Modulation of Riverine Concentration‐Discharge Relationships by Changes in the Shape of the Water Transit Time Distribution

Abstract

The concentrations of weathering‐derived solutes in rivers and their covariance with discharge are thought to reflect reactive‐transport processes in hillslopes and to reveal the sensitivity of solute fluxes to climatic change. It is expected that discharge‐driven changes in water transit times play some role in setting concentration‐discharge (C‐Q) relationships, but knowledge gaps remain. To explore the specific role of changes in the shape of the transit time distribution with discharge, we combine models to simulate C‐Q relationships for major cations and Si as example solutes with contrasting affinities to partition into secondary phases. The model results are compared with an analysis of C‐Q relationships using the Global River Chemistry Database. We find that changes in the shape of the transit time distribution with discharge can produce a range of cation‐Q and Si‐Q relationships that encompasses most of the range observed in real catchments, including positive Si‐Q relationships and variable cation to Si ratios. We find that C‐Q relationships (characterized by power law exponents) can remain approximately constant, even as the Damköhler Number (ratio of transport time scale to reaction time scale) is varied over 3 orders of magnitude. So, in our model analysis, C‐Q relationships are as sensitive to hydrologic variability as they are to reaction rates. Additionally we find that, depending on the storage‐discharge relationship, changes in rainfall patterns can influence C‐Q relationships. Altogether, our results suggest ways in which C‐Q relationships may be nonstationary in response to climatic change and/or vary in space and time due to catchment hydrologic properties.