Variability of transit time distributions with climate and topography: A modelling approach

The time that rainfall takes to reach the outlet of a catchment as discharge (transit time) is a fundamental and integrated measure of catchment hydrological processes and solute transport mechanisms. As such, many efforts have been dedicated to its understanding and quantification. However, defining and ranking which factors, internal and external to the system, control the distributions of transit time is still an open challenge. Here, we develop a two-stage approach to explore climate and topography controls on transit time, using a fully distributed hydrological model coupled with a transport component. Specifically, we apply the model to two synthetic topographies under five observed climate regimes. With this setup, water fluxes from two years of daily rainfall events are singularly tracked across the catchments to then derive the distributions of transit time and fraction of young water for each combination of topography and climate. Results highlight a considerable variability of transit times in all climates and a pronounced effect of topography within a given climate. They further reveal that for wet climates it is possible to define a curve describing water transit time 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 and not amenable to a simple summary. Despite simplifications, quantitative model-based inferences of transit time distributions are useful to better understand how climate and topography affect catchment functioning.