This project is innovative in that it uses a new modelling approach which overcomes some of the problems in traditional models. Distributed models traditionally divide the hillslope or catchment into compartments of smaller or larger spatial extent. Here we present the multiple interacting pathways (MIPs) model approach as a grid-free alternative which explicitly defines the transit times along specific flow pathways. This provides a basis for studying the controls of hydrological processes on water quality. In this proposed project we will evaluate MIPs models using detailed isotope data. Such data provide an important test to evaluate a model that predicts both water and conservative solute fluxes. The hydrometric, geochemical and isotope data which has been collected for Swedish research hillslopes and catchments during the past decades provides excellent test cases for this modelling approach. These data will be complemented by new tracer experiments to better challenge this model. These tests are important because realistic simulation of flow pathways and transit times is a prerequisite for extending the model approach to reactive solutes and issues such as weathering, and the cycling of carbon or nitrogen.