Catchment mixing processes and travel time distributions

This work focuses on the description and the use of the probability density functions (pdfs) of travel, residence and evapotranspiration times, which are comprehensive descriptors of the fate of rainfall water particles traveling through catchments, and provide key information on hydrologic flowpaths, partitioning of precipitation, circulation and turnover of pollutants. Exploiting some analytical solutions to the transport problem derived by Botter et al. (2011), this paper analyzes the features of travel, residence and evapotranspiration time pdfs resulting from different assumptions on the mixing processes occurring during streamflow formation and plant uptake (namely, complete mixing and translatory flow). The ensuing analytical solutions are analyzed through numerical Monte Carlo simulations of a stochastic model of soil moisture and streamflow dynamics. Travel and residence time pdfs are shown to be time-variant as they mirror the variability of the relevant hydrological fluxes. In particular, the temporal fluctuations of the mean residence time are shown to reflect rainfall dynamics, whereas the variability of the mean travel time is chiefly driven by streamflow dynamics, with lower frequency and higher amplitude fluctuations. Dry climates enhance the effect of the type of mixing on catchment transport features (e. g., mean travel times and seasonal dynamics of stream concentrations). The implications for the interpretation of tracer experiments are also discussed, showing through specific examples that models disregarding nonstationarity may significantly misestimate travel time pdfs.