Flow and transport at the catchment scale: hillslopes vs river network who plays the leading role?

Travel time distribution allows for an effective representation of all the physical processes occurring in hydrologic system and provides a stronger link between catchment-scale water flow and water quality models. The travel time distribution TTD has been extensively used in hydrological modeling and several approaches, with different level of complexity, have been developed in past.

The aim of our research is to investigate the travel time distribution in river catchments, specifically we would disentangle the relative contributions of the hillslopes and the river network in determining the TTD at the catchment outlet. Catchment is represented as a set of hillslopes connected to the river network; the effective recharge, moves first within the hillslopes and then is conveyed by the river network up to the outlet control section. Our theoretical framework combines a Boussinesq hillslope flow model with a GIUH-based formulation of transport along the river network. The TTD is characterized by the first two spatial moments of the travel times of the water particles crossing, at a given time, the catchment outlet section. Despite the assumptions required by the analytical development, the proposed approach is built on key physical processes and can be used to analyze the complexity governing the water age distributions and the travel time in catchments.

The analysis of a 10-year long series of synthetic recharge indicates that:

• the travel time moments strongly depend on the hillslope flow dynamics, as a consequence transmissivity is a key parameter;
• transport within river exerts a secondary effect;
• the sequence of wet and dry periods, is also of primary importance in determining the travel time moments and their temporal fluctuations;
• the variability of the travel times at the outlet, at a given time, is generally very large, with coefficient of variation typically larger than unity.