Scale effect on geomorphologic and kinematic dispersion

Geomorphologic dispersion relates to the fraction of the variance of residence time distributions that can be directly linked to the heterogeneity of the paths available to hydrologic runoff of a river basin in response to suitable rainfall pulses. It thus defines the dispersive characters of geomorphological origin. Hillslope transport processes and systematic variations of advective processes in the channel network define a kinematic component of dispersion. Here we suggest that channel-borne kinematic effects are likely to be clouded by hillslope dispersion in a surprisingly wide spectrum of basin scales. We find that the signatures of hillslope transport on travel time distributions do not fade away rapidly with the size of the catchment (i.e., when the basin scale becomes much greater than the mean hillslope size), in particular because of their major impact on the skewness of the travel time distribution. We suggest that an appropriate description of hillslope-channel transitions entails a combination of geomorphic and kinematic effects which plays a decisive role in shaping the main characters of the hydrologic response in cases of practical interest.