrological responses with the velocities (and age) of water particles. As a result of the increased availability of high-resolution tracer data characterizing hydrological functioning across the Critical Zone and entire landscapes, together with the rapid improvement of computing capacity, four major advances reshaped the capability of TAMs, which we review in this paper: (1) enhanced representation of spatial heterogeneity, (2) more explicit conceptualization of ecohydrological partitioning, (3) application to larger catchment scales, and (4) incorporation of non-conservative tracers in coupled water quality modeling. However, persistent challenges have also emerged, particularly in relation to data acquisition, mismatches between the information content of data and scale of application, uncertainties in model structures, as well as adaptation of multi-criteria calibration. In this review, recent advances and remaining challenges of TAMs have been summarized and discussed with a particular focus on conservative tracers and flux tracking models.