Tracer and timescale methods for understanding complex geophysical and environmental fluid flows

Understanding advective-diffusive transport of trace constituents in natural fluid flows is an important challenge in Earth and environmental sciences with many diverse applications, including simulating the fate of contaminants, inferring the location of their source, and model assessment (e.g., [7,21]). Eulerian and Lagrangian methods are routinely used, including novel representations of mixing processes that resort to the so-called fractional-order diffusion. Moreover, geophysical and environmental fluid-flow models routinely produce huge amounts of output, and to make sense of these results sophisticated interpretation methods are required. Among these methods, an approach that is becoming progressively more popular consists in using real, or hypothetical, tracers to tag fluid masses and estimate associated timescales, such as age, residence time, and transit time. These timescales lead to very useful diagnoses that are increasingly applied in interdisciplinary environmental studies (e.g. [2,5]). This special issue presents a number of studies that are relevant to the above-mentioned field of research. Groundwater, soil water, riverine, estuarine, marine and ocean flows are considered, as well as the transport of sinking particles in water.