Hydrological modelling and water flux tracking to quantify controls on water transit and residence time

All over the world, hydrological models at the basin scale have been extensively used to understand flow dynamics, runoff generation, and manage water resources. At the same time, the movement of water within catchments along with solute transport still requires an adequate modelling approach at large scale, that would have a high value to tackle water quality problems. Water and solutes move in catchments following different paths and can take from minutes to centuries to reach the catchment outlet. This spectrum of paths and time scales, along with their dependency on climatic forcing, catchment heterogeneity, and numerous other factors, represent the main challenge towards inferring their magnitude and dependencies, and ultimately determining the processes that drive streamflow generation and catchment-scale dispersion. Water transit time distributions are a crucial metric for capturing these processes and they can be ultimately used to address the environmental challenges related to water quality. With the aim of better understanding and estimating them and their controlling factors, a novel model and framework are here developed. Hydrological and conservative transport simulations are coupled to follow multiple water parcels in space and time through the catchment. The model is firstly tested and benchmarked on a catchment in UK with extensive datasets of discharge and chloride, a conservative environmental tracer. The dynamic transit time distributions (TTDs) both forward and backward in time are satisfactory reproduced by the model. Results show that TTDs conditional on a given rainfall time are mostly correlated to the season in which the rain event occurs, whereas TTDs conditional on a given exit time are mostly affected by catchment wetness. To further explore this, in a second step the model is applied to five catchments in different climatic regions and scenarios are developed to infer the influences of climatic and geomorphological characteristic on transit and residence time. Results reveal that for wet climates we can define a curve describing water transit as a function of cumulative discharge that only depends on topographic properties. On the contrary, in dry climates the variability of transit time and young water fraction is much larger in all the representations, thus a summary of climatic effects on transit times is not straightforward. Overall, the study achieved an improved understanding of catchment transport processes in different climates and topographies, and further research directions are identified aiming at an unifying catchment transport representation and a quantification of natural and anthropogenic impacts on water resources