Differential C-Q Analysis: Including the Impact of Lateral Storage and Hydrologic Transients on Solute Concentration within a Mountainous Headwater Catchment

Concentration-discharge (C-Q) relationships have been widely used as "hydrochemical tracers" to determine the variability in riverine solute exports across seasonal and annual time scales. However, these C-Q relationships are limited to investigations of solute transport dynamics at individual sampling stations, such that they create an incomplete understanding of the solute behavior upstream or downstream of the sampling station. The objective of this study is to apply differential C-Q relationships and assess spatial variability and solute behavior across multiple sampling stations, as well as investigate controls on solute variability. This study uses two years of water quality data from the East River—a high-elevation headwater catchment in Colorado—and compares applications of traditional and differential C-Q relationships. For the 2016 water year, ΔC-ΔQ analysis indicates that calcium concentration decreased from upstream values noticeably during the peak rainy season, instead of accumulating downstream, as is expected of weathered solutes. In contrast, sulfate underwent little change in concentration downstream during the same time period, despite its reactivity. It is likely that sulfate coming from upstream station during peak river discharge reduces calcium concentration through Ca-SO₄ precipitation. Later that same year when streamflow reduced to baseflow conditions, calcium notably accumulated downstream, further suggesting the influence of a tributary source of sulfate to the stream. Overall, the differential C-Q analysis is able to provide an improved understanding of the lateral inputs to and biogeochemical processing within the lower East River triangle.